Surface Finish by Dyeing, Staining, Pigmenting, Matting and Polishing - Course: Manual woodworking techniques. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.) 4. Selection of Chemicals 4.1. Acids 4.2. Solutions 4.3. Salts 4.4. Bleaching Agents

Surface Finish by Dyeing, Staining, Pigmenting, Matting and Polishing - Course: Manual woodworking techniques. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.)

4. Selection of Chemicals

4.1. Acids

As for wood surface treatment, many acids are of importance, which are used either, due to their caustic effect, to remove old coating material or, due to the salt formation, to produce staining agents.

- Acetic acid (CH₃COOH):

Acetic acid (caustic) is used to produce staining agents and as bleaching agent. It ranks as best neutralizing agent for wood surface treatment.

- Hydrochloric acid (HCl):

Chemically pure hydrochloric acid (caustic and toxic) is a good neutralizing agent for wood surfaces. It is used to brighten up staining agents and to bleach wood.

4.2. Solutions

Solutions are soapy and caustic and have other properties than acids. Salts arise in connection with acids. Solutions dissolve organic matters, such as textiles and paintbrush bristles.

Saponification

When organic matters (resins, waxes, oils) are decomposed by inorganic solutions, one speaks of "saponification".

- Caustic-soda solution (NaOH):

Caustic-soda solution dissolves fats, oil and varnish paint coats and is used as deresinifying agent.

- Liquid ammonia (NH₄OH - ammonia water solution):

Ammonia water solution (caustic) is used as an additive to water-soluble staining agents (except iron salt staining agents). It can also be employed for tanning wood and removing fat and acid spots, as well as saponification means for the purpose of deresinification.

Neutralization

When acids and solutions are mixed in a certain ratio, a reaction occurs that is neither acid nor basic. Here, water is set free and a salt being formed.

4.3. Salts

Salts are formed when acids and solutions, as well as acids and metals are brought into contact.

Salts are needed to produce colour shades on the wood surface. They react with tanning agents inherent in wood or added by re-staining.

- Potash (K₂CO₃ - potassium carbonate):

It is used as an additive for pre-staining and re-staining processes, as a bleaching agent as well as for removing solutions and extracting resins.

- Glauber salt (Na₂SO₄ - sodium sulphate):

It is used to stain coniferous wood and as a filler for dyes and staining agents.

- Soda (Na₂CO₃ - sodium carbonate):

It is used for wood surfaces as an agent to remove stains and resins.

- with boric acid and is used as a bleaching agent. It is also used to clean one's hands from dyes and to produce water-proof staining agents as well as shellac compounds.

4.4. Bleaching Agents

Various acids and salts are able to remove spots and streaks from the wood surface. After having been bleached, the surfaces have to be thoroughly washed and neutralized.

- Iron-free hydrochloric acid:

to be diluted with water as an aqueous solution in the ratio of 1: 2 or 1: 4.

Hydrogen peroxide (H₂O₂) -

30 % hydrogen peroxide with 10 % liquid ammonia results in a strong bleaching agent. The bleaching process starts under heat development and foam formation.

- Saccharic acid ((CO₂H₂)₂ - oxalic acid):

It brightens up all spots that have formed through oxidation. The wood must not be treated with oil afterwards, otherwise there would be greyish shades and spots. Oxalic acid is very toxic.