This is the precipitate which takes place when solutions of gold and tin chloride are mixed under proper conditions. The preparation of the purple of a constant composition is effected by the following process: - Gold bichloride is prepared by dissolving 20 gr. gold in 100 of aqua regia, made with 4 hydrochloric acid and 1 of nitric acid. The solution is evaporated to dryness in a water-bath, in order to expel the excess of acid, and the remaining gold chloride is dissolved in 75 gr. water. Pure granulated tin is then introduced into the filtered liquor, which after some time becomes brown and turbid. Alter standing several days, all the gold is in the state of stannate of protoxide, which is separated from the remainder of the metallic tin. The product is collected upon a paper filter, carefully washed, and dried at a gentle heat.
Cobalt-pink is a mixture of the oxide of this metal with magnesia. It is durable, and more or less pink according to the proportion of cobalt. It is an expensive pigment, used only for fine painting. Its preparation consists in making a paste of carbonate of magnesia with a concentrated solution of cobalt nitrate. The paste is dried in a stove, and then calcined in a porcelain crucible.
(a) Digest 1 oz. coarsely-powdered cochineal in 2 1/2 oz. each water and rectified alcohol for a week: filter, and precipitate by adding a few drops of tin solution every 2 hours, till the whole of the colouring matter is thrown down; wash the precipitate in distilled water, and dry. (b) Digest powdered cochineal in ammonia water for a week; dilute with a little water, and add the liquid to a solution of alum as long as any precipitate (lake) falls, (c) Boil" 1 lb. coarsely-powdered cochineal in 2 gal. water for4 hour; decant, strain, add solution of 1 lb. cream of tartar, and precipitate with solution of alum. By adding the alum first and precipitating the lake with the tartar, the colour is slightly changed.
Colcothar is a red iron sesquioxide, which forms a very durable and bright colour, and is obtained by the calcination of green iron sulphate (copperas) upon iron plates until it has lost its combined water and become white. It is pulverized, placed in stoneware pots, and submitted to red heat. During the operation, sulphurous and sulphuric acids distil over, and the residue of the retort is a hard mass, which is coarsely powdered, washed, dried, finely ground, and sifted. The finer qualities are obtained by levigation (floating). The latter, after drying, are sometimes calcined anew, in order to increase their brightness. Colcothar is also produced in the wet way by mixing a solution of iron sulphate with another of sodic carbonate or bicarbonate. There forms a soluble soda sulphate and a precipitate of carbonate of protoxide of iron, which is soon transformed into hydrated iron sesquioxide. This is washed, dried, and calcined at a red heat in clay crucibles. When precipitation is effected in hot liquors, the colcothar is finer, more velvety, and deeper in colour.
It may be mixed with other iron colours, or calcined with lampblack, for producing various tones.
(a) Iron sulphate is calcined until the water of crystallization is expelled, then roasted by a fierce fire until acid vapours cease to arise, cooled, washed with water till the latter has no acid reaction, and dried. (6) Calcine 11 parts common salt with 25 parts green iron sulphate; well wash with water, dry, and powder, (c) The finest Indian red, or " crocus," usually undergoes a second calcination at a higher temperature.
- (a) Tie 2 oz. madder in a cloth, beat it well in 1 pint water in a stone mortar, and repeat the process with about 5 pints fresh water till it ceases to yield colour; boil the mixed liquor in an earthen vessel, pour into a large basin, and add 1 oz. alum dissolved in 1 pint boiling water; stir well, and gradually pour in 11/2 oz. strong solution of potash carbonate; let stand Until cold, pour off the yellow liquor from the top, drain, agitate the residue repeatedly in 1 qt. boiling water, decant, drain, and dry. (6) Add a little solution of lead acetate to a decoction of madder, to throw down the brown colouring matter; filter, add solution of tin or alum, precipitate with solution of soda or potash carbonate, and proceed as before, (c) Macerate 2 lb. ground madder in 1 gal. water for 10 minutes; strain and press quite dry; repeat a second and third time, and add to the mixed liquors \ lb. alum dissolved in 3 qt. water; heat in water-bath for 3 to 4 hours, adding water as it evaporates; filter first through flannel, and when cold enough through paper; add solution of soda carbonate as long as precipitate falls; wash the latter till the water comes off colourless, and dry.
An earthy red haematite, found in all countries and most geological formations.
This is prepared on the large scale by the oxidization of metallic lead in a reverberatory furnace with two fire-hearths covered by an arched roof, situated at the extreme end, separated from the middle hearth, in which the lead lies, by fire-bridges, and fed with coke. The lead, about 10 per cent, being hard, is worked about by an iron tool as soon as melted, the " massicot" or protoxide formed being constantly pushed to the side. The temperature must be kept at low redness, or the oxide will melt. The treatment is sustained for 24 hours; the massicot is then removed, ground, levigated, and again exposed in the furnace to the same heat for 48 hours, or till it exhibits a bright-red colour on cooling. The furnace is then closed and allowed to cool as slowly as it will. The product is "minium," or "red-lead." Mercier finds that it is not desirable to manufacture red-lead in trays in the same furnace that serves for the production of the original massicot, owing to the fluctuation of the temperatures; the best degree of heat is that approaching, but not quite reaching, a dull red, and a special furnace has been constructed for the maintenance of the temperature for a long time, on the manufacturing scale.