Paints, coloring substances prepared so that they may be spread with a brush, to color or preserve surfaces.. The term is usually applied to mixtures of pigments with oil, but may include those with water and gum or other thickening material. The pulverized solid pigments are called the base, and the liquid is called the vehicle, while a third material may be added to give color. Paints which are used by artists are often called "colors," but they only differ from common paints in that their materials are selected with more regard to permanence of color and fineness. For ordinary house painting, especially for exteriors, white lead is generally regarded as the best base, and boiled linseed oil as the best vehicle. The boiling prepares it, especially when a small portion of litharge (protoxide of lead) is added, to become hardened by oxidation. If white lead paint is exposed to exhalations of gases containing sulphuretted hydrogen, the surface will become converted into a black sulphide, which would cause discoloration in any except a black or dark brown paint. Zinc white, or oxide of zinc, as it does not become tarnished in this way, is often used as a substitute in white paint, especially for interior painting.
White lead, however, makes the strongest paint, forming when dry a homogeneous solid, because it combines chemically with the oil, while the zinc white is only held mechanically by it; and lead is therefore to be preferred for outside work. It should be selected with care, because it is often largely adulterated with ground sulphate of baryta or heavy spar, which destroys its opacity and also its affinity for the oil, so that it soon washes away. Sulphate of lime, or gypsum, and chalk are also sometimes used as an adulteration for white lead, and zinc blende is sometimes used as a substitute, and is said to cover well. The finest white leads are made at Krems in Lower Austria, and their preparation is attended with great labor. Next to these the Dutch and English white leads are regarded with the most favor; the former have the greater opacity, but are said to be rather more apt to become yellow by age than the English. The most durable paints intended for covering ordinary objects are made with white lead as a basis, the coloring being given by other materials, such as lampblack, umber, ochre, red lead, vermilion, verdigris, etc.; although cheap paints are sometimes made by mixing oils, particularly those that are cheap, such as fish oils and petroleum, with fuller's earth, ochre, or lime.
It has been asserted that the use of zinc in place of white lead relieves the workman from disease; but the effects of zinc under similar circumstances have not perhaps been sufficiently observed to form a correct opinion. - The following are some of the finer paints, called artists' colors. Flake white is made of carbonate of lead with an excess of oxide, mixed with boiled linseed oil, to which is often added a varnish. By the Krems, and also by the Dutch process of manufacture, white lead is rendered amorphous and very opaque, in consequence of the oxide of lead being in excess, a portion of the carbonic acid being replaced by water. Silver white is a term applied to a similar kind of white lead, and some whites are called Roman and Venetian. The whites of bismuth, zinc, tin, and barytes are of too feeble opacity. Strontian yellow is a color in which the base is chromate of strontia, of a pale canary color, resisting the action of foul gases and light, and being perfectly durable. Cadmium yellow, sulphuret of cadmia, is a rich and brilliant orange, and also resists the action of foul gases and light.
There are other yellows, such as the chromates of lead; Naples yellow, made of the oxides of lead, antimony, and zinc; yellow orpiment, sulphuret of antimony, forming the base of King's yellow; the zinc yellows, and the chromates of cadmium, mercury, and barytes. These are objectionable on account of want of durability, either from blacking by sulphuretted hydrogen, or for changing color from disintegration and other causes. The native earths, or ochres, such as yellow, Oxford, Roman, stone, brown, and Sienna, consisting of silica and alumina, colored by oxide of iron, are permanent colors, and furnish the painter with some of his soundest materials. Indian red, made of a native silicate and oxide of iron, is also a durable and valuable color. Venetian red is an inferior kind of Indian red. Vermilion, cinnabar, or sulphide of mercury (HgS, formerly called bisulphide), is a permanent paint of a beautiful red, and not affected by acids or alkalies. The madder lakes, which are vegetable dyes mixed with earthy bases, are rich and beautiful colors; but all vegetable compounds should be regarded with suspicion, although madders have the best reputation. Palladium red, ammonio-chloride of palladium, is a rich, deep, beautiful red, and a valuable pigment, but not much used.
Ultramarine, made from native lapis lazuli, or artificially by heating together kaolin, carbonate of soda, and sulphur, is a brilliant purplish blue, not destroyed by foul gases or light, and, although affected by acids, is durable for the painter. There are many processes for artificial ultramarine, and a method has been found by Prtiekner (see "Colors in Painting," by Riffault, Vergnaud, and Toussaint, Philadelphia 1874) to determine their durability by treatment with hydrogen. Silicate of cobalt and potassa form cobalt blue, but it is not so strong a pigment as ultramarine. Prussian blue is liable to change, and indigo fades in the light. Chromium green, sesquioxide of chromium, made by strongly igniting chromate of mercury, is a strong, opaque, permanent light green; it is the coloring matter of the emerald. Scheele's green, arsenite of copper, verdigris, a mixture of sub-acetates of copper, and other mineral greens, are not considered as durable as chromium green; many of them form black sulphides by continued action of foul air. Terre verte, a native silicate of protoxide of iron, with water, potash, and magnesia, is a delicate green, resisting all ordinaiy destructive agencies, but destroyed, like ultramarine, by acids.
Vandyke, Rubens, Cassel, and Cologne browns are made of decomposed vegetable and bituminous matter, and are deep and rich, with a certain degree of translucency, for which they are valued. Ivory black, or burnt ivory, is a pigment much used by painters; also lampblack, and the blue blacks, made of burnt vine twigs, and that of the ancients made of the lees of wine. Manganese black, peroxide of manganese, and black ochres or earths are also used.