SCIENCE IN THE INDUSTRIAL WORLD 



lead chrome yellow the exact shade can be determined 

 by the amount of the two liquids used. By under- 

 standing this it is possible for the manufacturer to pro- 

 duce a lighter or darker shade at will. To produce the 

 numerous shades of yellow that cannot be made directly 

 by the initial process, varying quantities of white lead 

 are used. These may be added after the process of pre- 

 cipitating the chrome lead is completed, or may be pre- 

 cipitated at the same time. The white-lead salts add 

 greatly to the brilliancy of the pigment. 



In some instances there is a chemical combination 

 between the lead chromate and such a salt as the sul- 

 phate, so that the resulting light-yellow compound 

 is not merely a mechanical mixture of two lead salts, 

 but is a chemical compound. 



When the manufacturer wishes to produce one of 

 these pale shades of yellow he adds a certain quantity 

 of sulphuric acid to the solution of the chromate used, 

 and mixes this with the acetate-of-lead solution. In the 

 reaction that follows a certain quantity of the white 

 sulphate of lead is formed with the lead chromate, and 

 is precipitated with it, either as a chemical combination, 

 as we have just said, or as a mechanical mixture having 

 a perfectly uniform color. The shade of color so pro- 

 duced will vary with the amount of sulphuric acid used, 

 larger quantities of the acid making corresponding 

 lighter shades of yellow. The process of making zinc 

 chrome yellow, and barium chrome yellow, is very 

 much the same as for making the lead chrome yellow. 



Nature furnishes another lead pigment in the form of 

 lead monoxide (PbO) which, in its crystalline form, is 



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