Village Level Brickmaking (GTZ, 1989, 124 p.) Clay Preparation (introduction...) Tempering Mixing

Village Level Brickmaking (GTZ, 1989, 124 p.)

Clay Preparation

To make good quality bricks, the dry lumpy clay soil has to become a smooth, soft, moist mixture containing no hard lumps of soil or stone. To obtain this mixture, the clay soil is treated in two different steps:

1. Tempering, and
2. Mixing

Tempering

Tempering is the process of adding water to the clay and allowing it to stand undisturbed for a few days before mixing occurs. This will begin to soften and break down the lumps of clay making the mixing process easier.

Figure

At some brick sites, the producers flood the hole or pit where they are digging with water because it makes the digging or extracting of the clay easier. This method is not recommended because far too much water tends to get mixed into the clay which results in lower quality bricks.

It is better to dig or extract the dry clay and bring it to a special pit or tempering tank where you can add water in exact proportions. This tempering tank should be located close to the place where you mix the clay and mould the bricks.

In the tempering pit or tank, the clay is moistened with the correct amount of water. In general, for every three buckets of dry clay soil, you will add one bucket of water. The amount of water can vary and will depend on the type of soil and how damp it is. The dry clay soil should be in powder form or only small lumps before the water is added. This is to help the clay absorb the water quicker. The ratio of three buckets of soil to one of water is much less than what many producers are used to. It may seem slightly more difficult to mix the clay when using this ratio but the results will be far better with a stronger brick and a much lower breakage rate during drying.

Note: It is true that some very experienced people can judge the correct amount of water without measuring, but this is not recommended for first time producers. Because more water makes mixing easier, most people "forget" that the bricks will be a poorer quality and tend to add more and more water to make their work easier.

Normally a 100 mm layer of dry clay soil is placed in the tempering pit and the correct amount of water is sprinkled on top of it. To ensure that the layer is moistened well, turn it over with hoes or shovels. Next, add another 100 mm layer of soil and this is sprinkled with water as well. Again, turn the layer over with hoes or shovels. Continue this process until the tempering pit is full. When full, cover the pit with grass or leaves to prevent the clay from drying out.

Figure

The clay is left in the pit to temper for one to five days (the time will depend on the type of clay). During this tempering time, the water will enter the clay particles or lumps and begin to soften them.

Each of your tempering pits or tanks should have-the capacity to hold enough clay for one day's production. If you produce 1,000 bricks per day, you will need 3 cubic metres of prepared clay each day. Therefore, the tempering pit or tank should have a capacity or volume of 3 cubic metres. Possible interior dimensions of this tank could be 1.75 m in length and width and 1.0 m in height.

If the clay needs to be tempered for only one day, you will need two tanks, each with a capacity of three cubic metres. One tank will hold the day's supply of tempered clay which you will use to make bricks, and the other will be filled with more soil for the next day's production. If your clay needs to temper for 2 days, you will need 3 tanks, each with a three cubic metre capacity. If it needs to temper 3 days you will need 4 tanks. With 4 days of tempering, 5 tanks will be necessary.

Figure

The tempering tank can be as simple as a hole or pit in the ground or a specially built tank built into or on top of the ground. A good permanent tank can be made of fired bricks with a cement plaster coat.

Mixing

After the soil has been tempered well, it needs to be mixed and kneaded. The purpose-of mixing is to ensure that the clay is a smooth, soft, homogeneous mixture that contains no hard lumps.

There are various methods of mixing tempered clay for small-scale brickmaking. One system uses a simple mechanical mixer that uses an empty 200 litre fuel drum with a mixing paddle built into it. The mixing paddle is connected to a long pole and turned by a horse, donkey or ox. The tempered clay is put into the top of the barrel and the paddles mixes and pushes the clay out a hole cut in the side of the drum. This method is very good for mixing but for some brick units obtaining the materials for building it or the animal to operate it can be difficult.

Figure

A simpler method preferred by many first-time producers which will give very good results, is to mix the tempered clay with your feet. The tempered clay is taken from the tempering pit and spread on top of a hard surface where workers trample it with their feet until it becomes a smooth mixture with a uniform colour. One advantage with this system is that rocks, stones or lumps of hard clay can be felt with the feet and removed. This results in a smoother mixture for moulding. The one difficulty with mixing clay with feet is that it can be very tiring. To prevent the workers from becoming tired quickly, keep the depth of the clay on the platform to about 100 mm.

After mixing, the clay can be used immediately for moulding into bricks or returned to the tempering tanks where it can remain until needed.

A good sized mixing platform is 3 metres by 3 metres. The mixing area can be built on top of or into the ground. A hard flat area can be made with fired bricks and cement plaster.

Figure

Depending on the situation, the tools and equipment that you will probably find most useful for tempering and mixing the clay are the following:

- buckets with 20 litre capacity
- empty fuel drums with 200 litre capacity
- hoes
- watering cans
- shovels
- wheelbarrows

To build a permanent tempering tank with a capacity of 3 cubic metres, you will need approximately:

- 600 bricks (240 mm × 115 mm × 73 mm)
- 70 kg of lime (for mortar)
- 300 kg of sand (for mortar)
- 75 kg of cement (for plastering)
- 200 kg of sand (for plastering)
- brick mason's tools

To build a mixing platform, 3 m × 3 m, you will need approximately:

- 700 bricks (240 mm × 115 mm × 73 mm)
- 150 kg of lime (for mortar)
- 600 kg of sand (for mortar)
- 100 kg of cement (for plastering)
- 300 kg of sand (for plastering)
- brick mason's tools