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Coloring Finishing And Painting Wood
The white pigments which are largely inert, and show little or no chemical reaction with drying-oils which are acid in nature, are calcium carbonate or whiting, which is apt to saponify unless there is white lead in a mixture with the drying-oil, gypsum, barytes, China clay, silica, talc or French chalk, and asbestine. The inert pigments are often called "extenders," because their hiding power or opacity when mixed with oil is very small. The "extenders" are not considered as adulterants, pure and simple, by some manufacturers of prepared paints, who claim that small percentages give protective qualities and improve durability of the more active opaque pigments.
The colored pigments are sometimes subdivided into four classes: (1) colors containing lead, (2) colors from iron, (3) carbon colors, and (4) organic colors. Some of the well-known pigments in the lead group are red lead, chrome yellow or lead chromate, and its various modifications called chrome orange and chrome green. The iron colors have been known for ages; the oxides being contained in such earth colors as Turkey red, Venetian red, Indian red, red ocher, yellow ochers, which contain ferric oxide (Fe2O3,) in varying amounts; the black oxide of iron (Fe3O4); the umbers, which contain usually more than 36 per cent of iron oxide (Fe2O3); and the siennas which contain from about 55 to 79 per cent of ferric oxide (Fe2O3). Prussian blue, sometimes called Chinese blue, also contains an iron salt of a very different kind. The slow-drying carbon-black pigments are known as graphite or plumbago, lampblack, carbon black, charcoal black, and ivory or bone black. The organic colors are numerous and diverse in origin. They may be derived from animal, vegetable, or chemical sources.4
266. Vehicles.—Vehicles for paints are of three distinct classes: drying-oils, varnishes, and water. Vehicles really have two functions in paints: (1) if they be drying-oils or varnishes, they act as binders which on drying cement the particles of the pigments together and to the surface, and often penetrate into porous or absorbent materials, and on hardening form anchorages for the film of paint;- and (2) they form a fluid that helps to carry the powdered pigments to the places where they are wanted by adding brushing qualities not possible with dry substances. Where water is the vehicle a certain amount of cementing or binding quality should be given to the paint thru the addition of glue to the mixture.
The drying-oils used as paint vehicles are linseed-oil, which was practically the only oil commonly employed for the purpose until very recently; China-wood oil, also called tung-oil; perilla-oil, another fine drying-oil from the orient; walnut-oil, a pale oil used in artists' colors; soy-bean or soya-bean oil, a poor drying-oil which can be treated with driers; menhaden-oil, also called fish-oil, which is used on smokestacks, roofs, and on hot surfaces; and poppy-seed oil, which is a golden oil of slow-drying properties used in artists' paints.5
