PAINTS, DYES, AND VARNISHES 



dantly distributed throughout the mineral world, al- 

 though the quality of the product varies greatly in 

 different regions. The color of the ochres is due to the 

 presence of hydrated peroxide of iron; and while this 

 substance gives the color to the siennas also, it is modi- 

 fied in these pigments by the presence of small quantities 

 of manganese. With these substances there is always 

 a mixture of various "earths," so that the pigment is a 

 conglomerate mixture of several chemicals. Some of 

 these mixtures occur in such natural deposits that they 

 may be used as pigments of good quality without 

 treatment other than that of grinding to a fine powder. 



When ochres are heated they turn red, and are 

 sometimes marketed as Venetian red, or Indian red. 

 When the siennas are heated, however, a reddish- 

 orange shade, known as burnt sienna, is produced. 

 The cause of this change of color is probably the con- 

 version of hydrated iron oxide to an anhydrous 

 state; but as yet we do not know just why the ochres 

 turn red, while the siennas have orange color during 

 the process. 



That the composition of ochres is complicated is 

 shown by the following analysis of Oxford ochre, by 

 Hurst: 



Water, hygroscopic 6. 887 per cent. 



Water, combined 8. 150 



Calcium oxide o . 998 



Sulphur trioxide i . 321 



Alumina 6. 475 



Ferric oxide 12 . 812 



Silica 63 . 478 



