The Technique of FURNITURE MAKING
WOOD STAINS
These rightly require a complete textbook devoted to their manipulation, and therefore only general observations are possible here. Of the three categories, water, spirit/alcohol and oil, the water stains are most troublesome to apply but penetrate fairly deeply and yield the clearest finishes; they are therefore firm favourites with the hand-worker. The old tried recipes are still viable, i. e. Vandyke brown crystals dissolved in hot water with a little 0.880 ammonia to bite into the wood and a teaspoonful of household detergent to break down surface tensions; mahogany crystals for red staining, water black aniline dye and
potassium bichromate. As all photographers will know the last is light sensitive, and an aqueous solution of the orange crystals obtainable at most pharmacies will yield any shade from fierce red-brown to light tan on certain woods after exposure to light. The concentrated solution should be stored in brown or covered bottles, diluted and tested on scrap wood, for the results are never predictable. Both oak and mahogany are affected (some African mahoganies do not respond), and the results are fairly permanent although inclined to fade somewhat in strong light, as will most other stains. Other shades of brown are easily obtainable with mixtures of red, yellow and black anilines with the addition of a little blue for coldness. Modern anilines are reasonably fast to light and some pronouncedly so.
Before applying water stains the work should be flashed off with clean water, dried, sanded and the dust cleared from the fibres with a stiff brush. Even staining without streakiness is facilitated if the surfaces are first lightly swabbed with water; moreover this dilutes the stain and gives more time for levelling off. Two coats are always better than one, for one strong coat may strike too dark in places; moreover, a first coating gives another opportunity to ease off upraised fibres. The stained surface can be coated or finished with all types of polishes.
Painted work
So-called painted work—bedroom and living - room furniture, kitchen furniture, etc.—are spray coated with pigmented nitro-cellulose, acid catalyst or polyurethane finishes available in gloss and semi-matt formulas. The build up is the same as for clear lacquer finishes, with appropriate pigmented grain fillers, sealers and base coats, etc. and the final surfaces flattened with wet and dry silicon carbide paper, cutting down pastes and burnishing creams. Brush coating can be attempted but is likely to fail on large surfaces, for it is the dead smoothness of this type of finish which constitutes much of its charm.
Compatibility in wood finishes
As so many types of wood finish are now used it is useful to know which finishes will take over each other and which will not. The list below gives the average behaviour:
Oil finish will accept french polish, polyurethane or wax.
French polish will accept cellulose, polyurethane, oil and wax.
Cellulose will accept polyurethane and wax. Polyurethane will accept oil and wax only. Polyester will accept oil, polyurethane and wax.
Wax will accept none.
Discretion must be exercised, for whereas the solvent used in cellulose finishes will soften if not actually dissolve french polish and permit the partial fusion of the two coats, the methylated spirit solvent in french polish will not dissolve cellulose and both coats will remain separate; moreover, any oil used with the french polish will be prevented from soaking down into the wood and will inhibit final hardening. It should be noted that wax is inimical to all other finishes, and must be completely removed by stripping, scraping, sanding and degreasing before attempting any other form of finishing.
Safety precautions
Modern spray booths will ensure a fume-free working atmosphere in properly organized factory work, but the hand-maker using a portable spray outfit without fan extraction should watch not only the fire risk but the health hazard. Many of the synthetic finishes and solvents used today are highly toxic, and their inhalation over a period can do active damage to the lung tissue and to the nervous and digestive systems. An efficient respirator or face mask should always be worn while spraying, and this also applies to brushwork if working in a confined space. To quote only one illustration, the popular polyurethane lacquers are isocyanate cured and are therefore capable of releasing hydrogen cyanide and carbon monoxide if they catch fire.
