LEAD — MITMAN. 



609 



white lead is more extensively used for this purpose. Briefly, the 

 process is as follows : 



After the clay is molded into shape it is burned, when it becomes 

 known as " biscuit ware." The biscuit ware is then dipped in a 

 water pulp made of litharge (or white lead), clay, whiting, and zinc 

 oxide, which is deposited on the surface as a thin film and when 

 heated melts and forms a glaze. Enamelers' litharge is identical 

 with potters' litharge, both in character and in use. 



Litharge is also used by petroleum refiners. The crude petro- 

 leum is run into large tanks, where it is treated with diluted sul- 

 phuric acid. It is then washed with water to remove the acid and 

 other soluble impurities. A solution of caustic soda in which 

 litharge has been dissolved is agitated with the oil to remove sulphur, 

 during which time the sulphur combines with the lead, forming lead 

 sulphide, which settles out. Color-makers' litharge is dissolved in 

 diluted acetic acid to form soluble lead salts and these salts are used 

 to produce the paint pigments, chrome yellow and chrome green. 



Orange mineral is another lead compound identical in composition 

 with red lead but rather paler in color, finer in texture, and lighter 

 in weight. It is similar in character to red lead, too, has similar 

 properties, and is used chiefly in the manufacture of vermillion 

 paint, printing ink, and dipping paint. Its fineness, fluffiness, and 

 high degree of oxidation make it valuable for these purposes. 



Of all lead compounds, none is so universally used as white lead. 

 In fact, nearly one-half of all the metallic lead produced in the 

 United States enters into the manufacture of white lead and red 

 lead. White lead is pure, refined metallic lead converted by a chemi- 

 cal process into basic lead carbonate. It is a chemical compound 

 analyzing about 69 per cent carbonate of lead and 31 per cent 

 hydrated oxide of lead. 



White lead is of importance as a paint because of its opacity or 

 hiding power, its preservative properties, and its durability. The 

 hiding power is due not to the absorption of light but to the ability 

 of the fine particles of white lead to refract or turn aside the light 

 rays. This property of a substance to refract light rays can be 

 observed, and the power of such refraction as compared to the 

 similar power of some other substance taken as a standard, is des- 

 ignated the refractive index. Thus air, which transmits light prac-' 

 tically without bending or refracting the rays at all, is taken as 

 unity, and compared to air white lead has a refractive index of a 

 little over two, which is higher than that of any white pigment in 

 commercial use. 



White lead, of course, is of no value as a paint unless incorporated 

 in some vehicle, commonly an oil. The desirable feature of a paint 

 oil is the ability to set up or dry in a short period to a hard surface 



