These three heads form but one subject, and are appropriately discussed together, in their natural order, which is - (1) Pigments, the preparation of the dry colouring matters; (2) Paint, the compounding of these colouring matters with driers and vehicles ready for application; (3) Painting, the application of the prepared paint to various surfaces.
The term " pigments " is applied to colouring matters which are mixed in a powdery form with oil or other vehicle for the purposes of painting. They differ in this respect from dyestuffs. A very large proportion of the pigments are derived from the mineral kingdom. Organic colouring matters for use as pigments are mostly made in the form of " lakes,"by one of the three following methods; - (a) To a filtered solution of the colouring matter is added a solution of alum; the whole is agitated, and the colour is precipitated by a solution of carbonate of potash. (6) A solution of the colouring matter is made in a weak alkaline lye, and precipitated by adding a solution of alum, (c) Recently-precipitated alumina is agitated with a solution of the colouring matter as before, until the liquid is nearly decolorized, or the alumina assumes a sufficiently deep tint. The first method is generally adopted for acidulous solutions of colouring matter, or those injured by alkalies; the second, for those not injured by alkalies; the third, for those whose affinity for gelatinous alumina enables them to combine with it by mere agitation.
Alumina in a state suitable for the preparation of "lakes " may be produced in the following manner: - Dissolve 1 lb. alum in 1/2 gal. water, and add 75 gr. copper sulphate and about 1/4 lb. zinc turnings; leave the mixture for 3 days in a warm place, renewing the water lost by evaporation. The copper is first deposited upon the zinc, the two metals thus forming a voltaic couple. Hydrogen is disengaged, zinc sulphate is formed, and the alumina gradually separates in the state of very fine powder; the action is allowed to continue till no more alumina is left in solution, or until ammonia ceases to give a precipitate. If the reaction is prolonged beyond this point, iron oxide will precipitate if present. The alumina washes easily, and does not contract upon drying. (Dern. Prog, de rind. Chim.)
The most important of these are animal-black, bone-black, Frankfort-black, ivory-black, lampblack, and soot-black. They are mostly obtained by carbonizing organic matter in closed vessels or crucibles, or by collecting the soot formed by the combustion of oily, resinous, and bituminous substances.
This is almost identical with bone-black, but is generally in a more finely-divided state. Any refuse animal matter may be used in its preparation, such as albumen, gelatine, horn shavings, etc. These are subjected to dry distillation in an earthenware retort. An inflammable gas is given off, together with much oily matter, ammonia, and water, while a black carbonaceous mass is left behind. This is washed with water and powdered in a mill, the product being animal-black. It is largely used in the manufacture of printing-ink and blacking.
Frankfort-black is a powder obtained from dried vine-twigs, carbonized to a full black, and then ground very fine. On a large scale, it is prepared from a mixture of vine-twigs, wine-lees, peach-stones, bone-shavings, and ivory refuse. It varies in shade according as animal or vegetable charcoal is in excess; when the latter predominates, the powder is of a bluish colour; when the former, it has a brownish tinge. it is customary to wash the powder well when first made, in order to remove any soluble inorganic impurities. It makes an excellent pigment, and is extensively used by copperplate engravers in the preparation of their ink.
Ivory-black is a beautiful pigment prepared by carbonizing waste fragments and turnings of ivory. These are exposed to a red heat for some hours in crucibles, great care being taken to avoid overheating or burning. When quite cold, the crucibles are opened, and the contents pulverized, the richest coloured fragments being kept apart for the best quality. The powder is then levigated on a porphyry slab, washed well with hot water on a filter, and dried in an oven. The product is of a very beautiful velvety black colour, superior even to that obtained from peach kernels, and quite free from the reddish tinge which so often characterizes bone-black. Ivory-black is employed by copperplate printers in the preparation of their ink. Mixed with white-lead, it affords a rich pearl-grey pigment.
(1) Camphor smoke makes an excellent black, but has the disadvantage of coming off with the least touch or drop of rain. (2) A good and tolerably permanent black is made of 1 part stick lac, 1 of lampblack, and 6 of methylated spirit.
Kraus prepares a fine blue, rivalling ultramarine, and capable of giving beautiful green shades (equal to Schweinfiirth green, and without its arsenical character) when mixed with chrome yellow or with zinc chrornate, by adding a solution of yellow (ferro-) cyanide of potassium to one of antimony in aqua regia, 2 and filtering through ground glass, as long as a precipitate forms. This precipitate contains no antimony, the antimony salt simply facilitating the formation of the pigment; mercury salts will also give it. The blue is soluble in hydrochloric acid, which successively renders it green and yellow; on standing, the blue colour is restored. Alkalies immediately decompose it. In fact it is merely a variety of Prussian blue. (Dingier'sPl. J I.)