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2.1.1 Bonfire kilns
2.1.2 Sinde up-draught kiln
2.1.3
Bangladesh up-draught kiln
2.1.4 Permanent up-draught kilns
2.1.5 European
up-draught kilns
2.1.6 Down-draught kilns
2.1.7 Khurja kiln
2.1.8
Mayangone kiln
2.1.9 Bujora down-draught
2.1.10 Cross-draught
kilns
2.1.11 Tube kilos
2.1.12 Chinese chamber kiln
2.1.13 Champaknagar
chamber kiln
2.1.14 Sumve cross-draught kiln
A kiln may be described as an enclosure to contain heat. Potters use it to fire their pots and they have developed a countless number of different kiln types, each one reflecting the demands of local markets, tradition, skills and materials.
Even so the basics of all ceramic kilns are the same; heat is
introduced into the enclosure surrounding the pots. Some heat is lost through
the walls or is carried away with the combustion gases, but as more heat is
introduced than escapes, the temperature rises and the pots will mature.
2.1.1 Bonfire kilns
The oldest type of kiln, dating back more than 10,000 years, is the bonfire kiln. These kilns are still widely used for firing traditional unglazed red ware (terracotta) because they are still the most suitable for small-scale production of low-fired pottery. This is due to the fact that no investment is needed for a permanent kiln, that the firing at most takes a few hours and that cheap and readily available fuels such as straw, grass and cowdung can be used.
Fig.2-1: Bonfire kiln
Sukuma potters
The Sukuma women in Western Tanzania often use split roots of sisal as a fuel (fig. 2-2). The roots produce intense heat and the firing takes no more than half an hour. The pots are fired no higher than 700 °C. This is an advantage for pots made for cooking over an open file because the clay has not started to sinter and its open structure can more easily adjust to the thermal shock of being put over a fire.
The pots are dried in the sun the whole day so that moisture in the pots will not crack them when they are exposed to the sudden heat. The pots are raised a bit on a layer of broken pots and some sticks of sisal roots are placed in between. About two layers of roots are placed around the small heap of pots and set on fire. Another layer of roots is added during the fire and sometimes more where the fire consumes the roots too fast. Before the pots have cooled they are raked out of the smouldering fire and beaten with branches (fig. 2-3) dipped in a bark soup. The carbonaceous matter of the extract sticks to the pots and gives them a partly water-proof surface.
Nepalese potter
In fig. 24 a potter in Nepal is preparing his kiln for firing.
Behind him another kiln is opened and the pots are ready to be sold. The pots
are stacked in a big heap with straw and in the lower part firewood in between.
The pots are finally covered with straw, broken pots and an insulating layer of
ash on top. Holes in the bottom of the kiln allow air for combustion to enter.
The fire is lit in the bottom of the kiln and then gradually works its way
through the heap. This kiln illustrates a development from the Sukuma kiln as it
has the heat travelling up through the pots, vent holes making control of the
fire possible and an insulating layer for better containment of the heat. Firing
temperature may be 150 °C higher compared to the Sukuma kiln.
2.1.2 Sinde up draught kiln
The kiln of the Sinde potters (fig. 2-6) has no permanent structure. Four fireboxes, one on each side, are constructed by the setting of pots. A bottom layer of once-fired, partly broken pots works as flues through which heat from the fireboxes spreads to all corners. The green pots are stacked on top and other cracked pots are built into a kiln wall.
Straw, pieces of broken pots and clay form the outer layer. Vent
holes are left in the crown of the setting. Firing is carried out by stoking
firewood in the four fireboxes. The combustion gases and heat go up through the
setting and leave through the vent holes at the top. Kilns of this kind are
called updraught kilns. The use of fireboxes and flues, though simple, allows
much better control of the firing. In the beginning a very small fire allows the
pots to dry out completely and at the end of the firing heavy stoking will
ensure a high temperature. The hot gases and flames from the fire circulate all
over the kiln creating a more even temperature and utilizing the heat
better.
2.1.3 Bangladesh
up-draught kiln
In fig. 2-8 a simple up-draught kiln is nearly ready for firing. Once-fired pots are sewing as a kiln wall as with the Sinde kiln, but this one has a permanent firebox dug out under the kiln. Fuel is cowdung stuck on bamboo sticks as this area, the western part of Bangladesh, has hardly any firewood to offer.
Fig.2-9: Ancient up-draught kiln
from Greece
2.1.4
Permanent up-draught kilns
In the Near East up-draught kilns with permanent outer walls were developed (fig. 2-9) and this type of kiln spread with migrating potters from Persia to India. It is still widely used and fig. 2-10 shows an improved type of up-draught kiln which was constructed by Indian advisers in Tanzania. Stoking is done through firemouths at two sides and the hot gases enter the kiln chamber through the perforated floor and leave through holes in the crown. Great skill is needed when setting the ware so that space is left for the gases to pass in a way that ensures even temperatures. At cold spots more space is left so that more hot gases will pass there while the spots tending to overheat are stacked more densely. This kiln is fired to 900-1000 °C.
Fig.2-11: Setting of pots in an
up-draught kiln has to be done so that the hot gases rise evenly throught the
pots.
Glazed pots
The permanent structure makes packing of the kiln easier and the
walls retain and reflect the heat better so that higher temperatures can be
reached. The drawback, compared to the lighter kilns mentioned above, of the
heavy kiln structure is that a great deal of fuel is used for heating the walls
along with the pots. The permanent kiln chamber makes it possible to stack
glazed pots properly and this may be the main reason for constructing a
permanent kiln.
2.1.5
European up-draught kilns
The up-draught kiln originating in the Near East spread to Europe where it was further developed and reached its perfection with the bottle kilos (fig. 2-12). These kilns were widely used until the beginning of this century, when they were replaced by downdraught kilns. The bottle kilns could be fired up to 1300 °C. Dampers on top of the dome could be opened and closed for directing the draught. That enabled the skilled fireman to achieve fairly even temperatures. The ware was placed in saggars to protect it from the combustion gases. Often a biscuit chamber over the main chamber was added so that the otherwise wasted heat was used for biscuiting.
Fig.2-12: Bottle kiln with its
innovations: chimney, firebricks and iron grates for burning coal.
Refractories, grates, coal, chimney
These up-draught kilns were originally developed in Germany, by the beginning of the 17th century, in an attempt to produce porcelain which was then only produced in China. The 1300 °C needed for porcelain was reached by constructing the kiln with firebricks and by firing coal on cast-iron grates. The grates made it possible to speed up combustion of the fuel and reduce the intake of excess air. A chimney placed on top of the chamber creates the extra draught needed to draw combustion air through the grates.
Fig.2-13: Cross-section of a hovel
kiln
Hovel kiln
A variety of the bottle kiln is shown in fig. 2-13. It works in the same manner but a hovel encloses the kiln and protects it and the workers from the weather. The kiln itself was cheaper to construct as it did not need to carry the weight of the chimney and the hovel could be constructed entirely from common red bricks. The potteries of North Staffordshire, England, were famous for these kilns which literally dominated the skyline around Stoke-on-Trent. The hovels could be up to 21 m high.
Limitations of up-draught kilns
By the turn of the century the up-draught kiln was considered outdated. A ceramic expert Mr. E. Bourry wrote: "Intermittent kilns with up-draught ought to be condemned. They have the double effect of being wasteful and giving an unequal distribution of heat ... and only deserve to be forgotten."
Fig.2-15: Chimney effect creates hot
spots in an up-draught kiln.
The up-draught kiln is wasteful because the hot combustion gases rush too quickly through the kiln setting, so that the heat of the gases has little time to be transferred to the ware.
The bottom of an up-draught kiln tends to become hotter as the
hot gases strike here first. Furthermore, in the setting of the ware some places
will be more open and the hot gases will tend to pass that way. That makes these
spots hotter whereby even more gases will be pulled that way just like a hot
chimney pulls better than a cold one. The updraught of the gases simply creates
this tendency of making hot spots even hotter. These drawbacks led to the
invention of down-draught kilns.
2.1.6 Down-draught kilns
In a down-draught kiln the hot gases from the fireboxes circulate to the top of the kiln chamber, are then pulled down through the setting and leave through flue holes in the floor. Under the floor flue channels lead to the chimney (fig. 2-16).
Even temperatures
Hot air rises so the downward draught of the hot combustion gases tends to avoid the hot spots and seeks out the cold spots where the downward pull is stronger. In this way the draught will by itself even out temperature differences.
Fig.2-16: Down-draught kiln with flue
channels under the floor
Bag walls
A wall, named a bagwall, at the inlet from the fireboxes directs
the hot gases upward. In case the top of the setting tends to be too hot the
height of the bagwalls is lowered and vice versa. Sometimes holes in the bag"
wall help but the holes weaken the wall and it may collapse during
firing.
Fig. 2-17: The downnward draught
avoids the hot spots and seeks out the cold spots in the kiln seting.
Heat economy
The combustion gases spend a longer time inside the chamber, compared to the up draught kiln, simply because they have further to go. So more heat is transferred to the ware and consequently fuel is saved. As the hot gases leave the kiln chamber at ground level it is easier to let them pass through another chamber or several chambers before entering the chimney.
Chimney
A chimney for down-draught has to be tall to create a strong
pull, which is required to force the heat downward especially if more chambers
are added. The Bujora kiln (p. 53) has an up-draught biscuit chamber which at
the same time serves as a chimney.
2.1.7 Khurja kiln
The Khurja kiln is a typical example of a coal-fired down-draught kiln of European design (fig. 2-18).
Fig.2-19: Khurja down-draught kiln a)
side elevation, b) ground elevation showing flue holes and chanels. The details
for this kiln have been obtained from "Status Report on Ceramic Industry at
Khurja", published by Central Glass and Ceramic Research Institute, Calcutta,
India.
2.1.8 Mayangone kiln
The kiln in fig. 2-20 was originally woodfired but has recently been converted to oil. It was built in 1924 after a German design that has five flue channels in the wall. These help to transfer some of the heat of the flue gases back to the chamber through the wall (fig. 2-23). The outer kiln wall carries the weight of the top chamber and chimney and the kiln is reinforced with plenty of mild steel bands. The flue channel under the floor can be cleaned from the outside, which is a good idea.
The kiln fires to cone 7 (1250 °C) and the temperature
difference between top and bottom is with 1-1+ cones (30 - 50 °C).
2.1.9 Bujora down-draught
This kiln was constructed with a second chamber which works as an up-draught kiln and chimney. When the first chamber reaches 1240 °C the second chamber would be 800-900 °C which is sufficient for firing biscuit ware and common red bricks. The top third chamber can be used for calcining feldspar and quartz for glazes and clay bodies.
Chimney chamber
Fig.2-23: The Mayangone kiln is made
with flue channels in the walls.
As firebricks, of which a normal chimney would be built, were made of kaolin which was expensive, the chimney was expanded and turned into a second chamber. The wide inside diameter made it safe to build the chimney chamber of common bricks. In any case, a second chamber meant firing more ware for the same outlay. A drip-plate burner could be placed in the flue channel (fig.2-24) between the two chambers so that the temperature of the second chamber could be raised further in case the waste heat of the stoneware firing was not sufficient. A small fire was lit in the flue channel in order to increase the pull of the chimney when starting a firing.
Fig.2-24: Two-chamber Bujora kiln,
cross-section. The first chamber is provided with three fireboxes for oil firing
but fireboxes for firewood or coal could be used as well.
2.1.10 Cross-draught kilns
Cave kilns
The cross draught kiln originated in the Far East and as with the up-draught kiln this type of kiln must have developed gradually from the open bonfire. Potters found that by enclosing the fire higher temperatures could be reached; instead of building up a wall around the fire the potters hollowed out a cave into a bank of clay (fig. 2-25). The lower end served as a firebox and the hot gases were carried through the ware across the cave chamber and out through the flue hole. Cave kilns are not in use any more, but old kilns have been found by archaeologists.
Fig.2-25: Cave kiln dug out of a
clay bank.
Stoneware temperatures
Such simple kilns were capable of firing stoneware. The cross-draught through the ware transferred more heat to the ware compared to up-draught and the fully enclosed kiln chamber retained the heat well. The kiln developed into a variety of cross draught kilns. Fig. 2-26 shows a reconstruction of a kiln type which was used in Central Thailand 700 years ago for firing glazed stoneware, and similar kilns, though bigger, are still used throughout South-East Asia. Fig. 2-27 shows a wood-fired kiln used for firing celadon stoneware. It has no separate firebox, but the front part of the kiln is 0.5 m lower and serves as a fireplace. The floor for the setting of ware slants upward and the kiln chamber narrows towards the exit flue. That helps to create a more even firing temperature.
The faster the flow of hot air the more heat will be transferred to the pots. Close to the fireplace the air is hot but moves slowly, whereas towards the back the air is cooler but is moving faster due to the narrowing kiln chamber.
Some kilns of this type have stoking holes at the sides so that
stoking is done here towards the end of the firing.
2.1.11 Tube kilns
The cave kiln, supposedly, was made ever longer until it developed into the long sloping tube kiln about one thousand years ago. Tube kilns are up to 50 m long and ate used for both earthenware and stoneware. Tube kiln is seen from the firebox end. The kiln chamber is a long uninterrupted tube with an exit on top. The tube is filled with pots, traditionally in an open setting, but now also with saggars (fig. 2-28). The fire is started in the firebox and the combustion gases go through the whole kiln to the top exit and transfer all of their heat to the ware on the way. When the lower section of the kiln has reached maturing temperature stoking into the tube is begun through side holes just above the matured section (see p. 71). The combustion air enters through the firebox and is very hot when it reaches the firing zone. In this way the firing zone slowly moves upward until the whole kiln is fired. When the upper section is fired the lower section has already been cooled considerably by the intake of combustion air.
The difference in height of the exit flue and the inlet at the firebox is often enough to create sufficient draught through the kiln. However, some tube kilns have a low chimney as seen in fig. 2-29.
Figure
2.1.12 Chinese chamber kiln
In China the tube kiln was further developed by breaking up the long tube into separate but connected chambers (fig. 2-30). The fire is started in the firebox and the first chamber is fired as other kilns. When the desired temperature is reached in the first chamber, say 1280°C, the second may be around 1100 °C and the third around 700°C. Firing is continued by side stoking in the second chamber through openings in the door. The temperature in the second chamber rises rapidly because the combustion air is preheated from passing through the first chamber. The preheated air is so hot that thin sticks of wood fed through the stoking holes burn instantly.
Fig. 2-30: Cross-section of a chamber kiln
Setting
In each chamber a bagwall or saggars force the heat upwards
after which it is drawn down through the setting and across to the exit flues
leading into the next chamber. The draught is normally created by the upward
slope of the kiln. The slope of a chamber kiln is about 20°. The Chinese
chamber kilns could have up to eight chambers and would be stacked with ware
produced by many individual potters. The largest kilns could be up to 400
m³ in total kiln space.
2.1.13 Champaknagar chamber
kiln
Fig. 231 shows a three-chambered woodfired kiln at a pottery school in Champaknagar in Bangladesh. Identical kilns are built by groups of students when they set up their own potteries. According to the size of the group kilns are built with two or three chambers and more chambers can be added later as the production increases.
Earthenware
The kilns are fired to 1100 °C with en open setting on kiln shelves. Eeach chamber is about 3.5 ml and takes 600 mugs. Ordinary red bricks are used for construction throughout, but there are plans to provide the kilns with an inner lining of insulating firebricks in order to improve the fuel economy.
Firing
The firing is started at midnight so that the last part of firing takes place during the day in order to minimize the risk of fire in the villages. The pots have been biscuit-fired by traditional kilns similar to the type shown in fig. 2-8 and so there is no need of a smoking fire. During the first three hours a layer of embers is built up in the firebox. After that firing is done at full rate until the temperature reaches 1100 °C in the first chamber at 8-10 a.m. Three dampers in the bottom of the chimney are used for evening out temperature differences sidewise. The stoking is then moved up to the next chamber and the firewood is fed through stoke holes at both sides above the inlet from the lower chamber. Each additional chamber reaches 1100 °C after about two hours' stoking.
Firewood
The first two chambers consume 2200 kg firewood and each additional chamber about 500 kg. Unfortunately the firewood is not properly seasoned so heat is wasted drying out the extra water in the firewood.
Fig.2-33: Two-chambered wood-fired
kiln. Champaknagar, Banladesh.
Extra chamber
The small extra cost of firing additional chambers makes it tempting to add several more. The additional chambers would also reduce the size of the chimney which is 4.4 m for the two-chamber version. However, a huge kiln capacity would also mean longer periods between firings and would mean that more space for storing pots awaiting firing would be needed. It may also be difficult to set aside enough money for the production costs in the longer time between making pots, firing and selling the finished ware.
Construction
It is better to start with a few chambers while the pottery workshop is starting up; later as production and confidence grow additional chambers can be added without much interruption to production. It is better to plan for future expansion when designing and constructing the kiln, so that sufficient space is left to build on. In case the kiln is built on a slope it is-easier to add extra chambers at the firebox end as the chimney is a larger structure to dismantle and reconstruct.
Fig.2-34: Champaknagar firebox.
Firebox
The firebox shown in fig. 2-34 is made very wide because the unseasoned firewood has to spend longer time drying in the firebox compared to properly dried firewood. Other types of fireboxes as described under fireboxes (p. 73-87) can be used as well.
A more rational solution of course would be to season the firewood properly. However, small village potteries have no money to invest in a stock of firewood sufficient for drying 4-6 months. It is costly to be poor.
Self-supporting
The chamber kilns are constructed without any iron frame
supports. The structure supports itself as the chambers lean onto each
other.
2.1.14 Sumve
cross-draught kiln
The cross-draught principle of the chamber kiln is used in a small waste-oil-fired kiln constructed in a small village pottery in Sumve, Tanzania. The kiln is fired to 1250°C and uses an open setting. It is constructed with self-made insulating firebricks with an outer wall of common bricks.
The chamber is constructed as a catenary arch (see p. 102) which makes the structure self-supporting. The capacity of the kiln is rather small but for newly started workshops it is fine. This kiln could be expanded by adding more chambers as is done in Champaknagar.
Fig.2-36: Stoneware kiln with about 1
m³ capacity (cross-draught
kiln).
2.2.1 Firewood
2.2.2 Agricultural waste
2.2.3
Peat
2.2.4 Lignite
2.2.5 Coal
2.2.6 Oil products
Nearly everywhere the cost of fuel for firing kilns is the single biggest cost of ceramic production. In some areas the cost of fuel simply rules out the production of modern pottery and only traditional pottery fired with agricultural waste materials is economically possible.
Kilns heated by gas or electricity will not be described here because these fuels are seldom available or their cost is prohibitive. (This might change in the future when big hydro-electric or natural gas projects will make these types of energy more easily available and cheaper.)
That leaves us with three main sources of fuel: firewood (and agricultural waste), coal and oil.
Cost and supply
In many areas only one type of fuel is available for potters. However, those fortunate enough to be able to choose from several types of fuel should consider which fuel will serve them the best by comparing (l) the cost of the fuels and {2) how reliable the supply is.
Total cost
The cost of transport and the time spent on buying the fuel needs to be added to the actual market price, e.g. sawdust may be very cheap at the sawmill but if this is 50 km from the pottery the cost of hiring a lorry may make this fuel very expensive. Or in case coal can be bought from a government store the cost of employing a person to do the necessary paperwork, etc. will also add to the fuel cost. The different fuels have different heating values and this should also be taken into account, e.g. l kg of firewood may only produce half the heat of 1 kg of coal (see appendix).
Table 2-1 is an illustration of how to compare fuel costs. This comparison is based on the estimated fuel consumption for the firing of a two-chamber kiln of Champaknagar type. In this example firewood turned out to be the cheapest fuel, but if the source of coal had been closer it would have been less costly to transport and could in that case become the chosen fuel. The cost of coal in this example also includes $ 12 for employing a person to acquire the necessary licence to purchase coal from a government store. In areas with a higher cost of labour, firewood would become more costly due to the heavy work involved with felling the trees and cutting the firewood.
Table 2-1
Supply
A low-cost fuel which is seasonal or of unreliable supply may
turn out to be costly due to delayed firings. These in turn will cause cuts in
production and income. In order to secure a regular supply it may often be
better to accept a higher cost of fuel than suffer the results of an insecure
supply. If the supply situation is difficult it is better to arrange fireboxes
which can burn two or more different fuels. For example it is easy to place
drip-plate burners in a firebox for coal or firewood. The additional cost of
making two firing systems may be recovered in one or two firings.
2.2.1 Firewood
Formerly firewood was the main fuel all over the world but today in the industrialized part of the world firewood accounts for only 0.4% of fuel energy used, while in the developing world firewood still accounts for 25% of the energy used. In the poorest countries about 40% of the energy is from wood. That figure reflects the fact that oil products and coal are too expensive and often not available to the majority of people in the developing world. Therefore, many potters, especially those in rural areas, will continue to rely upon firewood for firing their kilns.
Ash colours
Firewood is easily capable of heating kilns beyond 1300 °C if desired, and it also produces long flames which help to even out the temperature inside the kiln. Firewood ash will not normally harm glazes, apart from slightly changing their colour. Some potters even try to promote this colour effect for its decorative quality. If this discolouration is not desired the ware should be fired in saggars.
Heat value
The softwoods such as fir and pine have slightly more heat value per kg compared to hardwoods such as teak and oak. The weight per volume of oak or teak is about double that of pine (see appendix).
The hardwoods burn more slowly while the light softwoods burn much faster and with longer flames. Usually hardwoods are burned in the initial stages of firing while softwoods are used near the end when a fast release of heat is needed to raise the temperature. The slower heat release of the hardwoods can be countered by splitting the wood into very thin sticks.
Fig.2-38: Special axe for splitting
firewood. The axe does not cut the wood but splits it by impact. It is not
suitable for splitting wood with long fibres.
Water content
When wood is freshly cut it contains 30-50% water. Wood with so much water not only burns badly but a lot of the wood's energy will be used to turn the water into steam. The wood should be stored until it is completely dry on the surface as well as within. In temperate climates this will take a year while in tropical countries the firewood should be allowed to dry throughout a dry season. Properly dried firewood still contains 10 - 15% water.
Storing
The potter will need to keep a stock of firewood big enough to last for six months or one year depending on the prevailing climate. The three-chambered kiln from Champaknagar (p. 58 f.) uses about 2.3 tons to fire to 1100 °C. It is loaded with 1800 mugs and is fired nearly every third week and so a stock of 20 tons of firewood will be needed in this case. Some potters may be able to buy wood which is partly dried but one can never be sure of this and so it is better to buy fresh wood which is easier to split. When settling the price for the purchase of firewood by weight, take into account that the fresh wood weighs about 30% more than dry wood.
Fig.2-39: Firewood stacked so that
air can pass easily throught the stack and dry the wood
Fig.2-40: a solid chump of wood
half-buried in the ground is the proper base for splitting firewood
Normally it is much cheaper to buy large amounts of wood by the lorry-load. This also allows the potters to cut the wood into suitable sizes. The cutting and splitting of wood is much easier while it is still fresh. Sticks about 60 cm long and 3 - 5 cm thick are needed for the last hours of stoking, while thicker ones will be fine until then. Splitting the wood makes it dry faster as does stacking the long sticks so that air can pass through the pile. Bamboo is an excellent fuel although it is usually reserved for construction purposes.
Fire hazard
Potters often place the firewood for the next firing on top of, or on shelves, above the kiln during a firing (fig. 241). The heat from the kiln dries out the wood completely thereby further reducing the cooling effect of the moisture in the wood. However, great care must be taken to prevent the wood from catching fret
Planting trees
In some areas there are large forests with plenty of trees and it may seem that there is no need to worry about a lack of wood for fuel. However, even potters in these areas may after only a short time find there is not enough firewood because the demand for wood is so great. The price of wood goes up as firewood is cut further and further away. A family uses about 4000 kg of firewood each year for cooking alone. Therefore' one single village may soon use all the trees in the nearby forest if no new trees are planted to replace the ones which were cut and burned for fuel. Even though the potter may face no trouble in getting firewood for the time being, these fortunate conditions are unlikely to last forever. If at all possible, potters should try to secure their source of firewood by planting their own trees.
Fig.2-42: The big stack represents a
family's annual use of firewood for cooking (drawing from: Aprovevho-Institute,
Fuel-Saving Cookstoves. GATE/Vieweg, 1984).
In heavily populated regions land is scarce and expensive and so potters will be unable to buy or lease land for growing trees. In other places, however, potters may through local authorities be able to lease fallow land where trees can be planted. Often local development authorities offer seddlings free of charge to villagers.
It may seem an overwhelming task to start planting a forest, but in fact the forest does not have to be very large. Where a three-chamber kiln like the one at Champaknagar. is fired every three weeks it will consume 40 tons of firewood annually. According to some estimates a forest covering 1-2 hectares of land will produce this amount of wood every year.
In tropical areas a tree such as a eucalyptus grows so fast that it can be cut for firewood after only two or three years Some trees will shoot again from the stem after cutting and so the trouble of replanting is dispensed with.
The right choice of tree species for growing firewood depends on
local climate and soils. All countries have a forestry department which should
be able to advise on which trees would be most suitable in a specific
area.
2.2.2 Agricultural waste
All agricultural waste products are bulky fuels. They take up a lot of space compared to the amount of heat they release on burning. The main attraction of waste products is their cheapness, transportation often being the only cost. In some areas agricultural waste products are already used for cooking or as fuel by local industry, whilst in many other areas these by-products are just dumped and are free for use.
Sawdust
Sawdust has the same heat value as the wood from which it was cut. It is an efficient fuel when used with the right firing system. The main problem is to keep it dry because it soacks up moisture like a sponge.
Rice husks
Milling of 66 kg of paddy produces 10 kg of rice husks. The heat
value is nearly as good as for sawdust but this fuel has a high ash content.
Some rice mills burn the rice husks for steam-powering the mill and drying the
rice hulls and so they may only have a little left over to
sell.
Other vegetable waste
The following materials can be used during the initial firing of the kiln as an additional fuel:
- Peanut hulls; very bulky but have the same heat value per kg
as wood.
- Bagasse; the crushed sugar cane after the sugar juice is
extracted. 130 kg sugar produces 100 kg wet bagasse. The bagasse has to dry for
a couple of months. With 15% moisture content it has a heat value close to
firewood.
- Sisal; production of 1 kg sisal fibre leaves 30 kg waste which
after drying is a usable fuel. The roots of the sisal plant also burn very
well.
- Straw from growing paddy, corn, etc. is often used for firing
traditional unglazed pottery and can be burned in the fireboxes of a pottery
kiln provided the straw is tightly bundled.
2.2.3 Peat
Peat is a spongy mass of vegetable matter formed by the decomposition of ancient forests. Peat can be described as the first step in nature's production of coal. It is normally only covered by a thin layer of soil.
Winning
The peat contains about 90% water when it is dug or rather cut. The soft mass is cut into blocks (20x5x5 cm) which are first laid out on the ground for drying. As soon as the blocks can be handled they are stacked into piles in order to accelerate their drying. Drying may take several months and the air-dried peat blocks will contain 20-30% moisture. Winning of peat requires only manual labour and a few tools.
Heat value
The properties of peat are still close to those of firewood and
peat can be burned on firewood grates although it needs slightly less secondary
air compared to firewood. The heat value of air-dried peat is close to that of
firewood.
2.2.4 Lignite
Lignite is the step between peat and real coal. Lignite may be brown or black and it still has a wooden-like structure. Some lignites are called brown coal. The [ignites differ considerably in their moisture contents and heat values with some being close to peat and some resembling real coal.
Winning
Sometimes lignite or brown coal can be won on a small scale by individuals as is the case with peat. However, usually the lignite seams are covered with so much overburden that the winning has to be left to commercial mining corporations. Lignite can be burned on coal grates.
Fig.2-43: Peat, lignite and coal are
created by forests growing in ancient times. The starting point for coal
formation is peat and as this is compacted more and more it is converted into
lignite and finally into coal. Peat found today may be only a few thousand years
old whereas lignite may be 50 million years old and coal up to 300 million
years.
Storage
Lignite is very friable and is therefore especially likely to
break up into small pieces if it is subjected to cycles of wetting and drying
Small fractions and dust cannot burn on the grates and so the lignite should be
stored in a dry place.
2.2.5 Coal
Coal represents the last stage of the transformation of vegetable matter and the term coal covers a wide range of heat values and moisture contents. However, all coals have a higher heat value and have less volatile matter than the other solid fuels. Ash content varies from 1 - 20% on average.
Saggars
Due to the high sulphur content in coal, glazed ware has to be protected in saggars. The consumption of coal when firing ceramic ware in kilns depends on the size of kiln, the firing temperature and the setting type, but it varies roughly from 0.3-1.5 tons of coal per ton of ware.
Storage
Coal is more economical to buy in large quantities. So several
potters may save money by buying their coal together. Coal does not need to be
stored in a shed but should be laid on clean ground. Coal oxidizes slowly when
exposed to air and this process will heat up the coal. This heating may cause
the coal to ignite by itself if it is piled in large heaps. The preventive rule
is not to pile the coal higher than 2 m, smaller piles being less likely to
ignite than big ones.
2.2.6 Oil
products
Some countries have many different types of oil products while others only a few, therefore, only the most common are mentioned here. Petrol cannot be used for firing kilns because it burns explosively. Kerosene is an excellent fuel but is normally more expensive than other oil products though in some countries it is subsidized by the government. Diesel oil and light fuel oil are rather similar when used for firing. Fuel oil is a very powerful fuel with a heat value of about 30% above good coal. However, it is also the most expensive fuel and will in many cases not be economical for potters.
Waste oil
Waste oil is as powerful as fuel oil yet cheaper. Waste oil can be obtained from garages, bus and transport companies and railways which are left with a lot of lubrication oil after servicing their vehicles. In some countries the lubrication oil is recycled, whereas in others no use is made of it and so it can be obtained for a low price. Some power-generating plants produce large amounts of waste oil when cleaning heavy oil for their diesel engines. This type of waste oil has a high viscosity and is often mixed with water. Waste oil should be screened before use. A 16 or 24 mesh screen is fine enough for drip-plate burners.
Contaminants
Waste oil contains various contaminants reflecting its life as a lubricating oil in engines, gearboxes, etc. A drain oil from a garage in Nevada was found to contain the following contaminants listed as parts per million (ppm):
|
iron |
50 |
|
copper |
18 |
|
chrome |
7 |
|
aluminium |
7 |
|
lead |
500 |
|
tin |
5 |
|
silica |
5 |
Some people regard burning of waste oil as a health hazard especially because of its lead content. However, the quoted amount of lead equals that in petrol while earthenware clay may contain more than 320 ppm and paper for candy-wrapping may contain as much as 7125 ppm. A potter firing with waste oil will be no less safe than if he had spent the day at the road side. At Bujora Pottery in Tanzania the waste oil gave a pleasant shine to the unglazed clay. This was probably due to the contaminants of the waste oil.
Pollution
Oil and especially waste oil are dirty to work with, and a more serious problem is that oil can cause great harm to the drinking water in the whole area if it is allowed to leak into the soil. Therefore, great care must be taken not to spill oil on the ground, and if this does occur, it should be cleaned immediately. If sawdust is always kept close by' it can be used to soak up the oil, and afterwards it can be used for preheating the fireboxes. The ground where screening and filling of the oil tank takes place should be covered by a layer of cement or bricks in order to prevent the oil leaking into the soil.
2.3.1 Combustion
2.3.2 Firewood firebox
2.3.3 Sawdust
firebox
2.3.4 Coal fireboxes
2.3.5 Oil drip firing
2.3.6 Pressure
burner system
It is not possible to learn how to fire a kiln successfully from
a book. That has to be done by participating in many, many firings as the
firemaster's assistant. However, it is helpful to understand the basic
principles involved in kiln firings.
2.3.1 Combustion
It is common knowledge that firewood burns as does charcoal, oil, coal and gas. The burning process is called combustion. All these fuels were originally green plants; firewood and charcoal are made from presentday trees but oil, coal and gas have originated from thick forests which covered the earth many hundreds of thousands of years ago.
Fig.2-45: Wood (carbon) combines with
air (oxygen) and heat is the result.
Carbon and oxygen
When watching a small fire we notice that the firewood slowly disappears leaving a little ash and that the fire needs plenty of air. But what is behind the magic? Wood and other fuels are mainly made of a material called carbon and the burning process takes place when the carbon combines with the oxygen in the air and forms a new material called carbon dioxide. The process produces a lot of heat. The carbon dioxide escapes and only ash is left. Ash is the part of the fuel which cannot burn.
Flash point
When a piece of wood is heated, initially water and carbon dioxide are given off. Above 280 °C volatile gases in the wood are given off. These gases will burn if they come Into contact with open flames and this temperature is therefore named the flash point of wood.
However, these gases will without open flames only bum at temperatures above 600 °C. This is called the ignition temperature. The temperature of wood flames are 1100 °C while fuel oil has a flame temperature of 2080 °C.
Fig.2-46: Temperatures of flash
point, ignition and flame of wood; Fig.2-47: Water in its three forms.
Solid, liquid, gas
Firewood and coal are solid matter while oil is liquid. However,
the burning will only take place when carbon is in the form of gas. All
materials exist in three different forms depending on the temperature. The three
forms of water are well known (fig. 2-47). So first we will have to turn our
fuel into a gaseous form and then mix it with air. This is the job done in the
fireboxes of pottery kilns and it is done differently according to the type of
fuel. For the potter, mainly three types of fuel are of interest: firewood, coal
and oil.
2.3.2 Firewood firebox
Firewood burns in two stages. When a new piece of wood is added to the fire, the wood will first give off volatile gases which will burn. (In wood the volatile gases amount to about 80% of the total mass, the remainder being in the form of fixed carbon (charcoal).) The flames of a fire are these burning gases and they will often not even touch the firewood. After the volatile gases have escaped only charcoal is left and it will burn with gentle blue flames.
Fig.2-48: Wood burns in two stages.
The first, seen to the left, is the burning of the volatile gases. The second,
seen to the right, is the burning of the charcoal.
In the ceramic kiln the two-stage burning takes place in the firebox which enables us to control the fire. The main problem is to ensure a good strong fire with just the right amount of air needed to combine with the carbon of the fuel. If we let in too little air some of the volatile carbon gas will go out of the chimney unburned which can be seen as black smoke. That means wasted firewood. If we let in too much air this excess air will cool the kiln. That too is a waste.
Fig.2-49: Function of a wood
firebox. The height if ash pit should equal the height above the gate.
Primary/secondary air
In fig. 249 air enters at the bottom of the firebox, passes over the embers and goes through the grate. Reaching the firewood it helps to burn the carbon gas being released rapidly due to the high temperature inside the firebox. Most of this air, primary air, is used to burn the charcoal and often there will be too little air left for the volatile gases released from the firewood. Secondary, air entering above the grate ensures complete combustion for these volatile gases. By thus dividing the air inlet less air is needed and thereby less cooling of the kiln takes place.
The volatile gases represent up to 30% of the heat value of the firewood. In case sufficient primary air passes the fuel the combustion will be complete, but it would mean an excess of air being 50-100% of the air used for combustion. This excess is reduced to 30ù 50% when secondary air completes the combustion.
Grate
The wood is spread out evenly on the grate so that air has easy access to it. For firewood the distance between the grates should be 1520 cm so that the wood will fall into the ash pit as soon as it is nearly burned out. Otherwise it will block the access of primary air.
The grate can be made of iron bars but they will soon wear out and grates made of fireclay bars (fig. 2-50) are more durable. The ash pit should be as big as or bigger than the space above the grate because a thick layer of embers is needed to preheat the primary air.
Fig.2-50: Fireclay bars made as long
solid firebricks.
A mousehole letting air into the bottom of the ash pit can regulate the thickness of the embers. The grate for firewood should be about 15-25 % of the floor area of the kiln chamber, the 15% sufficient for firings up to 1100 °C and the 25 % for 1300 °C and above.
Firing technique
Firing is started in the ash pit with big pieces of firewood so that firing begins slowly and it also helps in building up a good layer of embers. Later the firewood is placed on the grate and the secondary air inlet is opened. A properly designed kiln should be easy to take up to about 1000 °C but in order to save fuel care should still be taken to control the inlet of air and keep a steady stoking going. From 1100 °C to 1250 °C the kiln needs full attention. After stoking flames will come out of the blow-holes on top of the kiln chamber and the atmosphere inside the kiln chamber will be cloudy. The stoking will cause the temperature to fall and the kiln will be in strong reduction (p. 116). While the wood burns the temperature will rise and the atmosphere inside will become clearer. As soon as the inside is clear, stoke again! This technique produces an oxidizing firing. Reducing firing is done by stoking as soon as the blowhole flames have gone. Another point is to keep the grate covered with a thin layer of firewood all the time. That way air is not rushing through the open space on the fire grate. This would cool the kiln; only ten minutes' neglect might cost an hour's extra firing.
Fig.2-51: Iron grates will last
longer if they can be removed after firing has been finished (F.Olsen "The Kiln
Book" fastfire wood kiln).
Near the end of the firing thinly split firewood is used. This burns very fast and stoking will need to be done almost continuously by throwing in pieces wherever wood has burned out. However, the last firewood split should have burned out before a new one is thrown on top of it, otherwise the firebox and the ash pit will become choked. Stoking is done through a small hole not allowing in excess air.
Hob firebox
Conventional fireboxes have the primary air supply under the grate and the secondary over it. The hob firebox works the other way round (fig. 2-52). The firewood is fed from the top and the box over the hob can be filled up so that the firebox is kind of self-feeding.
Fig.2-52: Hob firebox.
2.3.3 Sawdust firebox
Sawdust, rice husks and other agricultural waste materials need special firing systems. The problem is that this type of fuel is very bulky and if used in a conventional wood firebox it would block the grate and only burn slowly on the surface. One solution is to let the fuel fall onto the top of a steep cast-iron grate provided with a lot of small steps where the fuel is burning (fig. 2-54). This system needs constant attention to ensure proper flow of the fuel evenly over the whole grate area. Otherwise areas without enough fuel will burn through and let in cold air. The system requires a rather big grate area compared to kiln size but can be used for smaller kilns fired up to 1000 °C. (A grate measuring 40 x 100 cm fired a 1 m¦ kiln to 1100 °C with rice husks in eight hours consuming 650 kg husks.)
Sawdust injection
In this system sawdust is sucked into a centrifugal blower via a pipe system and sawdust mixed with air is then blown into a conventional firebox. The firing is started with ordinary firewood in order to slowly heat the kiln. After smoking is over and when there is plenty of coal in the firebox the blower is started. In the beginning the firing uses only a little sawdust and firewood will still be needed to ignite the sawdust. When the firebox bricks are glowing red firewood stoking is stopped and the flow of sawdust is gradually increased. A 1.7 m kiln is quoted to use 3 m sawdust to fire to 1300 °C in 11 hours. For this kiln a 23 cm straight blade blower powered by a 0.3 HP electromotor running at 3400 rpm is used. The sawdust is fed to the suction pipe through a hopper (fig. 2-56) with an auger in the bottom. The sawdust has to be very dry, otherwise the firing will slow down and the fire may even be extinguished. In areas with no power supply the blower can be driven by a combustion engine and the auger in the hopper can either be connected to the engine by a v-belt or be driven manually.
Fig.2-55: Sawdust injection system.
The sawdust burns very fast almost like liquid fuels. The ashes will not remain in the firebox but will be blown throughout the kiln. Some will settle on the walls and saggars and form a glaze and the rest will leave by the chimney. Firings to above 1250 °C may be done in open settings because the ash will melt together with the glaze. Below 1250 °C the ash would produce a rough layer on the glazed ware.
Fig.2-56: Sawdust is fed to the
suction pipe by hopper. The speed of the auger can be used to regulate the
sawdust in take.
A hole in the bottom of the chimney will help to provide air for burning up unburned sawdust. The sawdust burning produces a lot of sparks which may start a fire.
Sawdust burner
In fig. 2-58 a modification of the injection system is shown. The sawdust is fed into the blower pipe in front of the blower outlet. This system is simpler but needs constant attention to ensure that sawdust does not clog the outlet from sawdust funnel to blower pipe.
Rice husks
The above system has only been tested with sawdust but may also work with other agricultural waste products. Rice husks are rather similar to sawdust except that they have a slightly lower heating value and leave much more ash. The ash has a high silica content which makes it unsuitable for open setting glaze firings.
Straw
Straw and other agricultural waste materials can also be burned with the injection sys- tem. However, the straw has to be cut into small pieces. If the straw is dry that can be done in a hammer mill with a coarse sieve.
Coal needs other types of fireboxes compared to wood mainly because coal has much less volatile matter and thus resembles the charcoal left after the volatile gases in wood have burned away. Fireboxes for coal-fired kilns are normally of the same size regardless of the size of the kiln. So bigger kilns simply have more fireboxes.
Grates
The grates are made of iron bars with 2.5- cm space in between. The bars are exposed to intense heat from the white-hot coals and wear out quickly. The cost of renewing the grate bars is a big drawback of coal firing.
Fig.2-60: Typical grate bars for
coal. The thickened parts adjust the open space between the bars.
A water container placed in the ash pit will cool the bars. The resulting steam will dissociate into ions when passing the burning coal. This action cools the temperature of the burning coal without a corresponding loss of energy. At the same time the flames will become longer and the coal is less likely to clinker.
The life of the iron bars will be much prolonged if they are designed to be removed as soon as the firing is stopped. Cast-iron bars are superior to mild steel bars.
Flat/inclined grates
Flat grates are mostly used for slow firings up to 1250°C mainly in an oxidizing atmosphere. Inclined grates are used for porcelain and faster firings. (Inclination is usually 15-25% but there seems to be no fixed rule, e.g. lignite should be fired on inclined grates instead of flat ones.) Types with steep inclination are called semi-gas producers because they are fired with a thick bed of coal which produces a lot of half-burned carbon gas. This gas is fully burned by an inlet of secondary air above the grate as in a firewood firing or inside the kiln in case long flames are desired.
Fig.2-61: Coal firebox with flat
grates.
Fig.2-62: Coal firebox similar to
the ones seen in fig.2-59. The firebox is within the outer wall of the kiln.
Secondary air is regulated by opening of the stoking shutter and by placement of
bricks just above the grate bars.
Fig.2-63: Stoking of a semi-gas
producer.
Semi-gas producer
A semi-gas producer is shown in fig. 2-63. The inclination of the grate is 50% and stoking is simply done by filling coal until the grate is covered with a coal bed of the desired depth. At full firing the whole stoking channel can be kept full of coal which will slide down by itself. The problem with this type of grate is to fire at a slow rate. That can be overcome by covering the upper part of the grate with a clay slab during the initial slow rising of the temperature. As more intense fire is needed the slab can be drawn out.
Preheated air
Channels for secondary air are built into both sides of the firebox so that when the secondary air enters above the coal bed it is preheated (fig. 2-64). This will ensure a better combustion of the volatile gas, thereby adding to a better fuel economy.
Stoking
Primary air enters under the grate and if the coal bed is thicker than 10 cm, secondary air is needed too for complete combustion. There are two basic stoking techniques:
(1) Frequent stokings are made to ensure that there is an even layer of coal all over the grate and that no place is burned through resulting in a rush of cold air entering the kiln.
(2) Stokings are made by first pushing the burned coal to the back of the grate and then filing fresh coal in front.
Fig.2-64: Channels in the walls of
the firebox preheat the secondary air.
The first method requires more experience and more frequent stokings whilst the latter tends to be less economical because it allows too much cold air to rush through the grate during stoking. Every now and then clinkers have to be cleared from between the bars with the help of a bent iron rod, which is inserted from under the grate.
Alternate stoking
Each time stoking is done the temperature will drop as the fresh coal is heated and it will then gradually rise again. It is therefore better to stoke the fireboxes alternately at an even interval, i.e. say the kiln has four fireboxes and each needs stoking every hour then stoking should be done to one firebox in turn every 15 minutes instead of all four once every hour. The interval between stoking is judged by looking at the smoke from the chimney and at the coal bed on the grate. It depends on the type of grate and firing technique. If stoking technique (2) is used stoking is done every 1-1+ hours up to 900 °C, then every 45 minutes to 1 hour up to 1200 °C and finally every 30 minutes.
The most efficient combustion takes place when the burning rate is about 95 - 145 kg per 1 m_ grate area per hour (Singer p. 908). The Khurja down-draught kiln burns on average 85 kg/m³ per hour.
Firebox size
The grate area of each firebox is within 0.3- 0.5 m². The length is normally 80-90 cm. The total grate area of all fireboxes should fall between 12% and 25% of the floor area of the kiln chamber. 12-15% would be sufficient in most cases, while 25% would be for fast firing and high temperature porcelain kilns. The top of the firebox arch should be around 80 cm above the grate. The lower part of the grate should be at least 50 cm above the floor of the ash pit.
Firebox location
The fireboxes should be spaced evenly around the kiln and the
firebox may fit within the outer wall due to its small size. The closer to the
kiln chamber the better and in the Khurja kiln (p. 51 ) the bagwall is made as a
part of the inner wall, thus saving setting space. The number of fireboxes for a
particular kiln is found by dividing the grate area of one firebox into the
total desired grate area.
2.3.5 Oil
drip firing
A simple and reliable drip firing system for oil can be made very cheaply. Oil is fed by gravity to a drip-plate burner and the air is supplied by the natural draught of the chimney. Forced air oil-burner systems are harder to construct and descriptions of such systems can be found in technical books (see bibliography).
As for all other firing systems the important thing is to change the fuel into its gaseous form so that it will ignite.
Water and oil
Ignition can be aided by adding water to the oil in the following manner:
From a valve above the firebox the oil drips down onto a straw, which is fixed to another valve, from where a thin squirt of water runs along this straw (fig. 2-66). The straw breaks the surface tension of the water making many small drops which mix more easily with the oil. This mixture drips onto a set of three iron plates placed inside the firebox. In the initial stages of the firing the iron plates must be kept hot by a small fret Hitting the hot plates the water will explosively turn into steam. This action will pulverize the oil into a mist which easily vaporizes and then burns. The system also works without water, but the energy spent on heating the water is negligible compared to the improved combustion of the oil.
Fig.2-66: Drip-plate burner.
Carbon clinker
Besides helping to atomize the oil the water also reduces the carbon clinker, which otherwise would build up on the iron plates during firing. The water works in the same way as for coal firing C + H2O = CO + H2 . This action cools the plates thus reducing corrosion but the energy is given back again when the carbon oxide and hydrogen burn.
The amount of water is judged by watching the flame. It is normally about one part water to five parts oil.
Burner plates
The iron plates can be made from any type of scrap iron, but the plates should not be too thin otherwise they will wear out fast. The plates are either welded onto a frame as shown (fig. 2-66) or they are placed in grooves in the brickwork of the firebox. The size of the plates is not crucial and the slope depends on the viscosity of the oil; a plate size of 10x20 cm and a slope angle of 15_ are good starting points for experimentation. A gradually decreasing slope ensures that the oil burns on all plates and a raised edge will guide the oil flow onto the next plate.
The drip-plate burner can easily be installed in fireboxes of other fuels. However, ample space (at least 0.6 m) should be allowed in front of the burner because the oil flames are very fierce. The capacity of drip-plate burners equals that of fireboxes for wood and coal. However, if the kiln is intended to burn oil only, more drip-plate burners could be installed in order to have a more even heat in the kiln chamber.
Oil tank
The oil flows to the burners by gravity. Heavy fuel oil and waste oil have high viscosity and will have to be heated to ensure a proper Dow in the pipes Waste oil often contains water and a heating system can be combined with de-watering. Two oil drums, welded together to form one long drum, are raised above a small firebox. Around the drum a chimney is made with an inside flue spiralling around the drum to the top. An outlet for water is made in the bottom of the drum and an outlet for oil is positioned about 0.5 m above the bottom.
Fig.2-67: Oil tank for separation of
water and heating of the fuel oil. In fig.2-35 a photograph of such an oil tank
is shown
Fig.2-68: Preheating of burner
plates
Oil is pumped or filled into the drum through a sieve at the top. As water is heavier than oil the water will settle at the bottom of the drum and is tapped off now and then. Only a small fire is needed to keep the oil warm. The level of the oil should be at least 1.5 m above the taps at the burners.
Firing
A small wood fire is made in front of the burner plates. If it is a biscuit firing, smoking should be finished before starting oil firing as it will raise the temperature too fast. After about two hours the burner plates will be hot enough to start the oil firing, but the preheating fire should be kept going until the firebox is hot.
Without a preheating fire the oil firing will produce black smoke during the initial stages of firing the kiln.
In the beginning when only a little oil is dripping the fire will take place on the upper plate. Later, the oil will flow to all the plates and produce an intense white flame.
Take care that all the oil dripping onto the plates is ignited and that a pool of oil does not form in the bottom of the firebox. This could cause an explosive burning which may even blow out the fire. A mousehole ending in front of the burner will collect the excess oil and by watching the outside end of the mousehole (fig. 2-66) it is easy to discover excess oil flow. The mousehole works as an emergency reservoir and when the oil flow has been adjusted the excess oil harmlessly burns from the mousehole opening inside the firebox.
Additional fuel
In order to save oil the firing up to say 1100 °C can be
carried out with cheaper fuels e.g. firewood. The last stages of the firing up
to say 1250 °C normally are the most difficult when firing with firewood.
By changing to the powerful fuel oil, the last few hundred centigrades are
easily reached.
2.3.6
Pressure burner system
This burner system is based on the same principle as the kerosene pressure stove, only expanded greatly in size (fig. 2-69). The burner is a double-walled cylinder of rolled steel, welded together and fitted with an orifice system. It works with kerosene, diesel oil and light fuel oil.
Function principle
The kerosene or oil is fed to the bottom of the double-walled chamber either by gravity or from a pressure tank. Initially the burner is heated by placing an oil-soaked rag or sawdust inside the cylinder. The heat will vaporize the fuel in the evaporation chamber and after a while the pressure of the oil gas will expel the vapours through the orifices (1.5 mm holes) at the rear of the burner. The oil gas is ignited by the burning rag and the burner is now operating. The flame of the gas from the orifices will heat up the evaporation chamber and the fuel valve can slowly be opened fully. The rate of firing is controlled by the valve at the oil pipe.
Back pressure
The oil has to be under a certain pressure to counteract the back pressure generated inside the evaporation chamber. Otherwise the vaporized oil gas will escape through the oil pipe instead of the orifices. The oil pipe is fitted to slope downward from the burner to prevent gas bubbles from entering and thereby blocking the oil pipe.
Fig.2-70: Large pressure burner.
Large burner
For firing medium to large kilns it has been found that a burner with an inner diameter of 18 cm, 35 cm long and with 9x1.5 mm orifices is sufficient to heat 2-3 m¦ of kiln up to 1000 °C. For this size of burner fuel can be gravity-fed from a minimum height of 4.5 m, which will be sufficient to counteract the back pressure in the burner.
Small burner
Small kilns can be fired with a version of the burner that has an inner diameter of 4 cm, length of 30 cm and a single 1.5 mm orifice. This side will be suitable for kilns up to 1 m (1000 °C). More burners can be added for each additional cubic meter of kiln volume. Because of greater back pressure generated in these small burners a pressurized fuel tank must be used. Since a I m¦ kiln can be fired to 1000 °C using approximately 25 l kerosene, a 50 1 tank pressurized with a bicycle pump to about atmosphere 3 will do. This burner can be scaled down by changing the 1.5 mm orifice with a 1 mm orifice.
Chimney and firebox
Because the burner produces a flame under pressure, a large chimney is not required. Air for combustion is drawn into the burner by the venturi effect of the pressurized fuel gas. A simple up-draught kiln can function with" out any chimney while a cross-draught or down-draught type will need a low chimney to pull the combustion gases properly through the kiln setting. The burner is placed with a gap of 3-5 cm from the firebox inlet in order to let in secondary air An opening in the kiln wall is sufficient but the flame should be allowed a free space of about 0.5 m in front of the burner. In general, several small burners will produce a more even temperature compared to one large burner.
Carbonization
However, small pressure burners tend to become blocked by carbon deposits inside the cylindrical chamber, especially if the temperature in the chamber becomes too high. These carbon deposits are very difficult to remove and so it is best to avoid overheating the cylindrical chamber by leaving a sufficient gap between the burner outlet and the firebox opening and by ensuring an additional natural draught of air.
Fig.2-71: 2 m³ down-draught
shuttle kiln with four small pressure burners. Exit flue is in the bottom of the
shuttle car.
Kilns with additional draught, powered by a chimney, show little tendency to cause carbonization problems compared to chimneyless kilns.
The cylindrical chamber should have an airtight lid that will facilitate cleaning out minor deposits with petrol between firings.
Steam
Several successful firings have been done with a modified small pressure burner in Nepal. Oil was substituted for water so that steam was generated in the cylindrical chamber and this was forced out under high pressure through a single small orifice. The oil was fed in front of the steam orifice by a brass pipe where it would be vaporized and blown through the burner by the steam. A back pressure valve is needed between the burner and the pressurized water tank. Initial results indicate that fuel consumption equals that of a similar type of oil burner.
2.4.1 Transfer of heat through air
2.4.2 Transfer of heat
through solids
2.4.3 Transfer of heat by radiation
2.4.4 Natural
draught
2.4.5 Flues
2.4.6 High altitude
The hot combustion gases will, after leaving the fireboxes, pass through the kiln chamber on their way to the chimney. The more heat the combustion gases transfer to the ware while they pass, the more efficient is the firing. If the gases leaving through the top of the chimney are still very hot this means that a lot of heat or fuel is being lost. The transfer of heat takes place in three different ways:
2.4.1 Transfer of heat through air
Convection
Heat is transferred to the kiln walls and to the saggars or kiln setting when the hot gases from combustion pass through the kiln. Heat transferred in this manner by circulating air is called convection. Increasing the speed of the air (velocity) will result in the transfer of more heat.
Kiln setting
Just as a river will run fast where it is narrow and more slowly
where it widens, air streams act in the same way. Therefore, a half-filled kiln
with a lot of space between the saggars would fire badly. For an efficient
firing the kiln should be set with many narrow passages through which the hot
gases will pass quickly.
2.4.2 Transfer of heat through
solids
Conduction
As the kiln setting becomes hot by heat transferred by convection this heat will also pass through the saggar walls and reach the pots inside. This transfer of heat through solids is called conduction. Heat transferred by conduction will take some time and so the air outside the saggars will at first be hotter than inside where the pots are. This should be kept in mind when firing and is one reason for soaking the kiln at top temperature so that the temperature throughout the kiln will even out.
Kiln walls
Heat will also pass through the walls of the kiln and be wasted.
The use of insulating firebricks for the inner wall will reduce this loss. The
many small holes in insulating firebricks will reduce the passageways for the
conduction of the heat. However, if the holes are too big the air inside the
holes will have room to circulate and transfer the heat by convection (see fig.
143). The weight of the insulating bricks is less than solid firebricks and so
the fuel that would be needed to heat that weight difference is also
saved.
2.4.3 Transfer of
heat by radiation
Finally heat can also be transferred by radiation from a glowing surface. The sun transfers its heat to the earth by radiation. The more a surface is glowing the more heat it will transfer to its surroundings. At the end of a firing everything inside the kiln glows white-hot and will radiate a lot of heat. This radiation helps to even the temperature in. side the kiln. The hotter parts will glow more and transfer heat to the cooler spots. A soaking period will give this radiation time to even out the temperature differences.
The insulating firebricks have here another advantage over solid firebricks; as less heat escapes through the insulating bricks their surface temperature will become higher and so they will radiate more heat back to the kiln setting.
Fig.2-72: At red heat the kiln walls
will radiate heat back to the kiln setting. Fig.2-73: The hot air the chimney
weighs much less than cold air of the same volume. The balance illustrates this
weight difference.
Kiln wash
Some potters paint the inside of the kiln with a feldspar
mixture which will melt and produce a shiny surface. This will add to the
reflection and radiation of the kiln wall and will make a considerable
difference especially in smaller kilns.
2.4.4 Natural draught
Chimney
One litre of hot air weighs less than one litre of cold air. So hot air will rise following the same law of nature that makes a piece of wood ascend when it is immersed in water. This law is used to produce a draught or pull of air in kilns by the help of a chimney. The column of hot air inside the kiln weighs less than a similar column of outside cooler air and this weight difference will produce a pull at the bottom of the chimney.
The force of the pull will increase with increasing height and volume of the chimney and with increasing temperature of the gases inside the chimney. The maximum weight of gases is pulled through the chimney when the temperature of the gases is around 300 °C. With higher temperatures more volume of gases is passing but it weighs less. The pull should be sufficient to create a good draught through the fireboxes and the kiln chamber.
Chimney dimensions
Some general rules exist for the dimensions of a chimney according to the size of the kiln. The following dimensions apply to high temperature firings and less height could be used for low temperature kilns:
3 m of chimney for every 1 m downward pull plus I m of chimney for every 3.5 m horizontal pull.
The height of chimney for the kiln in fig. 2-74 should be:
3 x 2.2 + (2 + 1.5)/3.5 = 7.6m.
Fig.2-74 Kiln with downward pull of
2.2 m and horizontal pull of 3.5 m.
The bottom area of the chimney should be approximately 10% of the area of the kiln chamber.
Dampers
A slight tapering of the chimney at the top will increase the speed of the air passing through the kiln. However, too strong a pull will cause irregular heating. The pull can be regulated by the use of dampers or by making an opening at the bottom of the chimney through which cold air will be drawn in and cause the pull to slow down.
Velocity
The pull will be slow at the beginning of a firing but when the temperature in the chamber is around 1100 °C the speed (velocity) of gases through the kiln should be 1.2-1.5 m per second. This can be checked by throwing an oil-soaked rag into the firebox. The time taken for the black oil smoke to come out of the chimney should be noted. The total distance covered by the gases is from the firebox up the kiln wall across the arch and down to the outlet fuel . The speed of gases = distance in m/seconds
Fig.2-75: Three different types of
dampers. Dampers are placed between the outlet flue of the kiln and the chimney.
The pull during wood firing is right when flames and smoke come
out of the blow holes (a sign of back pressure) right after stoking. If there is
no back pressure, dampers should be closed more to reduce the draught.
2.4.5 Flues
The openings or channels carrying the gases from the firebox to the kiln chamber (inlet) and from the chamber to the chimney (exit) are called flues. The size of inlet and exit flues should be of equal but generous size. One litre of air at 20 °C will expand to 4.5 litres when heated to 1250 °C and so the cold air entering through the primary and secondary air inlets needs 4.5 times larger openings after it has been heated in the firebox.
The size of the flues should be slightly bigger than the
cross-section of the chimney. Another measure suggests that for each l m¦
of kiln chamber there should be about 600 cm_ flue area In any case it is better
to make the flue area too big because later when the kiln is finished it is easy
to reduce the flue size but difficult to increase it.
2.4.6 High altitude
At high altitudes there is less oxygen in the air than at sea level. So kilns built in mountainous regions need to pull more air through the firebox for combustion of the same amount of fuel. Therefore, the height and diameter of chimney and size of flues should be made larger to allow for this extra air. Roughly the chimney and flue dimensions are expanded 1% for each 100 m altitude above sea level.
Example:
A kiln designed for sea level with a 6 m high chimney of 0.5 m diameter is to be constructed at an altitude of 1400 m.
New height of chimney = 6 + 6 x 14% m = 6.84 m
New diameter of chimney = 0.5 + 0.5 x 14%m = 0.57 m.
The added inside volume is nearly 50% in this case.
2.5.1 Site of the kiln
2.5.2 Foundation
2.5.3
Masonry
2.5.4 Floor and walls
2.5.5 Curved walls
2.5.6 Arches
2.5.7
Domes
2.5.8 Catenary arch
2.5.9 Arch construction without
support
2.5.10 Expansion joints
2.5.11 Insulation
2.5.12 Maintenance of
kilns
2.5.1 Site of the kiln
The site of the kiln should be selected carefully. At some
stages of the firing a lot of smoke will develop and sparks from the chimney
could cause fires at nearby houses. If possible select a site for your kiln at a
safe distance from neighbours. The kiln site should be dry and preferably
levelled above the surroundings so that the kiln, its foundation and flue system
will not come under water during rainy seasons. Ample space is needed around the
kiln for stacking and drawing the ware, for storing saggars or kiln shelves and
for storing fuel. Firewood is especially bulky (see fig. 2-76).
2.5.2 Foundation
Construction of a kiln resembles that of a common house built of bricks. The bricklaying technique only differs a little and ordinary skilled masons will easily adapt themselves to the task. For larger kilns it is recommended to consult local building specialists (engineers, architects, masons) with experience in major constructions, particularly for assessing the size of proper foundations according to the type of soil and climate of the kiln site. In particular, foundations for chimneys above about six metres need a good foundation to ensure the chimney will not lean. Apart from the chimney only the supporting walls need foundations. The foundation should be exactly levelled by using a spirit-level. Levelling over larger distances can be done with a hose filled with water (fig. 2-77). A clear plastic hose is fitted to each end of the hose. So the level can be easily observed. Take care that no air bubbles are trapped inside the hose.
Fig.2-77: Levelling of the
foundation by the help of a hose
Damp-proof layer
A damp-proof layer has to separate the kiln structure from the ground. Otherwise, moisture from the ground will slowly be absorbed by the brickwork and fuel will be wasted drying this moisture with every firing. This layer can consist of stone pebbles (not broken bricks) 30 cm thick and sealed with cement or common blocks on top (fig. 2-78). If available, plastic sheets with a top layer of about 0.5 m of sand may be cheaper to use (fig. 2-79).
Fig.2-78: A layer of stone pebbles
seals off the moisture - Fig.2-79: Plastic sheets with a protective layer of
sand work as a moisture seal.
2.5.3 Masonry
Mortar
The mortar (p. 38) should be mixed with water a day or two before use. Avoid any lumps and big pieces of grog. The consistency of the mortar should be soft, nearly sliplike. Some prefer to soak the firebricks in water before laying them. The joints are the weakest part of the structure and they should be as thin as possible. The mortar is not meant to stick the bricks together but rather to provide a level bed for the bricks by filing up the space between irregularly shaped bricks (fig. 2-80).
Fig.2-80: Mortar is used for filling
the spaces between irregularity shaped bricks.
Fig.2-81: Each brick should level
vertically with the wall. Fig.2-82: Cutting of firebricks
Laying
The mortar is laid out with a trowel but only for one or two bricks at a time. Each brick is tapped into position with the trowel or hammer until it is in line (fig. 2-81). Immediately check its position with the spirit-level both vertically and horizontally and readjust if necessary by tapping lightly. In case adjustment is done later the brick and mortar should be removed and fresh soft mortar applied. The joints should also seal the kiln chamber. Therefore, be sure that the joints are completely filled with mortar.
Cutting
Cutting of firebricks is done with the claw of a brick hammer by
tapping the brick all around along the line of the cut. After about two rounds
of tapping the brick is given a sharp blow on the edge while resting in the hand
or on a bed of sand. Some people prefer to use a brick chisel for the final
blow. Insulating firebricks can also be cut by a saw though the saw will
afterwards only be suitable for cutting bricks. Cutting by saw could be reserved
for specially shaped bricks such as skewbacks and arch bricks.
2.5.4 Floor and walls
Normally the floor is laid last, but flue channels passing under the walls will need to be made at the same time as the foundations. The basic rule for bricklaying is that joints should never be in line but always be bridged by the next brick course. There are two basic bricklaying patterns: header course and stretcher course.
Header course
Header courses are laid across the wall, thereby only exposing the smallest face of the firebricks to the high temperature inside the kiln chamber. For our self-made firebricks, often possessing limited refractoriness, this is an advantage.
Fig.2-83: Header course.
Fig.2-84: One stretcher course for
each three header courses. Fig.2-85: Streatcher courses in a Chinese-type tube
kiln.
Stretcher course
Stretcher courses are laid along the wall, but for straight walls stretcher courses alone will only be strong enough for unsupported walls less one metre's height (bagwalls).
One stretcher course for each 3-4 header courses is suitable for
23 cm walls. A 23 cm wall is safe up to about 2 m height and a 34 cm wall is
safe up to about 4 m.
2.5.5 Curved
walls
Round kilns
Curved walls are found in circular kilns which have proved very durable. A circular inner lining can stand alone because the bricks are wedged together in the same fashion as an arch and will not fall into the kiln. So a curved wall does not need to be bonded with the outer wall. Instead a gap of about 5 cm can be left between the inner and outer wall. This gap can be filled with ash or a mixture of 70% sawdust and 30% clay (volume). A curved wall is laid in the same manner as a straight wall. Normally only header courses are used.
Fig.2-87: Cross-cut of a circular
kiln wall.
Wall thickness
Circular kilns with diameters up to 2 metres are made with a 10 cm inner lining, 2 - 3 metres with 15 cm and above 3 metres with 23 cm. The inner lining should preferably not support the dome which instead is made to rest on the outer wall (fig. 2-88). That enables the wall to be replaced while leaving the dome in place and the wall, only carrying its own weight, can be made from light insulating bricks. The outer, dome-supporting wall should have twice the thickness of the inner lining and is made from ordinary red bricks laid in an ordinary mud/sand mortar.
Fig.2-88: The dome is resting on the
outer wall.
2.5.6 Arches
Arches are used to bridge doors, flues and fireboxes and to form roofs for rectangular kiln chambers. Square bricks can be used for laying arches if the joints are filled properly. However, if available, tapered bricks are preferable as they produce a more durable arch. In case firebricks are self-made a number of tapered shapes should be made for the construction of arches. The number of bricks and their degree of tapering can be calculated (see appendix). The main point is that the outer size of the arch bricks is bigger than the inner size so that each brick is prevented from falling in. That means the higher the rise of the arch the stronger it is. Normally a rise of 20 cm for each 100 cm span is reasonable for roofing kiln chambers. (12.5-25 cm rise per 100 cm span is within normal good practice.)
Fig.2-89: Arch.
However, in fireboxes, especially for oil firing, the firebricks are under severe conditions and a rise nearly half of the span is advisable if the firebricks are of poor quality. The firebricks will shrink causing the arch to sink and especially the joints are exposed to the fluxing action of ashes.
Relieving arches
A relieving arch is used to remove the load from the main arch. Normally it is rarely used. However, the firebox shown in fig. 2-92 benefits from having a relieving arch above, which would carry the load of the kiln wall. A relieving arch also makes it easier to replace the bricks of the main arch when these wear out.
The important feature is the gap between the relieving and the main arch which ensures that no weight is being carried by the main arch. A loose filler, such as asbestos or a sawdust/clay mixture can be used to fill the gap.
Skewbacks
The arch is resting on a skewback at both ends. The weight of the arch is transferred to the walls through the skewbacks. These are made from square bricks cut to the proper angle (fig. 2-91). The skewbacks should be laid very carefully because if they fail the arch will come down.
Fig.2-90: Firebox arch after
construction, and later after firing shrinkage of the firebricks has taken
place. The initial high rise saved the firebox arch from collapsing due to
firing shrinkage; Fig.2-91: Skewbacks cut from square bricks
Arch frame
While the arch bricks are being laid, a support frame is needed. This is made of wood (fig. 2-93). The frame is raised until its sides are level with the skewbacks.
Fig.2-92: Relieving arch.
Laying the arch (fig. 2-95)
Fig.2-93: Arch support frame
Fig.2-94: Bottom of the joints should be as thin as possible.
Construction of the arch is started from both sides working towards the middle. Each brick should be placed so that it follows the circle of the arch by pointing towards the centre of the circle. Extra care is needed to ensure that the joints are completely filled with mortar. This is done by applying a thin layer of mortar on both surfaces and after the brick is laid it is rubbed back and forth. With a hammer and a piece of wood the brick is given a few taps at the lower end. The bottom of the points should be as thin as possible. The outer part of the joints might be thicker depending on the tapering of the bricks. Thicker joints should have small pieces of broken firebricks forced in from above after the whole arch is laid.
Square brick arch
In case no tapered bricks are at hand ordinary square bricks can be laid as shown (fig. 2-96), but at least the key brick should be tapering and should be forced below the other bricks. The square bricks are laid so that the upper one is resting a bit inside the comer of the lower one in order to prevent it from slipping out. A square brick arch will have thick outer joints and these should be filled with broken firebrick pieces.
Fig.2-95: Laying the arch
As for all other brickwork the joints should, as far as possible, be broken as shown (fig. 2-97). In case one brick should fall out the bond will keep the arch from collapsing. When the key bricks are in place the wooden frame can be removed.
Fig.2-96: Square brick arch.
Fig.2-97: Bonded arch
Arch spans and thickness
The thickness of the arch depends on the span and rise of the
arch, the temperature of firing and the quality of the firebricks. The highest
temperatures are reached at the outlet of the fireboxes and the arch here should
be 23 cm thick. Generally, solid heavy firebrick arches spanning less than 1.5 m
are laid with 12 cm thickness, up to 4 m spans 23 cm thickness and above that 34
cm is used. Insulating firebricks are lighter and thickness above 23 cm is not
used for spans even above 5 m.
2.5.7 Domes
Circular kilns are roofed with a dome. A dome is a more stable structure than an ordinary arch and requires no supporting frame for its construction. The radius and rise of the dome circle are calculated as for arches (see appendix). Domes are normally made with a rise of 20-25 cm per 100 cm span. A stick with the length of the radius and thickness of the dome is tied at the centre of the dome circle (see fig. 2-99) in such a way that it can move both around and up and down. Take care that the stick is really pivoting at the centre of the dome and that it will not become displaced during construction.
Fig.2-99: Arrangement of dome ruler.
Skewback
The skewbacks on top of the circular kiln wall are then laid with the guidance of the stick. The skewback bricks are laid as headers (fig. 2-100) cut to the proper angle.
Fig.2-100: Skewbacks are laid as
headers.
Laying the dome
The laying of the first course of the dome is started by placing two bricks, also in header position, on the skewback. The joints should be as thin as possible. The stick is laid on top of each brick while tapping it into line with the stick and the lower edge of each brick should be in line with the mark on the stick (see fig. 2-98). Each brick will then be pointing towards the centre of the moveable stick. The stickiness of the mortar will prevent the bricks from slipping and as soon as more bricks are laid they will squeeze each other into place. The mason stands inside and works around until reaching the starting point (fig. 2-101). The last brick has to be cut so that the circle course of bricks interlocks tightly. A new course of bricks is laid in the same fashion taking care to overlap the joints of the former course. Each round of bricks will lock itself and there is not the same need for broken joints as with ordinary arches. As the circle of brick courses becomes smaller the bricks should be cut slightly at the edges on the inner side in order to give better lock.
When the mortar is dry it is safe to walk on the dome and from
the top all the joints should be gone over to ensure they are completely filled
with mortar. Normally a vent hole is left in the centre of the dome. Domes can
be constructed in similar ways for covering clay cellars or underground tanks
for water. In that case a lime mortar is sufficient.
2.5.8 Catenary arch
The shape of a catenary arch is found by hanging a chain or heavy rope between the ends of the span so that the rope touches the top point of the required arch. The curve is copied onto a wooden board which is used to form a wooden frame. An arch with this shape is self-supporting and so steelwork is not needed. A catenary arch can be laid with square bricks.
Fig.2-103: A chain forms the shape of
a catenary arch.
2.5.9
Arch construction without support
An alternative method of arch construction which does not require a wooden support is suitable for catenary arches or arches with a high rise like barrel vaults.
In this method, the face of the arch is sloped back from the vertical and the vault develops from back to front by first laying a complete arch, one brick thick, then adding one full arch at a time. The initial slope of the arch is built up against the end wall of the kiln as shown in fig. 2-105.
Mortar
The mortar should be plastic so that its stickiness will prevent bricks from falling until the key brick is set. A simple bent wood or bamboo guide, made to the inside curve of the arch, keeps the curve uniform.
Brick shape
Ordinary square bricks can be used for this construction method. However, specially shaped bricks, which are thinner but wider than standard, will make construction easier. Bricks like this are used for the large stoneware kiln shown in fig. 2-107.
Fig.2-108: Expansion joints in a kiln
wall. Side elevation. Fig.2-109: The same expansion joints as in fig. 2-108 seen
from the top of the wall.
2.5.10
Expansion joints
The firebricks might expand 0.5 - 1% each time the kiln is fired and they will shrink again while cooling. In small kilns (1 m¦) this is of no concern but in bigger kilns the expansion might cause the wall to crack and bulge if the construction is made too tight. Joints, + cm thick, are made without mortar for each 1 m length of wall. Such expansion joints are made starting from the corners in numbers according to the length of the wall.
Steelwork
However, the expansion joints are only needed for kilns firmly
supported by steelwork. Free-standing kilns without steelwork will just expand
as a whole and fall back again and circular kilns with an insulating layer will
need no expansion joints. If the kilns are laid with self-made firebricks these
will often shank additionally during the first firings of the new kiln. Only if
the firebricks have been fired at a much higher temperature will no more
shrinkage occur. This additional shrinkage makes room for the normal thermal
expansion and therefore expansion joints can be omitted. In conclusion,
expansion joints are only needed in big square kilns, laid with hard-fired
bricks and supported by steelwork.
2.5.11 Insulation
During firing heat goes through the kiln wall and is lost. Heat goes through solid matter (conduction) but is stopped by air. Good heat insulation means as much air as possible but the air should be in small pockets, otherwise the air will rotate and thereby transfer the heat (convection). The most practical solution is an insulating firebrick wall backed up by ordinary blocks. In some cases a gap of about 5 cm can be left between the inner and outer wall. This gap can be filled with some very light loose materials.
Loose layer
This can be a mixture of (by volume) 30% plastic clay and 70%
sawdust. The sawdust will slowly burn, out leaving insulating airpockets. Ash is
an even better insulator, but unless the ash has been calcined it will lose much
of its volume after a few firings A simple solution is to leave a few loose
bricks in the top of the wall to allow for the refilling of ash. If a loose
insulation is used spyholes and vent holes should be lined so that the
insulation does not fill the holes. A similar loose layer can be laid on top of
arches and a layer of flat tiles may be used to covet the insulation for
protection.
2.5.12
Maintenance of kilns
Nearly all kilns show cracks after the first few firings and it does not mean that the kiln is about to collapse. However, after each firing the condition of arches and dome should be inspected carefully for bricks beginning to sink in or for joints which have become loose or fallen out. These defects should be mended immediately to prevent the arch from collapsing. Flue channels, dampers and bagwalls should likewise be checked. It is an unnecessary waste of time and energy to stop a firing halfway through because a damper is blocking the draught or a bagwall has collapsed. The flue channels should be cleaned regularly.
Fig.2-110: Look out for bricks
falling in and refill mortar in loose
joints.
2.6.1 Loading biscuit firing
2.6.2 Loading glaze firing
Dry pots
When the raw pots are dry they are ready for firing. This can be checked by holding the pot to one's cheek; if any coolness is felt then it is not sufficiently dry to be fired safely.
The pots can be left in the sun for a day to ensure they are completely dry. Another method is to light a small fire in an empty kiln for half a day after which the pots are loaded and kept in the warm kiln overnight. The gentle heat remaining in the kiln walls will dry the pots and next morning the firing can start.
Biscuit/glaze firing
Some workshops glaze the raw pots and fire them only once while
others prefer to fire the green pots without glaze (biscuit firing), then glaze
the pots and fire them once mote (glaze firing).
2.6.1 Loading biscuit firing
Before loading a kiln it is preferable to have 1+ times the ware needed to fill the kiln. A wide selection of sizes and shapes makes it easier to fully utilize the kiln space. The same idea applies to glaze firings.
Stacking
Unglazed ware can be stacked on top of and inside each other. However, around 700 °C clay expands and then later it shrinks again Therefore the pots should not be set tightly inside each other and a gap of at least 5 cm should be left between the pots and the top of the kiln. Cups, bowls and pitchers should be placed rim to rim and base to base. Tiles saucers and other flatware will often show less breakage if placed vertically against each other.
Fig.2-114: Stacking like this may
cause breakage. Fig.2-115: Correct way of stacking
Fig.2-116: Flatware is better stacked on edge
Height of setting
The height of the stacked pots depends on the quality of the clay and the types of pots. The green strength of highly plastic clays is much greater than sandy clays and thin green pots will break before thick-walled ones. In case the pots cannot be stacked from bottom to top without breaking, the pots can be set in smaller stacks supported by kiln shelves resting on props (fig. 2-113). The same persons should always both load and draw the pots from the kiln so that they can learn from their mistakes.
Last check
While the pots are being placed in the kiln they should be given
a last check. It is a waste to fire broken pieces and at this stage it may still
be possible to correct a minor flaw in a pot before firing. This checking is
needed to improve the quality and correct mistakes done during earlier
production of the pots.
2.6.2
Loading glaze firing
Slabs or saggars?
A glazed pot will stick to anything it touches during firing so that pots will have to be placed separately either on kiln slabs or in saggars.
Provided the glazed pots are not harmed by direct exposure to combustion gases and ashes, kiln shelves are the best solution. For the same space a kiln setting with slabs carries more pots compared to saggars, which are also much heavier. The ratio of kiln furniture to ware is about 4: 1 for saggars but only 2: 1 for slabs. Sometimes saggars are set as a bagwall (fig. 2-30) behind which kiln shelves are placed.
Kiln wash
Before setting, glaze droplets from previous glaze firings should be chipped off the slabs or saggars and these should then be given a refractory kiln wash. A suitable wash can be made from a powder of silica sand or quartz mixed with water and gum arabic or another glue. Another suitable kiln wash can be prepared with l/2 kaolin and + silica powder mixed with water. Alumina, zirconium silicate and silimanite are excellent for high temperatures, though expensive and often not available.
Fig.2-118: Stacking with thimbles.
The kiln wash will prevent pots from sticking to the kiln furniture, which will also be less likely to stick to each other. Glazes accidentally running off the pots will also be easier to chip off after the firing. The wash can be painted on with an ordinary paint brush.
Lids
Pots with lids such as teapots and jars should be fired with the lid in place so that the lid and the pot will fit together after firing and the colours will be the same. This is especially important for high temperature firings. The faces of contact on the lid and the pot should be free of glaze and should be painted with a silica powder mixed with glue.
Sticking
The simplest way of placing glazed ware in the kiln is to remove any glaze from the foot of the pots and set the pots directly onto the shelf or saggar. However, at high temperature firings, especially when stoneware clays are used, the pots tend to stick to the kiln furniture. To avoid sticking one of the following methods should be used:
1. The pots can be set in sand, but care should be taken during
setting to avoid knocking sand into the glazed ware on the shelf below.
2. A
silica powder wash can be painted onto the feet of the Dots.
3. Small
hand-made clay balls of l/2 kaolin and + silica powder with the addition of a
glue (a cheap flour of cassava, maize or the like works well) can be stuck onto
the bottom of the pots before they are placed in saggars or on shelves. The
balls are made just before setting is done (fig. 2-119).
Fig.2-119: Freshly rolled balls are
stuck to the pot and it is then set in the kiln. Fig.2-120: Setting with stilts
Earthenware setting
Pots made from clay which does not soften or warp during firing can be supported on only a few points. Stilts and spurs are used for setting pots with glazed bottoms and they can also be used for placing glazed pots inside each other (fig. 2-120).
Fig.2-121: Spurs resting on the
unglazed bottom of the plates. Fig.2-124: Glazed pots should be placed 3-5 mm
apart to prevent them from "kissing".
Flatware such as saucers, dinner plates or tiles can be placed vertically and supported at the top with thimbles (fig. 2-122). These can also be used for stacking bowls or plates horizontally in which case a cover for supporting the stack of thimbles is helpful (see page 29 f.). Bowls and plates could also be placed rim to rim and base to base if their rims and bottoms are left unglazed. If bowls are made to accurate measure and with a thick rim, they can also be stacked hanging inside each other (fig. 2-123).
Fig.2-123:Bowls only glazed inside
can be stacked hanging inside each other
"Kissing"
The glazed pots should be placed carefully 3 - 5 mm apart. This will prevent them from "kissing" each other when the glaze and clay expand during firing.
Cold spots
All kilns will have some cold spots, where the correct maturing glaze temperature is not reached. To gain maximum use of the kiln space and to avoid second-rate, underfired ware being produced in these spots, a lower melting point glaze should be applied to pots which are to be stacked in these areas of the kiln. A good understanding of the kiln and careful stacking will ensure maximum results from each firing.
Saggars
Saggars are normally filled with glazed ware outside the kiln and stacked on top of each other inside. Before placing the saggar its outside base should be dusted to prevent any dust settling on the glazed ware in the lower saggar. Each stack of saggars, called a bung, should be set straight and not rock. The bungs are set a bit apart to allow a proper draught through the setting and if the bungs are tall a few lumps of clay are squeezed into the gap between the bungs so that they form an interlocking bridge support for each other. In case the firing temperature is close to the softening point of the saggars special fireclay bars could be set between the bungs and the kiln wall for additional support (fig. 2-126).
A wad of clay laid on top of the saggar rim (fig. 2-127) will seal the saggar and provide a safer setting if the saggar rims and bottoms are not even. New green saggars should only be used for the upper 3 - 5 layers of saggars.
Kiln slabs
The structure of kiln slabs and supporting props or firebricks is normally rebuilt with each new setting of ware. Each slab should be supported at three points only because it is unlikely to rest evenly on a four-point support and the slab would then be under tension, which could cause it to break. The kiln setting is placed layer by layer and it should be done evenly so that dense layers of (for example) cups are mixed with more open layers of larger pots. Having all cups on one side and large pots on the other side of the kiln chamber could result in uneven firing temperatures. Slabs that sagged in the former firing are placed with their sag upwards in the next firing.
Fig.2-128: Slabs should be supported
at three points only if possible.
Door
The brickwork of the kiln door should have the same insulating quality as the wall of the kiln. Bricks are to be laid in a refractory mortar? which has a high proportion of sand in order to make it easy to break up the door afterwards. Spyholes should be left at the top and bottom of the door, with neatly fitting bricks provided for closing the holes.
2.7.1 Biscuit firing
2.7.2 Glaze firing
The firing is the last step in the production of pottery and all of the potter's previous efforts during production may be either rewarded or ruined in this final process.
Firing routine
All potters feel excitement and anxiety when opening their kiln and inspecting the outcome of the firing. However, it is possible to reduce the anxiety once a successful firing routine has been developed. Each kiln has its own peculiarities and it often takes up to ten fringe to break in a new kiln and learn its secrets. This should be done in a systematic way and recorded so that mistakes are not repeated and experience is gained from successes.
Firemaster
At each firing there should be only one person in charge, the firemaster. The firemaster supervises all work from setting the kiln, firing, cooling until drawing. All major decisions such as when smoking is over or when the firing is finished are taken by the fire" master. A kiln during firing needs constant attention, and even when using a convenient fuel such as oil the firing process must be carefully watched. When a kiln is fired with solid fuels such as coal or wood a steady stoking is needed. A good firemaster listens to the breathing of the firing and checks the conditions in the fireboxes and inside the chamber many times every hour.
Kiln log-book
Each firing should be recorded in detail in a log-book, so that afterwards it is possible to trace the reasons behind possible misfirings.
Fig.2-130: Kiln log-book.
The following things should be noted down:
- repairs or alterations carried out,
- number and type of
ware in the kiln,
- pattern of setting of saggars, slabs and pots,
-
condition of fuel (such as wet firewood),
- date and time of lighting fires
in the different fireboxes,
- smoking time,
- start of real firing,
-
stoking intervals and rise of temperature, ùbending of cones or drawing of test
pieces,
- position of dampers and draught,
- condition in fireboxes during
firing,
- finishing time,
- cooling time and time of opening spyholes and
kiln door,
- fuel consumption.
While the ware is being drawn, it should be noted where the different pots were placed in order to get a picture of where the kiln was too hot or too cold. Finally the ware could be rated as first-, second- and third-class and the estimated value of the fired pots should be recorded too.
After that the firemaster and the firing crew should discuss the outcome of their firing and they should try to pinpoint problems and decide upon how these problems could be solved at the next firing. Some of the problems may originate from an earlier stage in the pottery production.
Only by experience can a successful firing routine be
established and this sort of practical understanding is difficult to gain from
books. However, it is hoped that some of the instructions given here will be of
use for the firing itself. This chapter deals with the general techniques of
operating kilns. Information on how to stoke the fuel and operate the fireboxes
is given in the previous chapter: Combustion and Fireboxes.
2.7.1 Biscuit firing
Smoking
Even if the pots felt dry when they were loaded into the kiln they will still hold some water. Pots are normally dried in the open air, but air always contains some water and during the rainy season the air can become very humid. The air left in the space between the clay particles contains the same amount of water as the air outside. This pore water will expand explosively if it turns to steam before leaving the pot.
Very plastic clay has small clay particles, which will only leave small openings for the pore water to escape through. Sand or grog added to such a clay will make big holes through which the water can more easily pass. Therefore, sand and grog will improve a clay which tends to crack during drying or smoking.
The pore water will only dry out when the pots are heated to temperatures of 50100 °C inside the kiln. If this heating goes too fast the pore water will turn to steam before it can get out of the pots. The pressure of the steam will cause the pot to crack or even explode (fig. 2-131).
Therefore, a long period of very slow heating is needed until all of the pots are completely dry. This period is called smoking and it should count for approximate one third of total firing time. That means 3 - 12 hours or even more depending on the nature of clay, types of ware and size of kiln. The first firing of a new kiln could mean smoking for days to enable the kiln itself to completely dry out.
Vent holes
During smoking a very gentle fire is kept in a few or only one firebox and in the beginning this fire may even be placed outside the firebox mouth (fig. 2-132). All spyholes and vent holes are left open during smoking to let out the resulting steam before it condenses at colder spots of the setting.
Fig.2-131: Clay shown in four stages.
The clay crystals are made 100 000 times larger than their real size.
a) Plastic clay. Water surrounds all the clay crystals and these
can move easly.
b) Leatherhard clay. The clay crystals touch each other but
there is still lubricating water in the clay.
c) Air-dried clay. All the
lubricating water has evaporated and only pore water remains between the clay
crystals.
d) The pore water turned to steam inside the clay and caused the
clay to crack because the clay was heated to above 100°C too suddenly.
It must be emphasized: Go slow. The kiln will contain the result of weeks of hard work and it is very easy to ruin it at this stage.
Condensed water
The smoking period is finished when the air coming out of the spyholes contains only a little or no moisture. This can be tested by holding a piece of glass which is of room temperature in the air stream above. the spyhole. If water forms (condenses) immediately on the glass smoking should be continued until only a little moisture is formed after holding the glass at the spyhole for a number of seconds.
Ceramic change
The temperature inside the kiln on completion of-smoking should be 120 - 200 °C and then the real firing can start. The spyholes and vent holes should be closed, and the firing time, from the end of smoking until a dark red colour is visible inside the kiln, should take more than four hours. Between 350 - 700 °C a ceramic change takes place, which permanently changes the plastic clay into something rock-like that can never again be softened by water and formed into a pot. The clay particles contain water in their crystal structure (Al2O3 2SiO2 2H2O).
This chemically bound water is released from the crystal structure and changes the crystals so that they cannot go back to their former shape. This chemically bound water amounts to 14% of bone-dry clay and its release will normally not cause the pots to crack. But the free silica or sand in the body expands suddenly at 573 °C and both actions combined may cause problems.
The ceramic change is at its peak around 600 °C. Chemically bound water from within the clay particles is driven out and often white steam can be seen at the chimney at this point. If the firing goes too fast, around 600 °C this rapid release of water may also cause the pots to crack.
Burn-out
The carbon or vegetable matter in the clay will start to burn out when a red glow is reached, but the burning out will only be completed at around 900°C. Clays containing a lot of carbon should be given a firing with excess air from 800-900 °C. A fast biscuit firing may close the surface of the clay before all the carbon gets out. This may later cause blistering of the glaze and the clay may even bloat at higher temperatures from the pressure of the trapped carbon gases.
A black core in a broken piece of a biscuitfired pot is a sign of unburned carbon.
Top temperature
At around 800 °C the clay starts to harden. The clay hardens as soda, potash, lime and other impurities in the clay begin to melt, thereby glueing the clay particles together. This will give strength to the clay body and more so as the temperature rises. This process is called vitrification. Traditional pottery fired in pit kilns is only fired up to this temperature. Ceramic ware that is going to be glazed after a first firing is normally not fired higher than 900-1000 °C because the pots should remain porous for the application of glazes. Earthenware always has problems with crazing of its glazes. A higher biscuit firing reduces the problem but also increases the danger of overfiring the biscuit ware. For such firings the top temperature should be judged by the help of cones (p. 120) or by drawing a test piece and checking its ability to absorb water.
However, normally the firemaster can judge when the desired biscuit temperature is reached by the colour inside the kiln. Stoking is then stopped and when the inside is clear of combustion gases the dampers are closed.
Cooling
The air intake of the fireboxes, spyholes and vent holes should
be sealed completely after firing and the kiln left to cool by itself. It is
tempting to speed up cooling, but if the kiln is cooled too fast pots
may
crack. The clay contracts suddenly at 573 °C and if the cooling is uneven
it will cause cracking. The free silica (quartz) of the clay changes its crystal
shape suddenly at 573 °C When heated free silica will expand and when it
cools it shrinks again and this may cause cracking of the pots during cooling.
During firing this expansion does not cause problems as the clay is still very
open and has room for this movement. These cracks may not become obvious until
after the glaze firing as they may be very fine. The crack pattern will often
look like a brick wall. Remedies are: a) to cool more slowly, b) to raise
biscuit temperature slightly, c) to decrease the amount of free silica (sand) or
substitute it with fine grog.
2.7.2
Glaze firing
If the ware has been biscuit-fired already there is no need for an extended water smoking, but the water from the glazing should be allowed to dry out slowly especially if the glaze shows a tendency to crawl. The glaze crawls away from rims or forms islands where the glaze has crawled away. This may be caused by firing the glazed pots before they have dried properly. The firing rate would normally be 100150 °C per hour.
However, when firing large pots it is prudent to slow down the firing around 573 °C. Apart from this the firing can proceed at a steady rate until the maturing temperature of the glaze is reached.
Oxidizing/reducing
The mixture of gases inside the kiln affects both the clay and the glazes. A firing with excess amount of air intake (oxygen) is called oxidizing and a firing with too little air for complete combustion of the fuel is called reducing. The main difference between these two firing conditions is the change of colour in clay and glazes. For example, the metal iron has a grey or black colour but if it is exposed to air it turns to rust, which is red.
Rust is iron + oxygen or iron oxide and is present in most clays. A clay with a small iron oxide content will fire to a yellow or buff colour when the firing is kept oxidizing throughout, whereas it will turn to a grey colour if the fire is reducing. What happens is that the reduction firing produces a lot of half-burned carbon that has been starved of air. On its way through the kiln this carbon picks up the oxygen in the iron oxide and the "rust" is turned back to its original metal, iron, which is grey.
It is more economical to keep a lightly oxidizing or neutral firing throughout as a reduction firing means that some fuel is not being completely burned. If the special colour effect of a reduction firing is desired it should be done at the right time. For changing the colour of the glaze the reducing firing should be started at 100ù150 °C below the maturing temperature of the glaze, whereas for changing the colour of the body reduction will only be effective before the covering glaze starts to melt.
The firing is kept oxidizing until the right temperature for reduction is reached. Reduction is then started by letting in less air at the fireboxes and by partly closing the dampers. Reduction is in process when flames come out of the spyholes. However, a heavy reduction is not needed and is a waste of fuel. For ware that has not been biscuit-fired reduction should not be started before the temperature is above 1000 °C when all carbon in the clay has been allowed to burn out.
Test draw
The firemaster should be able to judge the approximate temperature of the kiln by the colour of the glow inside. When this colour indicates that the maturing temperature is soon to be reached a test piece from the kiln should be drawn using a crooked iron rod (p. 119). The glazed test piece will show the condition of the glazed ware inside the kiln. If the glaze surface is still rough the firing should be continued. From the look of the test piece the firemaster can judge when to draw the next test piece. It may be necessary to reduce or stop stoking while drawing and care should be taken to draw the tests quickly in order not to cool the kiln unnecessarily.
Soaking
Tests should be drawn both from the top and bottom part of the setting. If the temperature is uneven the maturing temperature should be kept for one or two hours, allowing the glaze all over the kiln to melt properly. Such a period is called soaking.
Cones
A set with three different bending temperatures of cones can provide a warning 60 that when the first cone bends it indicates that there is only 30 °C to go before reaching maturing temperature. If, for example, the cone at the bottom spyhole is not bending at the same time as the upper cone the firemaster can start stoking more slowly in order to allow the bottom temperature to catch up with the top. The maturing temperature is reached when the second cone bends. The last cone, with a bending temperature 30 °C above the maturing point, should not bend but gives the firemaster a warning in case the temperature should rise during the soaking period.
Finishing
When the firemaster, from the look of test draws and cones, feels confident that the glazes have matured the firing can be stopped. The stoking of fuel is stopped and the dampers are left open until the inside of the kiln is clear of combustion gases. The dampers are then closed completely and all spyholes and firebox inlets are sealed. In case iron grates or drip-plate burners have been used their life will be prolonged if they are pulled out at this stage. If the firing has been reducing,then a period of 10-20 minutes oxidizing at the end will brighten the glazes without changing the reduction effect on the colour of the glaze.
Cooling
The kiln should be left to cool and only when the temperature is definitely below 200 °C can the door be opened. The temperature is low enough if it is possible to hold an arm inside the kiln for a short while. Too sudden a cooling will not only damage the ware but also the kiln structure and the saggars or kiln slabs.
Above 1100 °C a lot of free silica is formed in the clay body. Much of it will take the form of a crystal called crystoballite, which shrinks 3% at 230 °C during cooling. This shrinkage will cause many pots to crack if cooling is too sudden. These cracks will be long clean cracks of the body and the glaze will have a sharp edge because at this low temperature the glaze had solidified when the body cracked.
The problem OCCUIS often with stoneware, which is fired to 1250 - 1300°C. Saggars and slabs may suffer even more because they have developed a higher content of crystoballite due to their many cycles of firings.
Remedies are:
a) slower cooling around 230 °C (and 573
°C),
b) to reduce silica (sand) in the clay body by substituting
grog,
c) to add more feldspar to the clay body.
The feldspar will fuse
together with the free silica and crystoballite and the resulting glass will not
have sudden shrinkage points. All potters are eager to see the result of their
work but, if they cannot wait for the kiln to cool slowly, only broken pots may
be the reward for their
haste.
2.8.1 Thermometers
2.8.2 Colour
2.8.3 Test draw
2.8.4
Cones
2.8.5 Pyrometer
2.8.6 Self-made cones
2.8.1 Thermometers
The temperature inside the kiln cannot be measured with ordinary
mercury-in-glass thermometers above 550 °C. Thermometers may still be
useful for measuring temperatures during smoking and cooling periods and they
can be used to read the temperature of the flue gases in the chimney.
2.8.2 Colour
The colour inside the kiln is a good indicator of the temperature. The lowest visible red, which is visible only when it is dark outside the kiln, is seen at 470 °C but during daylight a red colour may not be seen before around 600 °C. The higher the temperature, the brighter the colour becomes as shown in the table.
|
Colour of temperature |
°C |
|
Lowest visible red, night-time |
470 |
|
Lowest visible red, daylight |
550-650 |
|
Dark red-cherry red |
650-750 |
|
Cherry red-bright cherry red |
750-815 |
|
Bright cherry red-orange |
815-900 |
|
Orange-yellow |
900-1090 |
|
Yellow-light yellow |
1090-1315 |
|
Light yellow-white |
1315-1540 |
|
White-bluish white |
1540 |
The experienced firemaster will be able to accurately judge the
temperature by its colour. This skill is one of several additional senses
developed during countless hours of tending firings and even a firemaster who is
fortunate enough to have other more sophisticated measuring methods such as
those described below should compare these with the colour reading before
trusting them completely.
2.8.3 Test draw
Glazed test pieces made from the same clay body and with the same glaze as the rest of the ware fired in the kiln can be used to judge when the glaze has matured. The test pieces should be placed close to the spyholes from where they can be drawn out with the help of an iron rod (fig 2-138). When the colour indicates that the firing is close to completion the firemaster draws a test piece and from the extent of fusion of the glaze the condition of the rest of the ware is revealed. The colour of the-glaze though may differ from that of the finished ware. The test pieces can be shaped as rings (fig. 2-137) or can be small cups with a hole in the bottom for easy fetching.
Fig.2-137: Two examples of glazed
test pieces: a ring of clay and a cup with its bottom pierced.
2.8.4 Cones
Cones are slim three-sided pyramids 5-7 cm high and made from various mixtures of kaolin, feldspar, quartz, limestone and other minerals. The cones soften like clay and glazes and bend depending on the melting point of the mixture they are made from.
Commercial cones are available for measuring temperatures of 600-2000 °C within steps of about 20 °C. The cones have a printed number corresponding to their bending temperature (see table of cones in appendix).
Set of three cones
Cones are normally used in a set of three placed in front of a spyhole from where the firemaster can see them during firing. In the example (fig. 2-139) where the maturing of the glaze corresponds to the bending of cone 7 (1230 °C) another one, a cone 6 (1200 °C), is placed in front of cone 7 so that the bending of cone 6 provides a warning. Behind cone 7 a cone 8 is placed which shows whether the kiln is overfired.
Fig.2-139: Setting of cones: a) Cones
should be set similarly at each firing; b) Cone 6 has started to bend, warning
the fire-master that the firing is soon to finish; c) Cone 7 is on its way down.
When the tip of the cone touches the base the maturing temperature has been
reached.
Setting of cones
The cones should be set in a well grogged clay at a slight angle and with the flat side of the pyramid away from the bending direction. The firemaster should make sure that the cones are set in the same way at each firing. They should not be placed too close to the spyhole, otherwise they will be cooled by air entering through the spyhole. Before firing the firemaster should look at the placement of the cones through the spyhole and remember their position for later. At temperatures above 1100 °C it can be very difficult to see the cones properly and one cone can easily be mistaken for another. It helps to watch the cones through a dark (smoked) piece of glass or through dark sunglasses.
Heat-work
Cones do not really measure temperatures but rather heat-work or the combined effect of heat and time. If cone 7, for example, is heated at about 250 °C per hour it will bend at 1230 °C but if heated at 50 °C per hour it may bend at only 1200 °C.
Cones with the same number but from different factories may not
bend at the same temperature and so before cones from a new supplier are used
they should be fired in the kiln alongside the old cones to make sure that they
bend at the same time.
2.8.5 Pyrometer
A pyrometer reads the temperature directly by measuring the electric current which is produced in a thermocouple. The thermocouple is made from two wires of different materials which are joined at one end. This end is inserted into the kiln chamber where it is protected by a ceramic tube from corrosion (fig. 2-140).
Fig.2-140: Pyrometer
For temperatures up to 1100 °C the joined wires are made from 90% nickel + 10% chromium for the positive wire and 98% nickel + 2% aluminium for the negative wire.
These metals are usually cheap but for temperatures up to 1500 °C the two wires need to be made from very expensive materials: platinum (negative wire) and 87% platinum + 13% rodium (positive wire).
The wire leading from the thermocouple to the pyrometer is made from metals electrically similar to the ones used in the thermocouple and if ordinary wire is used the pyrometer will not be accurate.
A pyrometer shows the actual temperature and is able to show the firemaster how fast the temperature is rising as well as the rate of cooling. It is a very helpful instrument but is expensive and spare parts can be difficult to get. Therefore, its use in many countries will be limited to pottery training centres and bigger factories.
Warning
A firing should never be measured by a pyrometer only. The final
judgement of when the firing is completed should still be based on cones or test
draws. A pyrometer only shows the temperature and does not record the condition
of clay and glazes as cones and test draws do. Furthermore, a pyrometer may
fail.
2.8.6 Self-made cones
Cones are made by a few large factories. These guarantee a consistent quality so that cones of the same number from the same factory should bend simultaneously even if their years of manufacture are years apart. This is only possible with a very strict control of raw materials and production. Even so, cones are inexpensive and so it would normally not make sense for potters to make cones themselves. However, in many remote areas, or where overseas imports are difficult, cones are often not available and potters could produce them in their own workshop.
Cone body
It takes patience to find the right mixture of clay, sand and melters (fluxes) which will bend at the desired temperature. The first problem is to get raw materials that do not differ much from batch to batch.
Clay
A pure kaolin clay which should contain the same proportion of sand and clay with each batch is the most reliable. Alternatively a stoneware clay or the plastic production clay of the workshop can be used. Clay with a high iron content is less suitable because the iron oxide acts as a melter in a reduction firing but not so in an oxidizing firing.
Silica
Silica sand or quartz is needed for adjusting the bending or softening temperature. The more silica, the higher is the bending temperature. The sand particles also help to open up the cone mixture so that it is less likely to crack during drying.
Melters
Melters (fluxes) which lower the bending temperature are added to the mixture of clay and sand. Melters such as feldspar, whiting (or limestone) and talc are sufficient for the higher temperatures above 1200 °C. Below that, melters such as borax or boric acid are added.
Potash and soda can replace these if they are not available. Potash, soda, borax and boric acid are all soluble in water. When these materials are added directly to a moist cone body they will to some degree leach out of the cone body and settle on the surface of the cones. This will cause the cones to bend at higher temperatures than intended. The materials could be made insoluble by melting them together with the silica sand, feldspar, whiting and talc of the con body and then, after crushing the resulting glass to a fine powder, adding this to the clay. That is a laborious process and a simpler method would be to mix and form the cones in a semi-dry state by adding a glue such as starch or gum arable.
Cone mixtures
There are many factors determining the melting point of a ceramic mixture besides the proportion of the various materials in the mixture. Therefore, the cone body recipes given next column are only meant as a starting point for further experimenting.
Kaolin, whiting (limestone, chalk), talc and feldspar all contain silica in varying degrees. One kaolin clay may be very pure while another source may contain high amounts of silica sand. These recipes are based on pure kaolin while the recipes using stoneware clay have smaller amounts of silica sand in them to compensate for the sand in the stoneware clay.
|
Cone 05a, 1000 °C | |
|
Potash feldspar |
10 |
|
Talc |
6 |
|
Kaolin |
22 |
|
Quartz, silica sand |
20 |
|
Whiting |
8 |
|
Borax |
34 |
|
Potash feldspar |
8 |
|
Talc |
6 |
|
Stoneware clay |
29 |
|
Quartz, silica sand |
16 |
|
Whiting |
8 |
|
Borax |
33 |
|
Cone 1a, 1100 °C | |
|
Potash feldspar |
18 |
|
Talc |
3 |
|
Kaolin |
22 |
|
Quartz, silica sand |
32 |
|
Whiting |
10 |
|
Borax |
15 |
|
Potash feldspar |
19 |
|
Talc |
2 |
|
Stoneware clay |
27 |
|
Quartz, silica sand |
27 |
|
Whiting |
10 |
|
Borax |
15 |
|
Cone 7,1230 °C | |
|
Potash feldspar |
27 |
|
Kaolin |
17 |
|
Quartz, silica sand |
44 |
|
Whiting |
12 |
|
Potash feldspar |
23 |
|
Stoneware clay |
26 |
|
Quartz, silica sand |
39 |
|
Whiting |
12 |
All recipes by weight.
A simpler solution is to make the cones from a mixture of the
clay and the glaze used in the workshop.
Example of a mixture for cone 8,
1250 °C:
|
stoneware clay |
22 |
|
glaze for 1250 °C |
51 |
|
silica sand |
27 |
(the glaze recipe: 4 feldspar, 3 quartz, 2 whiting, 1 kaolin)
Experimenting
A series of mixtures has to be tested before the right bending temperature it arrived at. The simplest way is to start with the recipes given above and vary the amount of clay and silica sand in, say, three steps of 5%. Cones of these three different mixtures are then fired in a normal production firing and through the spyhole an eye is kept on the bending of the cones. The bending is then compared with either the bending of commercial cones fired along with the self-made ones or with the state of the glaze on draw tests.
In case none of the cones bend, new mixtures should be made with less clay and silica sand but if the cones bend too soon more clay and silica sand should be added.
The test mixtures should be prepared as described and it is important to use the same raw materials and the same procedures during testing and also later when the actual cones are produced.
Preparation of cone mixtures
The clay' send, feldspar, limestone or whiling should be sampled so that later, when a new batch of cones is to be made, it will be possible to obtain raw materials of similar quality. In all deposits of raw materials there will be a variation in particle size, sand content' etc. This variation can be reduced by taking samples from many different places in the deposit and mixing these thoroughly. A much bigger portion than needed is collected and the amount required is divided from this by quartering (see testing, p. 39). All the materials are dried and screened (100 mesh) before weighing out the different amounts according to the recipe. This should be done before experimenting so that the same materials can be used for testing and production. After the materials are weighed they are mixed well and sieved (100 mesh) again. About 6 g material is needed for one cone.
Shaping of cones
The cone mixture is easier to shape if a starch or glue is added to the dry mixture together with water. If borax is used, the water content must be kept low. All cones must be of exactly the same shape. A mould is made of plaster' hardwood or iron according to the dimensions shown in fig. 2-141. The cones are three-sided pyramids with a triangular base. The base is cut so that it facilitates a setting angle of 70_ with the horizontal.
To begin with, only a few cones are made and tested so that if these cones do not bend at the right time the rest of the mixture can be adjusted.
Fig.2-141: Shape and dimension of
cones.
Biscuit
Then the whole mixture is shaped into cones and dried. If an organic glue such as starch or sugar syrup has been used it will with time go rotten, stink terribly and the cone made with it may be easily broken when handled. To prevent these problems the cones could be given a low temperature biscuit firing, ensuring they are all fired at the same time and at the same place in the kiln. However, it should be noted that a biscuit firing will slightly lower the final bending temperature of the cones. Cones which are bonded by an artificial glue can be stored without biscuiting.
New batch
Once the right mixture is established cones are easy to make. So it pays to make enough for several years of firings. Five hundred cones of about 6 g could be made from 3 kg of raw materials. Note down carefully how the cones are made and from which materials so that when the first batch is finished a new batch of cones of the same bending temperature can be made. Remember that with cones it is important to expose them to the same conditions at each firing; setting should be at the same distance from spyholes and at the same angle. A new batch of cones should be made well in advance of finishing the old ones so that the bending temperature of the new cones can be tested in the kiln alongside the old cones.
Use of self-made cones
Self-made cones will not be as reliable as good-quality commercial cones and so test draws of glazed pieces should also be done. The cones will help to give the firemaster a warning of when to draw tests. They can also be placed at the top and bottom or front and back of the setting so that the firemaster can change the damper setting or slow the firing rate to compensate for temperature differences within the kiln chamber.
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