PAINTS, CHEMISTRY OF. The character and value of paints may be considered in relation to their use, either for artis tic and decorative purposes, or as preservative coatings. In the former case, for a given medium, the choice of pigment rests primarily upon the colour effects desired; in the latter case the medium plays a more important part, whilst the choice of pigment depends on its chemical and physical properties and on a consider ation of cost. Through scientific progress an almost unlimited variety of colours is now available.
The opacity or transparency of a pigment in a medium is de termined by the amount of refraction and reflection which light undergoes in a paint film. This depends upon the difference between the refractive indices (to air) of the pigment and the medium respectively, the pigment approaching transparency as such difference diminishes. As the refractive index of oil is higher than that of water, the opacity of a pigment will be greater in the latter vehicle than in the former. In the case of oil colours the refractive index of oil increases as the drying proceeds, and thus produces a lowering of tone owing to the increased transparency of the pigment ; in water colours the refractive index of pigment to water being less than that of pigment to air causes an increase in opacity of the colour on drying. Opacity is generally intensified by increased fineness of the pigmentary particles. Reduction in the particle size may also modify the colour of a pigment as is shown in the manufacture of lead chromes and antimony sul phides. For this reason colours such as vermilion are materially altered during the process of grinding in oil, unless special care is exercised in the operation. Pigments ground in oil exhibit a great variation in behaviour. This is manifest in the amount of oil required to grind them into a paste (known as their "oil absorp tion"), and in their influence on the drying of the oil and on the subsequent character of the paint film, rendering it soft and pli able or hard and brittle. Differences in oil absorption are well illustrated in the cases of vegetable black and white lead, the former requiring about twelve times as much oil as the latter.
Pigments classified according to their colouring principles fall into two main groups : mineral or inorganic colours, and (2) colours produced from the natural or synthetic dyes (q.v.). As
the manufacture of mineral and inorganic colours is directly associated with their chemical constitution, which varies over a wide range, their preparation cannot be generalized. Broadly a division might be made between those pigments, e.g., the chromes, made by the "wet process" and involving the precipitation of the colour from solution, and those pigments, e.g., the cobalt colours, where the desired result is obtained in a furnace at a high tempera ture. Some colours, such as lithopone, require treatment by both processes. By whatever method the colour is obtained, adequate washing is essential. This is usually carried out in the "striking" or precipitating vats by decantation, the process being repeated until the wash water is free from salts. The washed colour is pumped through a filter press tc remove excess of water and dried in a drying-room at a low temperature or in a vacuum stove.
The manufacture of organic colours is equally complex and diverse. The term lake is applied to all pigments prepared by the precipitation of dyestuffs upon a "base" or "carrier." This base, which contributes largely to the physical properties of the lake and is selected according to particular requirements, either adsorbs the dye, enters into direct chemical combination with it or forms the substratum on which the dye is fixed by a precipitant. The choice of dye is naturally governed by the demands of the lake user; such considerations as price, permanency and insolubility in the medium may serve as determining factors.
As bases, are used the hydrate and certain salts of alumina (for transparent lakes), barium sulphate, china clay, zinc oxide, lead sulphate, red lead, litharge, green earth and others. Besides entering into chemical relationship with the dyestuff, these bases may further serve as extenders (or diluents) or combine both properties. Extenders are employed in both classes of pigments and cannot be considered only as adulterants; thus, when used for producing paler shades, for increasing opacity and obscuring power ("body") of colours, or for modifying the properties of a pigment to suit a particular need, they serve a special purpose.