CRUDE MINERAL PIGMENTS 321 



iron oxide. The gypsum, or white mineral reported in 

 1902 was mined in Cape Girardeau county, Missouri. 



In addition to the natural iron oxide pigments, made 

 by a simple washing and grinding of pure crude ores, there 

 are two very important artificial iron oxide pigments made 

 by roasting the residuum obtained from making copperas, 

 or green vitriol (FeSO^jVHoO). One is Venetian red, com- 

 posed almost wholly of artificial iron sesquioxide, and the 

 other Indian red, which is comprised of about 40 per cent 

 of iron sesquioxide, the balance being mainly calcium sul- 

 phate, made by adding a certain proportion of lime during 

 or before the roasting. 



There are two methods of manufacturing these artificial 

 reds, the dry and the wet. In the former method which 

 is the cheaper for the low grades of oxides, the impure cop- 

 peras is roasted in a furnace with lime or similar material, 

 in order to neutralize the acid in the sulphate until the 

 desired strength and color of the product are obtained. 

 Occasionally copperas alone is roasted until the acid con- 

 stituent has been completely expelled, leaving the residuum 

 in the form of pure iron sesquioxide; a filler (whiting or 

 gypsum) is then added in proportions to yield the desired 

 grade of Venetian red, some containing as little as 10 per 

 cent of iron oxide or coloring power. 



The wet method is cheaper for the manufacture of the 

 general grades of Venetian red, but yields a less uniform pro- 

 duct than the dry method. In principle, the waste liquor 

 from the cleansing or pickling process in preparing iron wire 

 or plates for galvanizing by immersion in weak sulphuric acid 

 is treated direct by the addition of milk of Ume, sodium car- 

 bonate or similar reagents, which precipitates the iron as a 

 hydrate or carl^onate to the bottom of the tank. This precip- 

 itate is subsequently separated from the liquor b}' filtration 

 through a press, the dried cakes remaining therein being sub- 

 sequently roasted in furnaces, cooled, and packed for the 

 market without grinding. The product made in this manner 

 is not uniform in quality, and has but a hmited use. 



Hematite ore is sometimes ground and sold as Venetian 



Vol. 0-21 



