VITRIFIABLE PIGMENTS 1085 



but no longer ; otherwise, the golden-yellow colour is converted into a dirty grey, as 

 in the case of the lemon-yellow pigment. 



Dark Yellow, 2. 20 parts minium, 2i white sand, 4J antimoniate of potash, 1 part 

 peroxide of iron (caput mortuuni), and 1 part oxide of zinc, are well mixed and fused 

 in a Hessian crucible. Long fusion is less injurious in this case than in the preceding. 

 Iron-red pigment may be laid on and near this dark yellow 2, Avithout its being 

 destroyed, or the harmony of the tints injuriously affected. 



For landscape and figure painting, the above-mentioned yellow pigments should be 

 made less readily fusible, in order to paint with them upon or beneath other colours, 

 without any fear of what has been painted being dissolved by the subjacent or super- 

 posed pigment. This property is given to it by the addition of Naples yellow, which 

 is best prepared for this purpose by long-continued ignition of a mixture of 1 part 

 tartar-emetic, 2 parts of nitrate of lead, 4 parts of dry chloride of sodium, in a Hessian 

 crucible, and washing the pounded residue with water. Very useful yellow colours 

 are likewise obtained by mixing this Naples yellow with lead-glass ; they are, how- 

 ever, more expensive than those above given. A very excellent yellow for landscape- 

 painting may be prepared, for instance, by mixing 8 parts Naples yellow and 6 parts 

 lead-glass (obtained by fusing 2 parts of minium with 1 of white sand and 1 of cal- 

 cined borax). 



The yellow pigments obtained with antimony, after being burnt-in upon the 

 porcelain, appear under the microscope to be mixtures of a yellow transparent sub- 

 stance (antimoniate of lead ?), and a colourless glass, and not homogeneous yellow 

 glasses. 



Uranium Yellow. 1 part oxide of uranium, 4 parts lead-glass (prepared by fusing 

 8 parts minium with 1 part white sand), are intimately mixed and ground upon a 

 glass plate. This colour is not adapted for mixing with others, with which it produces 

 discordant tints. It may be shaded with dark purple or violet. 



Uranium Orange. 2 parts oxide of uranium, 1 par' chloride of silver, and 3 parts 

 bismuth glass, (prepared by fusing 4 parts of oxide of bismuth with 1 part of crystal- 

 lised boracic acid), are intimately mixed and ground upon a plate glass. This orange 

 is not adapted, any more than the yellow pigment, for being mixed with other colours. 

 When examined under the microscope, after being burnt-in upon porcelain, the ura- 

 nium pigments appear as pale yellow-coloured glasses, in which unaltered oxide of 

 uranium is suspended. Only a small portion, therefore, of the oxide of uranium has 

 dissolved in the fusing. 



Green Pigments for painting upon Porcelain. Slue Green. 10 parts of the chromate 

 of protoxide of mercury and 1 part of chemically pure oxide of cobalt are ground upon 

 a glass plate, in order to produce as intimate & mixture as possible ; the mixture is 

 then heated in a porcelain tube, open at both ends, until the whole of the mercury is 

 expelled. The beautiful blueish -green powder thus obtained is then transferred into 

 a porcelain crucible, and the lid cemented to it with glaze. The full crucible is 

 exposed to the highest temperature of the porcelain furnace during one firing, the 

 crucible carefully broken after the cooling, and the pigment washed with water, to 

 remove fc. small quantity of chromate of potash. In this manner a compound of oxide 

 of chromium and oxide of cobalt is obtained in nearly equivalent proportions, which 

 possesses the bluish-green colour of verdigris. 



The blue-green pigment consists of a mixture of 1 part of the above compound of 

 oxide of chromium and oxide of cobalt, part of oxide of zinc, and 5 parts of lead- 

 glass (prepared by fusing together 2 parts minium, 1 part white sand, and 1 part 

 calcined borax), which are mixed and ground upon the glass plate. By mixing this 

 blue-green with lemon-yellow, any desired intermediate tint may be produced. 1 part 

 of blue-green to 6 parts of lemon-yellow furnishes a beautiful grass-green. 



Dark Green. The chromate of mercury is treated separately in the same way as 

 the mixture of it with oxide of cobalt for the blue-green ; and 1 part of the beautiful 

 green oxide of chromium thus obtained is mixed with 3 parts of the same lead-glass 

 as given under blue-green, and ground xipon the glass plate. 



Green for Shading. 8 parts chromate of mercury and 1 part oxide of cobalt are 

 intimately mixed, and exposed in a shallow dish to the strongest heat of the porcelain 

 furnace, during one of the bakings. In this manner a compound of oxide of chromium 

 and oxide of cobalt is obtained, of a greenish-black colour, which, mixed with twice 

 the weight of the lead-glass directed for the blue-green, furnishes a very infusible 

 blackish-green colour, for shading other green colours. 



When thin splinters of the green pigments of chromium, burnt-in upon porcelain, 

 are examined under the microscope, it is distinctly seen that particles of the oxide of 

 chromium, or of the oxide of chromium and cobalt, are suspended, undissolved, in the 

 colourless lead-glass. 



Blue Pigments for -painting upon Porcelain. Dark Blue, 1 part chemically pure 



