VITRIFIABLE PIGMENTS 1087 



Indium Grey. 1 part of the sesquioxido of iridium, 4 parts of oxide of zinc, and 

 22 parts of lead-glass (prepared by fusing together 5 parts of minium, 2 parts of sand, 

 and 1 part of calcined borax) are intimately mixed and ground fine upon a plate of 

 glass. On microscopical examination of the iridium pigments after they have been 

 burnt-in upon porcelain, the sesquioxide of iridium is seen to be suspended in the 

 transparent fused lead-glass. It is owing to the unalterability of the sesquioxide 

 \f iridium that it admits of being mixed with all other verifiable colours without 

 injuriously affecting the tints, as is the case with all the other vitrifiablo grey and 

 black pigments. 



Black from Cobalt and Manganese. 2 parts of sulphate of cobalt deprived of its 

 water of crystallisation, 2 parts of dry protosulphate of manganese, and 5 parts of nitre, 

 are intimately mixed, and heated to redness in a Hessian crucible until the whole of 

 the nitre is decomposed. The calcined mass, exhausted with boiling water, furnishes 

 a deep black powder, which consists of a combination of oxide of cobalt and oxide of 

 manganese. 1 part of this compound is mixed with 1\ parts of lead-glass (prepared 

 by fusing together 5 parts of minium, 2 parts of sand, and 1 part calcined borax), and 

 ground fine upon a plate of glass. 



Grey from Cobalt and Manganese. 2 parts of the above compound of the oxide of 

 cobalt and manganese, 1 part oxide of zinc, and 9 parts of lead-glass (prepared by 

 fusing together 5 parts of minium, 2 parts of sand, and 1 part of calcined borax) are 

 mixed and ground fine. 



These black and grey pigments are far less expensive to prepare than those from 

 iridium, and are not inferior to them in colour; but they do not mix so well with 

 other colours, and when baked several times they vary their tint somewhat, which 

 renders their application less certain. When these colours burnt-in upon porcelain 

 are examined iinder the microscope, it is seen that the oxide of cobalt and manganese 

 is not dissolved by the load-glass, but merely suspended in it. 



Besides these colours, a very infusible black is used in painting, which is not acted 

 upon by the superposed colours in the fusion ; it is the 



Ground Black, which consists of 5 parts of blue violet (gold-purple), Ifrds part of 

 oxide of manganese and cobalt, and l|rds part of oxide of zinc ; these are intimately 

 mixed and ground fine upon a plate of glass. 



White for Covering. 1 part minium, 1 part white sand, and 1 part crystallised 

 boracic acid, are well mixed, and fused in a porcelain crucible. This white enamel has 

 the peculiarity of forming a colourless clear glass when quickly cooled, for instance, when 

 poured into water ; while, when slowly cooled, it remains perfectly white and opaque. 

 On heating the clear glass to its melting point, it loses its transparency, and becomes 

 opaque as before. This property it possesses in common with the enamels, the opacity 

 of which is produced by arsenic or tungstic acid ; probably the opacity in the present 

 case is produced by the separation of silicate of lead, as in the white enamels by 

 arseniate or tungstate of potash, or by oxide of zinc. It is, however, of excessive 

 minuteness ; for under the microscope, even with the highest power, the glass merely 

 exhibits a yellowish turbidness, and no individual particles are visible. 



This white serves for marking the lightest part of the pictures, where it is impos- 

 sible to produce them by exposing the bare surface of the white porcelain ; it is also 

 frequently mixed in small quantity with the yellow and green pigments, to make them 

 cover well. 



Lead Flux. A colourless lead-glass for touching-up those parts cf the painting 

 which have remained dull, and for mixing with those pigments which are not easy 

 of fusion, is obtained by mixing together 5 parts of minium, 2 parts of white sand, 

 and 1 part of calcined borax. 



Eed and Brown Vitrifiable Pigments derived from Peroxide of Iron for painting upon 

 Porcelain. Yellow-Bed. Anhydrous sulphate of the peroxide of iron is heated to 

 redness on a dish in an open muffle, and constantly stirred with an iron spatula until 

 the greater portion of the sulphuric acid has been expelled, and a sample mixed with 

 water upon a glass plate exhibits a beautiful yellowish-red colour ; after cooling, the 

 peroxide of iron is freed by washing with water from any undecomposed sulphate, 

 and dried. To prepare the pigment, 7 parts of the yellowish-red peroxide of iron 

 are well mixed with 24 parts of lead-glass (prepared by fusing together 12 parts of 

 minium, 3 parts of sand, and 1 part of calcined borax), and ground fine upon a plate 

 of glass. 



Brown Eed. When the persulphate of iron is heated to redness until the whole of 

 the sulphuric acid is expelled, and a sample exhibits a dark red colour, the peroxide 

 of iron is well suited for a brownish-red pigment, which is prepared in tho same 

 manner as directed for the yellowish-red. 



Bluish Eed (Pompadour). When the persulphate is heated still more strongly, it is 

 deprived of its loose consistency, becomes heavier, and acquires a bluish-red colour. 



