CU/ton College Scientific Society. 43 



white oxide of antimony mixed with sand and oxide of lead are 

 employed, the latter substance serving as a flux to the others. 

 Very great carefulness is required in the preparation, as the lead 

 frequently approaches by reason of the heat to a metallic state, 

 when it appears in the form of black on the ware. Oxide of tin 

 is sometimes added ; and when a livelier colour is wanted 

 approaching to that of saffron, red oxide of iron is added, the 

 too great redness of this being subdued by the action of tlie 

 lead in the fusion which the ingredients undergo previous to 

 their application to the porcelain. The colours thus produced 

 are not susceptible of change, but if exposed to the full heat of a 

 porcelain furnace would be entirely dissipated. 



For the production of blue, well-prepared and very pure oxide 

 of cobalt is employed, mixed with a flux. Oxides of tin and 

 of zinc, added in different proportions, give different shades, 

 from a deep rich colour to a light blue. The rich smalt known 

 under the name of azure-blue is only the glass of cobalt mixed with 

 sand. This colour must be fused in a crucible, and reduced to 

 an impalpable powder in an agate mortar ; after which it may 

 be used in combination with flux. Green oxide of copper is 

 usually employed for the production of a green colour. On pre- 

 cipitating in different vessels by means of potass solutions of 

 copper which are equally pure and concentrated, it is perceptible 

 that the precipitate is formed more quickly in some vessels than 

 others ; and if these different products are separately collected, 

 those which are most promptly formed are when dry of a fine 

 blight green, and produce a corresponding colour on porcelain. 



Gilding on porcelain or on glass is performed either with or 

 without the addition of a fluxing material, the gold being made 

 to adhere to the surface by the incipient fusion either of the 

 glazing on the porcelain or on the surface of the glass or flux 

 employed. Gold is used for this purpose sometimes in the form 

 of leaf-gold, and at other times in that of a powder prepared 

 either mechanically or by some chemical precipitation. When 

 the first of these two methods is employed, leaf-gold must be 

 ground with honey or gum-water of an equal consistence, the 

 honey or gum being afterwards washed away. The gold may be 

 kept for use in paper or in shells, and the use of these latter 

 recipients has occasioned this powder to be known among artists 

 as shell-gold. This precious metal, about £4 an oz,, is precipi- 

 tated from its solution in aqua regia by adding to it a dilute 

 solution of green vitriol (proto-sulpbate of iron) or strips of 

 metallic copper. Gold powder may also be obtained from the 

 same solution by distilling it to dryness ; but this process is not 

 so convenient as precipitation. When gold powder is used, it 



