The most important group of painters' colors are the white pigments. White is the basis of nearly all opaque painting designed for the laying and covering of grounds, whether they be of woodwork, metal, stone, plaster or other substances. It should be as pure and neutral in color as possible, for the better mixing and compounding with other colors without changing their hues, while it renders them of lighter shades, and of the tints required; it also gives solid body to all colors. It is the most advancing color; that is, it comes forward and catches the eye before all other colors, and it assists in giving this quality to other colors, with which it may be mixed, by rendering their tints lighter and more vivid.

White is the nearest among colors in relation to yellow, and is in itself a pleasing and cheerful color, which takes every tint, hue and shade, and harmonizes with all other colors, and is the contrast of black, added to which it gives solidity in mixture, and a small quantity of black added to white preserves it from its tendency to turn yellow.

The most important of the white pigments is White lead, which may be obtained either pure or mixed with various substances, such as sulphate of baryta, sulphate of lead, whiting, chalk, zinc white, etc. These substances do not combine with oil as well as does white lead, nor do they so well protect any surface to which they are applied. Sulphate of baryta, the most common adulterant, is a dense, heavy, white substance, very like white lead in appearance. It absorbs very little oil, and may frequently be detected by the gritty feeling it produces when the paint is rubbed between the finger and thumb.

Oxide of zinc, or zinc white, is durable in water or oil; it dissolves in hydrochloric acid; it does not blacken in the presence of sulphuretted hydrogen, and it is not injurious to the men who make it, or to the painters who use it; but on the other hand, it does not combine with oil well, and is wanting in body and covering power, and is difficult to work. It is easily acted upon by the carbonic acid in rain water, which dissolves the oxide, and it therefore is unfit for outside work. The acids contained in unseasoned wood also have a great effect upon it. When pure and used for inside work, it retains its color well, and will stand washing for many years without losing any of its freshness. When dry it becomes very hard, and will take a fine polish. This paint is suitable for any place that is subjected to vapors containing sulphur, or in places where foul air is emanated from decaying animal matter.

Camel's Hair Lacquering Brushes.

Fig. 18. Camel's-Hair Lacquering Brushes.

The purity of white lead is ascertained by dissolving a sample of it in pure dilute nitric acid, 1 part of acid to two parts of water. On adding dilute sulphuric acid to the solution, after diluting it with water and filtering off the precipitate of lead sulphate thus obtained, no further pre-cipitate should be formed on successively adding ammonia, ammonium sulphide and ammonium oxalate to the filtrate.

The purity of zinc white in oil may he tested by burning out the oil by means of a blast lamp, on an iron spoon or ladle. Take of the zinc white a piece about the size of a pea, place it in the center of the spoon and direct the blast on it until it is burned white and perfectly dry. Crush the white cinder which is left to a fine powder and drop this into a glass of diluted sulphuric acid, 1 part of acid to 10 parts of water. If the powder be fine and very little dropped in at a time, it will, if pure, dissolve completely before reaching the bottom and without effervescence. If there be any effervescence it indicates the presence of whiting, which will precipitate as sulphate of lime, which is, however, sparingly soluble, barytes is insoluble, and a considerable adulteration of terra alba is not readily soluble; clay is insoluble.

Gypsurn mixes well with either water or oil, and, being neutral in its properties, it can be mixed with all other pigments without affecting them or being affected by them. It is used very largely by paper stainers and makers of wall paper, who prefer it to barytes on account of its having more body when used for that class of work. It is used in finishing of cotton goods, in paper making, and for a variety of other purposes where a cheap white pigment is required.

Whiting is sold under a variety of names, such as Spanish white, Paris white, English white. Whiting is the carbonate of calcium, purified by washing. It is prepared by grinding chalk under water to a very fine powder by passing it through several mills. The powder is run into tanks in which the coarser and heavier particles settle, while the liner chalk passes on to other tanks in which it settles. When the settling tanks are full, the chalk or whiting is dug out and dried. When partially dry it is cut into masses of a cubical shape and dried. When dry it is ground.

Paris white is a finer quality of whiting, hut the grinding is more thoroughly done. Spanish white is a name given to Paris white sold in a cylindrical form prepared by moulding the wet material into that form, and allowing it to dry in the open air.

Whiting is a dull white powder of an amorphous character, and soft to the feel. It is quite insoluble in pure water, but is soluble in water containing carbonic acid gas.

Kaolin or China clay is essentially a hydrated silicate of alumina. It is a natural product and only requires levigating and drying to prepare it for use as a pigment. It occurs in large deposits along with other constituents of undecomposed granite, the china clay usually forming from 15 to 20 per cent of the whole deposit.

Kaolin is a fine, white amorphous powder, having slight adhesive properties and adhering to the fingers when moist. The best qualities have a very soft unctuous feel and a pure white tint, while the common qualities are rather rougher and of a more or less yellowish hue.

As a pigment kaolin is quite permanent, resisting exposure to the atmosphere and to light for any length of time. It is, however, not much used as a pigment. In oil it loses its body and becomes more or less transparent. It can be used in water colors and in distemper work with good results, and is employed in paper-making and paper-staining.