320 JOSEPH STRUTHERS 



present as well as the quantity of water combined with 

 the iron oxide. A red ocher, improperly called iron min- 

 um, very rich in iron oxide, is made by calcining and pul- 

 verizing limonite that is free from clay. 



There are few pigments more free from adulteration 

 than the ochers, for the reason that any filler that can be 

 used advantageously is more costly than the ocher itself. 

 Sometimes a little chrome yellow is added in order to im- 

 prove the tone of a poor colored ocher, but the presence 

 of this adulterant is very easily detected. Oxford ochers 

 are the brightest and best of the mineral pigments of this 

 class. They are obtained from Oxford, England. The 

 German pigments are often called ochers, although im- 

 properly so, for the reason that they are ligneous earths 

 and not ferruginous clays. The manufactured product 

 varies greatly in quality and value, and some of the grades 

 pass insensibly into umber or sienna. 



Umber and sienna in reality are varieties of ocher which 

 have been isolated on account of the brown color which 

 is imparted to the natural clay material by the addition 

 of iron and manganese oxides; raw umber is of a brown 

 color, while burnt umber is of a somewhat richer and redder 

 hue. Raw sienna is of a brownish yellow shade which 

 affords a rich russet brown when burned. Intermediate 

 shades of color are obtained by mixing natural products 

 with various properties of iron and manganese oxides, and 

 sometimes by mixing both raw and calcined materials to- 

 gether. 



A few tons each of slate, shale, and soap stone (the last 

 named being a variety of the mineral talc) are annually 

 ground in the United States to produce a gray colored pigment 

 used chiefly as a filler for mineral paints, especially those 

 of the variety called fire retarding. 



Gypsum, known also as terra alba and mineral white, 

 is a natural hydrated calcium sulphate (CaS04, 2H2O). It 

 is used to a minor extent as a pigment for printing wall 

 paper. The method of making this pigment consists in 

 grinding the mineral and treating it with acid in order to 

 remove any tint or color resulting from the presence of 



