Pigments

Pigments are insoluble coloured substances which, when ground up with a suitable medium, usually oil or water, can be employed for painting. In their nature and composition they vary greatly; many are natural mineral substances, others are artificially-pre pared inorganic compounds, whilst others again are organic colouring material prepared usually by synthetic reactions. The natural substances, and frequently also the artificial products, require to be very finely ground before use. For artists' colours this grinding has to be much more carefully done than in the case of the pigments for house-painters, etc. The grinding is usually done by means of revolving stones, the pigments being ground either dry or mixed with oil or water. Artists' colours, after careful grinding, are usually stocked and sold in collapsible tubes, or, in case of water-colours, as hard cakes or softer moist colours.

White pigments may consist of ii number of different compounds. Amongst these some of the most common are time, used as a coarse whitewash for walls, ceilings, etc.; white lead (q.v.), one of the most common of all pigments, although it blackens if exposed to sulphur fumes and has certain dangers in its preparation; zinc oxide or Chinese white (q.v.); and permanent white (q.v.), which consists of sulphate of barium either natural or, better, prepared artificially.

Blach pigments are usually more or less impure forms of carbon obtained by heating various vegetable or animal substances, and indicating by their names the source whence obtained, as e.g. ivory-black, bone-black, lamp-black, etc. Oxides of manganese are also used, and aniline black; while tar is also frequently employed for painting wood, etc., possessing a powerful anti-corrosive action. Yellow pigments consist chiefly of oxides of iron and some salts of chromium and lead. The first class constitute the pigments known as ochres (q.v.), varjing in shade from yellow to brown, according to the quantity and nature of the oxides of iron contained. Chromates of strontium, and lead form valuable yellow pigments, while a compound of lead and antimony oxides forms the well-known Naples yellow. Gamboge is a natural organic pigment, as is also Indian yellow. Blue pigments comprise a number of important colours, as indigo, Prussian blue, and ultramarine (all of which see). Besides these, certain salts of cobalt find application as pigments under names of smalt, cobalt blue, azure blue, etc. Green compounds include chiefly compounds of copper and arsenic. The latter are highly poisonous; and, although bright and brilliant, their use on wall-papers, etc., is highly to be condemned. Of late years their use has largely decreased. The commonest are Scheele's green and Schweinfurth's green. Brunswick and Bremen green'are copper compounds, as is also verdigris. Cobalt green possesses good colour, and consists of compounds of cobalt and zinc. Sap-green is a natural organic green obtained from blackthorn berries, but possesses little body. Greens may also be obtained by proper admixture of yellow and blue pigments. Red pigments comprise a number of important compounds. Amongst these are the lalies (q.v.) from madder, cochineal (forming crimson lalte), and lac lake. Carmine is obtained from the cochineal insect. Cinnabars and vermilion are brilliant pigments, consisting of the sulphide of mercury, but are frequently adulterated with red lead or minium, itself a common red pigment. Venetian red, rouge, Turkey red, and some others, are all oxides of iron, the first being mixed with some earthy matter, the two latter being pure oxides. Brown pigments consist chiefly of ochres. Some browns are obtained from bituminous soils, while the commonly-used sepia consists of a secretion of the cuttle-fish. A large number of substances not named also enter into various pigments, while many also are especially prepared for certain purposes, and hence contain special ingredients, as e.g. to prevent corrosion, decay, etc. For more detailed descriptions, also see the articles treating on the various pigments, Vermilion, Red Lead, etc.