PIGMENTS. Positive prints may now be obtained in various pig ments, by mixing them with an organic substance, and bichromate of potass,—applying the mixture evenly to the entire surface of a sheet of paper, drying it, and exposing it under a negative,—then washing it in water or a suitable solvent, which removes the pigment from those parts of the paper which have not been acted on by light, and leaves it firmly cemented to the paper in the parts which have been so acted on. The process of printimg in pigments has not yet received much atten tion and the results are at present more or less imperfect as compared with those by the old processes ; but since prints by the methods in common use are extremely liable, if not certain, to fade, it is of the utmost importance that the methods of printing in carbon and permanent pigments should be so far improved as to yield results artistically equal to the others.
The following is a brief account of some of the common pig ments :— Black. Ivory black is made by calcining ivory dust in a close crucible. Lamp black is the soot produced by the combustion of oils, resins, and other vegetable substances.
Umber. A brown mineral found in the island of Cyprus ; it is composed of silica, alumina, and oxide of iron and manganese. When calcined for half an hour at a red heat the pigment called burnt umber is produced.
Asphaltum. A fine rich brown pigment. See " Asphaltum." Sienna. An argillaceous mineral found in Italy, and also near Wycomb. By calcination it becomes burnt sienna.
Small blue. A glass coloured with oxide of cobalt, and pul verized.
Cobalt. Hydrate of alumina mixed with hydrated oxide of cobalt, dried and calcined.
Sulphate of Indigo. Chemic blue, Saxony blue. Indigo dissolved in about six times its weight of sulphuric acid, then diluted with water, and neutralized with potass.
Prussian blue. A compound of cyanogen and iron. It is not considered a permanent pigment.
Stone blue. Finely powdered indigo mixed with starch paste, and made into lumps.
Copper blue. A mixture of carbonate of copper and chalk, ex posed to the air until it assumes the proper colour.
Ultramarine. A pigment composed chiefly of a costly mineral called Lapis lazuli, brought from China and Persia.
Artificial ultramarine. A pigment containing sulphide of sodium, obtained by fusing together in a crucible, porcelain clay, sulphur, and carbonate of soda. French photographic papers are tinted with
this villanous alkaline sulphide, which is enough of itself to cause the fading of any photograph.
Blue verditer. Nitrate of copper mixed with chalk.
Copper green. Native sub-carbonate of copper.
Brunswick green. Carbonate of copper mixed with calcareous matters.
Vienna green. A mixture of arsenious acid and verdigris.
Green verditer. An accidental variety of blue verditer.
Sap green. The juice of the berries of buckthorn, black alder, or ever-green privet, mixed with lime water and gum arabic, and eva porated until quite thick.
Iris green. The juice of the petals of the iris added to quick lime.
Carmine. An extract from the cochineal insect.
Lake. The colouring matter of raw shellac.
Brazil-wood lake. A mixture of a decoction of logwood, alum, and chloride of tin, to which carbonate of soda is added to form a precipitate.
Madder. A colouring matter obtained from the root of the Rubia tinclorum, which grows in the South of Europe.
Brown pink. To a decoction of French berries and fustic, boiled with potass in a tinned vessel, alum is added. The precipitate is " brown pink." Dutch pink. Turmeric is substituted for fustic, and whiting for alum, in the preceding formula.
Orange red. Sandix. White lead calcined.
Red lead. Minium. Litharge (oxide of lead), roasted in a re verberatory furnace.
Indian red. Peroxide of iron.
Red chalk. Clay iron-ore.
Venetian red. Oxide of iron.
Alum white. A calcined mixture of honey and alum.
White lead. Basic carbonate of lead.
Permanent white. Carbonate of baryta.
Zinc white. Oxide of zinc.
Chrome yellow. Chromate of lead.
Indian yellow. A concretion formed in the intestines of the camel. King's yellow. Sulphide of arsenic.
Naples yellow. A calcined mixture of lead, antimony, alum, and salt.
Patent yellow. Chloride of lead.
Queen's yellow. Turpith mineral, or sub-sulphate of mercury. Yellow lake. French berries boiled with potass, and precipitated with alum.
Ochres. Native oxides of iron mixed with argillaceous and cal careous earths.
Verdigris. Acetate of copper.
Indigo. A product obtained from the indigo plant.
Sepia. The black liquid contained in the cuttle fish. It consists of carbon, along with albumen, gelatine, and phosphate of lime. Vermilion. Cinnabar. Protosulphide of mercury.
Terra verte. Silicate and phosphate of protoxide of iron.