322 JOSEPH STRUTHERS 



red. The natural products, however, are inferior to the arti- 

 ficial and do not command as high a price. 



Formerly the iron oxide pigments made in the United 

 States did not contain more than 50 per cent of iron oxide, 

 the higher grades being imported. Recently, however, the 

 use of a crucible furnace of special design, which gives good 

 control of the heat and of the oxidation, has raised the grade 

 of the product to practically 100 per cent of iron oxide. 



Improvements have also been made in the wet process, 

 chiefly in the substitution of mechanical appliances for hand 

 labor, which has lessened the cost of production and given a 

 more uniform product. 



Among other minerals used in part in the manufacture 

 of paints are barytes, asbestos, graphite and asphaltum. 



The mineral is generally prepared as follows: The crude 

 ore is hand sorted and the limestone rock and other foreign 

 materials removed; the select ore is then crushed and boiled 

 in dilute sulphuric acid in order to remove any remaining im- 

 purities, chiefly iron oxide, which may impart a tint or color 

 to the product. The material is then thoroughly washed after 

 boiling to free it from acid or soluble salts and is finely ground 

 and put on the market in four varieties: No.l, No. 2, No. 3, 

 and floated or water sorted. 



Barium sulphate, known as blanc fixe, which has been 

 precipitated artificially as a by-product in some chemical 

 industries, is used to a considerable extent as a filler and 

 pigment. It has more body and greater capacity and covering 

 power than the native mineral barite, for the reason that it 

 is amorphous, while barite is crystalline. 



The chief use of barite is as a pigment which is usually 

 mixed with white lead. In the United States it is regarded 

 as an adulterant which depreciates the value of the paint, 

 but in Europe it is considered a valuable addition, and in many 

 cases a pigment composed of a mixture of barite and white 

 lead is considered more serviceable than white lead alone, for 

 the reason that, owing to the insolubility of barite in acids, 

 it imparts elasticity to the mixture, gives a greater body to 

 the paint, and resists the influence of the weather better than 

 white lead alone. 



