CEMENT FOR POTTERY AND GLASS. 33 



The orange contains, in addition to oxide of iron and flux, 

 some oxide of zinc, with traces of alumina; and Nos. 8, 9, 

 and 10 have oxide of manganese as part of their composition. 

 The traces of alumina found in some of them do not act any 

 important part, as its presence is not necessary in the prepara- 

 tion of vitrifiable pigments. 



These tints are not equally permanent. The strength and 

 blueness of tone increases with the temperature to which the 

 pigment is subjected ; the yellow tint predominating at low 

 heats. The greatest purity is insured by using coloring mat- 

 ter prepared so carefully that every particle has been heated, 

 uniformly, to the same temperature. 



, Gray Enamel for Porcelain. — Salvetat (Ann. de Chira. et 

 de Phys. xxv. 342) has given a recipe for a new gray color for 

 porcelain. It is more durable, and more certain and constant 

 in its results, than the usual grays ; and, on account of its 

 agreeable tone, greater economy, and facility of preparation, 

 has been introduced into the works at Sevres, as a substitute 

 for iridium gray. It is prepared by mixing together 1 pt. of 

 platinum powder with 3 pts. of glass, formed of 1 pt. sand, 3 

 pts. minium, and a half part calcined borax. 



Cement for Pottery and Glass. — Wachter describes a fusible 

 cement for glass or pottery, which consists of 3 pts. red-lead, 

 2 pts. white sand, and 3 pts. crystallized boracic acid. They 

 are well mixed in powder, fused in a Hessian crucible, poured 

 out on a metallic plate, and ground fine. When used, it is 

 mixed with tragacanth paste and applied to the parts to be 

 joined, and the piece is then heated in a muffle at a low heat, 

 not quite sufficient to melt the enamel. 



3. Hydroplastics, 



Or making and using mortars, plaster casts, and artificial 

 stone. Under this head may also be included lime and lime- 

 kilns, hydraulic cements, asphalt pavements, and mastic used 

 for coating walls. There is not much of novelty to offer in 

 relation to this subject. 



