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Coloring Finishing And Painting Wood
15. Classification of Wood-Stains.—So many kinds of stains are used for wood that any intelligent and useful classification of them is difficult. The problems which are apt to be met in applying a stain depend to a very large extent upon the solvent which is used to dissolve the coloring-matter. For this reason stains are generally classified according to the solvents used in making them.
The four classes of stains for wood are:
1. Water-stains.
2. Oil-stains.
3. Spirit-stains.
4. Chemical stains.
Two other classes of so-called stains are found on the market, known as varnish-stains and wax-stains. These are not transparent stains, as they obscure the grain and leave a layer of pigment on the surface. Wax-stains and varnishstains are really in a class midway between true stains and enamels or paints. They are not often used in furnituremaking, but are occasionally suitable for a home-repair job where the transparency of the grain of the wood is not considered. Colored varnishes are not popular with expert wood-finishers because they always leave muddy, unattractive effects.
The four classes of stain—water, oil, spirit, and chemical —may be subdivided generally into two classes, acid and alkaline, depending upon their chemical reaction with other substances.
Water-soluble acid-stains are very often made of coaltar dyes, which dissolve in water, and can be used in an acid bath; or they may be produced from some of the weak acids, such as tannic or pyrogallic, which give little color unless used in connection with an alkali. Chromic acid which comes in red crystals was formerly used; but on account of its corrosive action, its expense, and uncertain effects, it has largely been replaced by its well-known salt, bichromate of potash. Picric acid which produces a good yellow on wood is also sometimes used with a water-soluble nigrosine to produce an olive-black tone known as Early English. The strong acids are little used by the wood-finisher.
Alkaline stains which were quite commonly used a few years ago were frequently preceded or followed by an acid treatment. A volatile alkali, such as ammonia, or some fixed alkali, like caustic soda, caustic potash, or bichromate of potash, when used alone or in mixtures where practicable, was generally the alkaline element employed in producing the so-called alkaline stains.
16. Water-Stains.—Many up-to-date wood-finishers consider coloring-matter dissolved in water as in many respects the most satisfactory stain to use on wood. Water passes easily from cell to cell2 in the growing tree. Wood that has become thoroly dry will quickly reabsorb moisture ;3 and, if used in a wet place, it will take up water until its cell walls are at the saturation point; then, with still more moisture, it will become water-soaked as the cell lumina or cavities are gradually filled. This great readiness with which wood absorbs water is very important in wood-finishing, and is one reason for the great popularity of waterstains.
