PAINTS, DYES, AND VARNISHES 



OTHER YELLOW MINERAL PIGMENTS 



The principal yellow mineral pigments are the 

 chromes and ochres, although there is a long list of 

 yellows and oranges from other sources that are oc- 

 casionally used. The base of all the chrome pigments 

 is the chromate of lead (PbCrO) and its basic modifica- 

 tions. The lead chromates are readily obtained by 

 mixing a solution of potassium bichromate with a solu- 

 tion of lead acetate, and vary in color from light yellow 

 to deep red. As placed on the market commercially, 

 the darker shades are likely to be pure lead chromates; 

 while the lighter shades represent a mixture of the darker 

 base with some white pigment, such as sulphate of lead, 

 or enamel white. These mixtures do not necessarily im- 

 pair the quality of the pigment. Indeed the "pure 

 chromes" of commerce are really mixtures with the 

 lead sulphate. Thus "pure" lemon yellow is made 

 from the following formula : 



Lead acetate (or nitrate) 100 parts 



Bichromate of potash 25 " 



Sodium sulphate 35 " 



If this formula is varied a little, such as by increasing 

 the amount of bichromate of potash five parts, and the 

 sodium sulphate diminished fourteen parts, "pure" 

 chrome yellow is formed. And if the sodium sulphate 

 is omitted entirely, "deep" chrome yellow is made. 

 Still other shades may be produced by using sulphuric 

 or nitric acid in place of the sodium salt; and these 

 may be varied again by using a barium salt with the 

 lead acetate and potassium bichromate. 



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