Clifton College Scientific Society. 41 



will only be produced with glaze that melts with difficulty, and 

 when the heat has been raised precisely to and not beyond tlie 

 point that is requisite for its fusion. 



The art of painting on earthenware, though of comparatively 

 recent introduction into England, is by no means a modern 

 invention. It is well known that the ancients manufactured 

 coloured enamels, and some specimens of the art made by 

 the Egyptians more tlian three thousand years ago have been 

 preserved to the present day, which is an evidence of this fact. 

 In this branch of the art there are various objects to be con- 

 sidered, a proper acquaintance with which is necessary to success. 

 Such are the Composition of colours ; the fluxes which are 

 necessary to render these fusible, which unite them to the wares, 

 and in many cases impart brilliancy to their tiuts ; the vehicle 

 employed in laying on the colours, and the course to be pursued 

 in fixing them on the porcelain by means of heat. 



Metallic oxides form the bases of all vitrifiable colours, but 

 every metaUic oxide is not proper for being employed in deco- 

 rating porcelain. Some are very volatile, as the oxides of mercury 

 and of arsenic. Others part so freely with the oxygen they hold 

 in combination that their colour proves uncertain, and varies 

 with every application of heat ; such are the pure-coloured and 

 red oxides of lead, and the yellow oxide of gold. Oxides which 

 are .susceptible of great variations are very seldom emjiloyed. 

 Black oxide of iron is not used alone for producing that colour 

 on porcelain ; and the green oxide of copper, as formerly pre- 

 pared, was so uncertain that it was very rarely employed, but 

 the defect has of late been greatly remedied. Oxides uncom- 

 bined with other substances are not susceptible of fusion ; and 

 although they may be attached in thin strata to vitrifiable bodies 

 by a very violent heat, yet their coloiirs, with the exception of 

 lead and bismuth, would in such cases become dull, and possibly 

 be even destroyed. In order to promote their fusion a flux is 

 therefore added, the composition of which varies, according to 

 the means employed for diluting the colours at the time they are 

 used. Where a volatile oil is chosen for this dilution, a flux 

 composed of glass, nitre, and borax is most proper ; but when 

 gum-water is substituted for this volatile oil, the flux must be 

 varied, because borax cannot be properly diluted in gum-water. 

 A compound of glass, lead, and silex is therefore prepared by 

 some. Another compound extensively used consists of — powdered 

 glass, 40 parts ; calcined borax, 22 parts ; refined nitre, 44 parts. 

 It is essential that not only the borax and nitre be as pure as 

 they can be rendered, but also that the glass shall not contain 

 the smallest particle of lead in its composition. These in- 



