SALTS. 



With the addition of borax, it is also employed for im- 

 itations of precious stones. 



What is ena- 819. ENAMEL. Enamel is an opaque 

 md? glass, produced by the addition of some 



material which does not dissolve in the fused mass. 

 Binoxide of tin is the material commonly employed. 

 Various tints may be imparted to enamel, as to ordi- 

 nary glass, by the addition of small quantities of me- 

 tallic oxides. A thin surface of enamel is often baked 

 on to a metallic surface, as in the case of watch dials, 

 and various objects of jewelry. 



How is glass 82 0. COLORED GLASS. Glass is colored 

 colored? an( j stained by the addition of various me- 



tallic oxides. The peculiar coloring effects of these 

 substances have already been mentioned, in the sec- 

 tion on Oxides. 



EARTHENWARE. 



What is the 821. Clay is the basis of all earthenware, 



basis of all from the finest porcelain to the coarsest 



earthenware? 



How is porce- brick. Being first fashioned by moulds or 



lain made? ^_ means j nto t h e 



proper form, it is dried, baked, and 

 subsequently glazed, to render it 

 impervious to water. In the man- 

 ufacture of porcelain, glazing is not 

 essential. Sand and chalk are ad- 

 ded to the original material, and 

 the heat is carried so high as to 

 bring the whole mass into a semi- 

 vitreous condition. This is also 

 the case in certain kinds of stone- 



