62 PAINT TECHNOLOGY AND TESTS 



Aluminum Silicate (China Clay). China clay, or aluminum 

 silicate, is a permanent and valuable white pigment showing 

 very little hiding power in oil. It is found widely distributed 

 in granitic formations. It is very stable, with a gravity of 2.6. 

 Particles are found in many shapes and sizes, showing sharp 

 and definite angles. 



Ochre. Ochre is a hydrated ferric oxide permeating a clay 

 base, largely used as a tinting material. It has a specific gravity 

 of about 3.5, and a decidedly golden yellow color. A good 

 quality should contain 20% or over of iron oxide. The par- 

 ticles of this pigment are flocculent and very uniform in 

 appearance. 



Sienna. Sienna, like umber, is essentially a silicate of iron 

 and alumina, containing manganic oxide. It contains, however, 

 a lower percentage of the latter than in the case of umbers. The 

 photomicrograph of the burnt variety shows clearly the fine 

 condition of the pigment, while large particles are shown in the 

 raw variety. 



Umber. Umber, another naturally occurring pigment, con- 

 sists of iron and aluminum silicates, containing varying propor- 

 tions of manganic oxide, its color and tone varying according 

 to the percentage of the latter. The raw variety is drab in 

 color, which in burning changes to reddish brown. A marked 

 percentage of large-sized particles exist in this pigment. 



Indian Red. Indian red is the term applied to natural hema- 

 tite ore pigments and to those produced by the roasting of 

 copperas (iron sulphate). They generally contain 95% or 

 more of iron oxide, with varying percentages of silica. The 

 pigment is heavier (specific gravity 5.2) than that of Metallic 

 Brown. The crystalline, mineral-like structure of the particles 

 differ greatly from the amorphous particles of Metallic Brown. 



Metallic Brown. The natural hydrated iron oxide or carbonate 

 as mined largely in Pennsylvania, yields, when roasted, a ses- 

 quioxide of iron known as Metallic Brown. It contains a high 

 percentage of alumina and silica, and has a characteristic brown 

 color with a gravity of 3.1. It finds wide application as a pig- 

 ment for protective purposes. The particles when enlarged 

 show the usual appearance of a natural compound which has 

 been roasted and ground. 



