Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/97

Rh The oxides of antimony, tin, bismuth, &c. , form white pigments ; but these possess no peculiarities which render them valuable for painters use. The carbonate of lime, more or less pure and in various degrees of pulverulence under several names, such as Chalk White, Paris White, Whiting, &c. , is very extensively used in distemper work for walls, roofs, &c. , and in paper-staining, occupy ing in these relations the important place held by white lead in oil painting. Mineral white or satin white consists of powdered gypsum and alumina, a preparation very largely used by paper- stainers for their glossy satin bodies. There are several other white earths of relatively little importance as pigments. BLUE PIGMENTS. The list of blue colours of real importance is not extensive, comprising, as principal items, ultramarine, Prussian blue, the cobalt blues, and indigo. The following list embraces the names and varieties ordinarily recognized in commerce : ultra marine (native), powdered lapis lazuli ; ultramarine (artificial), silicates of alumina and soda with sulphide of sodium ; Prussian blue, cyanide of iron ; Paris blue, modified Prussian blue ; Antwerp blue, fine Prussian blue ; smalts, a cobalt glass ; azure blue, a preparation of smalts ; cobalt or Thenard s blue, sub-phosphate of cobalt ; crcruleurn, stannate of cobalt and sulphate of lime ; mountain blue, native carbonate of copper ; lime blue, carbonate of copper and lime ; A T erditer or Bremen blue, hydrated oxide of copper; indigo from species of Indigofera; indigo carmine, prepara tion of indigo. Apart from the important colours ULTRAMARINE, PRUSSIAN BLUE, and INDIGO, separately noticed, these blues, which are not of much value for painters, owe their colour principally to cobalt and copper. The principal cobalt colour is Smalts, called also strewing smalts, cobalt glass, zaffre, Saxony blue, &c. It is prepared by smelting together the mineral arsenide of cobalt, pure sand, and carbonate of potash into a glass. The molten glaSs is cast into cold water, tlien ground fine and levigated. Smalts is chiefly available for distemper and fresco painting, and is not much used as an oil colour. Azure Blue is generally recognized as a preparation of smalts, but the name is given to several compounds. Cxruleum is a light blue colour of durable quality with a greenish tinge, consisting of a combination of cobalt oxide with stannic acid ; and Cobalt Blue, the subphosphate of cobalt, a colour discovered by Thenard, possesses a purple tinge. Carbonate of copper, either in the form of the mineral azurite or artificially prepared, is a principal source of the copper blues, which, however, possess little value as pigments owing to their tendency to blacken under exposure. Blue Vcrditer, a greenish blue which passes into green verditer, is a hydrated oxide of copper. YELLOW PIGMENTS. The following list includes the ordinary yellow colours of commerce : ochres and sienna earth, native earths tinted with iron ; Mars yellow, hydrated ferric oxide ; chromes, chromates of lead and other metals ; massicot, protoxide of lead ; Naples yellow, antimoniate of lead ; mineral yellow, basic chloride of lead ; aureoliu, nitrate of potassium and cobalt ; cadmium yellow, sulphide of cadmium ; orpiment, trisulphide of arsenic ; Indian yellow, ui io-pliosphate of calcium ; gamboge, resin of Garcinia ; Dutch pink, a vegetable lake ; yellow lakes. Of these colours the more important are the ochres and the various combinations containing chromium. The Yellow Ochres are native earths coloured with hydrated ferric oxide, the brownish yellow substance that colours, and is deposited from, highly ferruginous water. These ochres are of two kinds one having an argillaceous basis, while the other is a calcareous earth, the argillaceous variety being in general the richer and more pure in colour of the two. Both kinds are widely distributed, fine qualities being found in Oxford shire, the Isle of Wight, near Jena and Nuremberg in Germany, and in France in the departments of Yonne, Cher, and Nievre. The original colour of these ochres can be modified and varied into browns and reds of more or less intensity by calcination. The high heat expels the water of hydration from the iron oxide, changing it into red ferric oxide. The nature of the associated earth also influences the colour assumed by an ochre under calcina tion, aluminous ochres developing red and violet tints, while the calcareous varieties take brownish red and dark brown hues. The well-known ochre Terra da Sienna which in its raw state is a dull- coloured ochre, becomes when burnt a fine warm mahogany brown hue highly valued for artistic purposes. Yellojy ochres are also artificially prepared Mars Yellow being either pure hydrated ferric oxide or an intimate mixture of that substance with an argillaceous or calcareous earth, and such compounds by careful calcination can be transformed into Mars Orange, Violet, or Red, all highly important, stable, and reliable tints. The metal chromium owes its name to the intense coloration produced by the combination of its oxide, chromic acid, with various metals and alkaline earths. Several of these salts are soluble, but those which form pigments are insoluble compounds. The principal chrome pigments the various shades of lemon and yellow chrome deepen ing to orange tints are composed of the neutral chromate of lead, the difference of hue depending on the greater or smaller proportion of lead used in the preparation. The basic chromate of lead has a 87 deep orange colour passing into the minium-red-like hue of chrome red. Strontia Chrome, the chromate of strontium, is a pale lemon pigment of line quality and permanence. With zinc, chromic acid forms two combinations, neutral and basic, both possessed of an intense yellow colour ; and chromate of barium also furnishes a useful yellow colour. Lead itself, without chromium, is the basis of several valuable yellows. Massicot, the protoxide of lead, is a clear yellow pigment deficient in body. Naples Yellow, a colour highly esteemed by early artists, is an antimoniate of lead which in early times was obtained from native sources; and Mineral Yellow is an oxychloride of lead. The sulphide of cadmium forms the fine durable Cadmium Yellow, a colour now much appreciated for artistic use. The arsenical yellow, Orpiment, is now little used as a pigment, although formerly, under such names as King s Yellow, Imperial Yelloiu, and Chinese Yelloiv, it was held in high esteem by artists. Aureolin, a nitrate of potassium and cobalt, is a colour of recent origin which has come into high favour among artists. Indian Yellow is a colour of animal origin of no perma nence, and Gamboge is a gum resin yielded by trees of the genus Garcinia, principally employed as a water colour. The yellow lakes are comparatively unimportant, but some, known, rather absurdly, as Dutch, English, or Italian Pink, are largely used in paper-staining. RED PIGMENTS embrace two distinct series of substances the reds of inorganic origin, and red lakes obtained from animal and vegetable colours. The principal commercial varieties are as follows : rouge, Turkey red, and Indian red, red ferric oxide ; Venetian red, ochreous feme oxide ; ochres, earths coloured by ferric oxide ; vermilion and cinnabar, sulphide of mercury ; antimony vermilion, red sulphide of antimony ; Derby red, a form of chrome red ; red lead or minium, red oxide of lead ; chrome red, basic chromate of lead ; realgar, bisulphide of arsenic ; madder lake, alizarin and alumina ; madder carmine, preparation of aliza rin ; carmine lake, cochineal red and alumina ; carmine, prepara tion of cochineal ; wood lakes, from various red dyewoods. The principal mineral reds owe their colour to oxides of iron and to compounds of mercury. The reds due to iron are closely allied to the yellow ochres and other ferruginous pigments. As already explained in connexion with these yellows, tints passing through, orange to deep purple reds are obtained by calcination of yellow hydrated ferric oxide, and in this way a great variety of ruddy and red tints are prepared. The proportion of ferric oxide in these compounds ranges from pure oxide to combinations in natural ochres containing not more than 2 or 3 per cent, of iron. Rouge or Mars Red, Crocus, Indian Red, and Turkey Red are all pure ferric oxide, varying in depth of tint from having undergone different degrees of calcination, or from being made from different artificial or natural sources. The other iron reds are all of the nature of ochres some of them, such as Venetian Red, being artificial compounds. These reds form exceedingly useful durable colours which do not injuri ously aifect the tints with which they are associated. Of red colours from mercury, Cinnabar and Vermilion are the most import ant, the former being the native and the latter an artificial sul phide of mercury (see MERCURY, vol. xvi. p. 34). Vermilion is one of the most pure, brilliant, solid, and durable of all colours. Its beauty is largely affected by the smoothness of the powder to which it is reduced, and in this respect that obtained from China is of the highest excellence. Being a costly pigment, vermilion is freely adulterated with other reds, a fraud easily detected by the perfect volatility of the genuine substance. From mercury com bined with iodine is prepared a pigment of unequalled vivacity and brilliance, Iodine Scarlet, but unfortunately as fugitive as it is bright, and consequently not available for work requiring perma nence. The principal red colour from lead is Minium or Red Lead, a pigment of great antiquity obtained as a product of the oxidation of massicot, or by the calcination and oxidation of white lead. It is orange red in colour, of good opacity and body, but it has the fault of white lead and lead colours generally, blackening in con taminated air and injuring colours with which it comes in contact. By itself it is a valuable paint for first coating exposed iron surfaces to prevent their oxidation, and it is an excellent dryer, on which account it is much used in preparing boiled oil for painters. Chrome Red, a basic chromate of lead known also as Persian or Derby Red, is a brilliant pigment ranging in tone from orange to a deep vermilion hue. It is obtained by precipitating a solution of acetate of lead with bichromate of potash, with the addition of more or less of caustic potash or soda, the proportion of the latter addition determining the depth of resultant tone. Antimony Vermilion is the red variety of the sulphide of antimony which, as found in nature (stibnite), is a dark grey body with metallic lustre. This, when fused and kept some time at a high heat and suddenly cooled, by allotropic modification becomes a fine vermilion red. The colour is artificially prepared by acting on solutions of the butter of antimony (antimony chloride) with hyposulphite of soda or lime. It is a colour of excellent purity and body as a water colour, but unfortunately it becomes brown by exposure. The lakes form a numerous and important class of red pigments. A lake is a com bination of a colour of organic origin with a metallic oxide or salt,