As a pigment it possesses the same features and faults as its close ally orpiment.
Siennas.—Another name for ochres, described on p. 272.
CHAPTER IX.
LAKES.
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.
(b) 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 decolorised, 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 the pigments known as “lakes” may be produced in the following manner:—Dissolve 1 lb. of alum in ½ gallon of water, and add 75 grains of sulphate of copper, and about ¼ lb. of zinc turnings; leave the mixture for three 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 sufficiently strong. Hydrogen is disengaged, sulphate of zinc is formed, and the alumina gradually separates in the state of a very fine powder; the action is allowed to continue until there is no more alumina left in solution, or until ammonia ceases to give a precipitate. If the reaction is prolonged beyond this point, oxide of iron will precipitate if present. The alumina washes easily, and does not contract upon drying.
Brazil-wood Lake.—(a) Digest 1 lb. ground Brazil-wood in 4 gal. water for 24 hours, boil ½ hour, and add 1½ lb. alum dissolved in a little water; mix, decant, strain, add ½ lb. tin solution, again mix well, and filter; to the clear liquid cautiously add a solution of carbonate of soda while a precipitate forms, avoiding excess; collect, wash, and dry. The shade will vary according as the precipitate is collected.
(b) Add washed and recently-precipitated alumina to a strong filtered decoction of Brazil-wood.