VITRIFIABLE PIGMENTS 1089 



different brown colours also show that the dark compounds are simply suspended in. 

 the lead-glass, and not, or merely to a small extent, dissolved. The direction above 

 given for preparing the coloured combinations of the oxides in the dry way, for the 

 bodies which constitute the different brown pigments, is cheaper and more certain 

 than the precipitation of the mixed solutions by carbonate of soda and calcination of 

 the washed precipitate, which also answers. If, however, the several oxides were to 

 bo mixed with the lead -glass separately, instead of combined, the colours would not 

 be pure, that is to say they would exhibit after the firing different tints in a thick and 

 thin layer ; they would moreover possess a totally different colour before the burning 

 from that which they acquire after that operation, and would thus contribute to deceive 

 the artist. 



Gold purple is obtained, according to the process of Ladersdorff, by mixing a solu- 

 tion of 1 part ducat gold, in 4 parts aqua regia, with 1 drachm of tin-salt dissolved in 

 4 oz. distilled water, and a solution of 1 drachm of gum in 3 oz. of water, in the fol- 

 lowing proportions : 



Distilled water . . . . . . 3 oz. 



Solution of gum arabic . . .28 grs. 

 of tin-salt . . . . . 14 

 of gold 23 



and adding alcohol of 0'863 spec, grav., until the liquid begins to grows turbid. The 

 purple is deposited and washed with spirit of - 958. The dried precipitate has a 

 brownish colour, and furnishes, when all the gum has been carefully removed by 

 washing, a very beautiful purple after the firing. 



According to Fuchs, 1 oz. liq. ferri muriat. oaydati, Ph. bor., is mixed with 3 oz. 

 of distilled water, and a solution of 1 oz. protochloride of tin in 6 oz. distilled water, 

 and 10 drops of muriatic acid added until the whole has acquired a greenish colour, 

 when a further addition of 16 oz. of distilled water is made. 



On the other hand, some ducat gold is heated to boiling with pure nitric acid, until 

 all the gold is dissolved. An excess of acid should be avoided. 360 parts of distilled 

 water are added to this solution of gold ; and then the above solution of iron and tin 

 gradually poured into it until the whole of the purple is precipitated. This precipi- 

 tate has likewise a brownish tint after drying, but furnishes a beautiful purple after 

 burning. 



It has been found, however, that purple prepared according to the following 

 process is preferable, especially as regards the external appearance. A mixture of 4 

 parts pure nitric acid of 1'24 spec, grav., and 1 part pure muriatic acid, which is mixed 

 with half as much pure alcohol of 0'863, and chemically pure tin, gradually added in 

 small portions until no more is dissolved ; the solution must be effected slowly, on 

 which account the vessel containing the mixture should be placed in snow or cold 

 water. The carefully decanted solution is diluted with 80 times its weight of distilled 

 water, and mixed with a solution of gold, prepared according to the above directions. 

 The precipitate is purple-red, and remains so after drying. The tin-solution for this 

 purpose cannot be preserved long, otherwise nitric ether is formed ; and the higher 

 oxidation of the tin-salt no longer furnishes such beautiful precipitates with gold as 

 the recently-prepared solution. 



For mixing with the purple in order to produce a rose colour, the author does not 

 employ a carbonate of silver, but the metal in a very minute state of division, obtained 

 by mixing the finest silver-leaf with honey and a few drops of ether, and well grinding 

 it, when the honey is washed out with water. Mr. Waechter uses as a flux for the 

 purple colours a lead-glass, consisting of 6 parts minium, 2 parts silica, and 2 parts 

 calcined borax. 



With respect to the chrome colovirs, he observes, that the expensive method for their 

 preparation by means of the chromate of the protoxide of mercury is still the only one 

 by means of which a fine colour can be obtained. 



Cobalt Colours. In purifying the cobalt for porcelain colours, the removal of the 

 whole of the arsenic is of less consequence than that of the iron. Cobalt ores from 

 various localities, Tunaberg, Saxony, and Thuringia, are treated in the following 

 manner. The mineral is reduced to a fine powder in an iron mortar, kept for the 

 purpose, and mixed with fth its weight of charcoal-powder; then exposed in Hessian 

 crucibles to a red heat under a chimney with a good draught or in the open air, and 

 roasted as long as arsenical vapours escape, a very disagreeable operation, which lasts 

 several hours. The ore thus prepared is now boiled over the fire with a mixture of 

 4 parts nitre and 1 part muriatic acid, 1 part of which is diluted with 3 parts of water. 

 This operation is repeated about 3 times, with less acid. The liquids are allowed to 

 settle, the clear portion decanted, the remainder diluted with water and filtered, and 

 the solution evaporated to dryness. The dry mass is mixed with some water, heated, 



VOL. Ill, 4 A 



