CRUDE MINERAL PIGMENTS 319 



dried, pulverized, and passed over a screen of bolting cloth 

 or through some type of pneumatic separator, from which 

 the final product is classified and packed for shipment. 

 At times the ground ore is levigated and the settled prod- 

 ucts dried and packed for shipment. 



The name ocher is applied to clays and other earthy bases 

 containing in their natural state sufficient ferrous or ferric 

 oxide or hydroxides to impart to the mass a bright red or 

 yellowish red tint. The color varies from a golden yellow 

 to a dark red, occasionally possessing various tints of blue 

 and green. 



The ochers reported w^ere mined in Clay county. Ark. ; 

 Calaveras and Stanislaus counties, Cal.; Bartow and Rich- 

 mond counties, Ga.; Berks, Lehigh, Luzerne, and North- 

 ampton counties, Pa. ; Rutland county, Vt.; and Page county, 

 Va. The sienna reported w^as mined in Washington county, 

 N. Y. ; and the umber in Lawrence county, Pennsylvania. 



The ochers have been used as paints from a very early 

 date, the oldest applications positively recognized having 

 been made in Italy, though it is believed that some vari- 

 eties were in use still earlier by the Egyptians and Greeks. 

 In modern times the ochers were first mined and prepared 

 in Italy, and the siennas and umbers derive their names 

 from the Italian towns in which they were manufactured 

 into pigments. 



Ochers are classified in man}^ ways, according to the 

 locality of occurrence, the composition, and the special 

 shade of color. Practically they may be grouped into yellow, 

 red, and brow^n. Yellow ocher is that which is colored 

 by a ferric hj^droxide. Red ocher owes its tint to ferric 

 oxide, and it is therefore evident that red ocher m^ay be 

 prepared artificially by expelling the water from yellow 

 ocher by calcining in a furnace or kiln. Brown ocher is 

 red ocher modified by the presence of black manganese 

 dioxide, which in various proportions yields a large range 

 of brown colors, notably sienna brown, umber, Vandyke 

 brown, and manganese browm. 



The variation of ocher in shade and in quality depends 

 chiefly, but not entirely, upon the proportion of iron oxide 



