42 Transactions of the 



gredieuts must be first well mixed in a glass mortar with a 

 pestle of the same material for an hour, and then exposed in a 

 crucible to the heat of a charcoal fire until the swelling, which 

 for a time accompanies the fusion of the mass, has ceased. By 

 means of this flux the colours are fixed on the porcelain, and 

 made to assume a resplendent appearance. The metallic oxides, 

 being enveloped by the flax, are preserved from all contact with 

 the air, and this colour is rendered permanent, the fusion having 

 been promoted at a temperature too low for the destruction. 



Enamel is glass rendered opaque by the oxide of tin and 

 by the oxide of lead. All glazes that contain lead participate in 

 the properties of enamel. Raw glazes used for covering tender 

 porcelain are of this nature. The colour's employed in painting 

 this porcelain are those which serve for painting in enamel ; 

 they require less flux than others, because the surface to which 

 they are applied becomes soft enough to be penetrated. Hard 

 porcelain, whose nature is identical with those of China and 

 Saxony, has two kinds of colours applied to it. The liquid 

 matter which serves as a vehicle in laying on the colours is 

 rubbed with them upon a glass palette until the whole is inti- 

 mately united. The mixture must be brought to that state of 

 dilution which is most proper and convenient for its application 

 with a hair-pencil on the surface of the porcelain. Purple and 

 violet colours are procured by dissolving gold in aqua regia 

 (nitro-muriatic acid), and immersing a bar of pure tin in the 

 solution. The product thus obtained is called, from its inventor, 

 the ' precipitate of Cassius,' and is used generally for giving the 

 above-mentioned colour to porcelain. A preferable way of pre- 

 paring this precipitate is to dissolve the two metals separately, 

 and by then mixing the solutions together in different propor- 

 tions, various shades of carmine, violet, and purple are obtained. 

 The first of these three tints is seldom used in porcelain works ; 

 for although extremely beautiful, it is also very transient, and 

 may be easily spoilt by a small excess of heat, or by contact 

 with carbonaceous vapours — a circumstance which is the less 

 important, as its place may be well supplied by a rose colour 

 obtained from iron, which is not liable to the same disadrantage. 

 Carmine colour, when used for tender porcelain, is prepared with 

 fulminating gold (made by dissolving the metal with aqua regia, 

 and precipitating with ammonia) and muriate of silver. This 

 compound being without any admixture of tin, shows that a 

 union of the oxide of tin with that of gold is not necessary for 

 the production of purple, A violet colour is also made with the 

 purple oxide of gold. 



To obtain a yellow colour for both hard and tender porcelain, 



