66 PAINT TECHNOLOGY AND TESTS 



Prussian Blue. On oxidizing the precipitate resulting from 

 the interaction of solutions of prussiate of potash and copperas 

 (iron sulphate), Prussian blue as used in the paint trade is pre- 

 pared. It has a specific gravity of 1.9. The pigment shows 

 an amorphous structure, the particles varying greatly in size. 



Ultramarine Blue. This bright blue pigment is prepared by 

 burning silica, china clay, soda ash and sulphur in pots or fur- 

 naces. It has a specific gravity of 2.4. It is of little value as 

 a paint pigment on account of its sulphur content, which causes 

 darkening when mixed with lead pigments, and corrosion when 

 applied to iron or steel. The darkness of the photograph is due 

 to the massing of the pigment particles. 



Chrome Green. Chrome green is prepared as a paint pigment 

 from nitrate of lead, Chinese blue, and bichromate of soda. 

 It has a gravity of 4 and is liable to contain slight traces of lead 

 salts. The particles when magnified appear very fine and 

 flocculent. This color is often precipitated on pigments, such 

 as barytes, which do not reduce its tone. 



Bone Black. By grinding the carbonaceous matter resulting 

 from the charring of bones; in iron retorts, the pigment bone 

 black is prepared. It contains about 15% of carbon and 

 85% of calcium phosphate. It has a gravity of 2.7. Com- 

 paratively large particles of charred bone can be seen scat- 

 tered throughout the mass, resulting from the difficulty of 

 grinding to a uniform size. 



Carbon Black. This form of very pure carbon results from 

 the combustion of gas. Its gravity, 1.09, is lower than that 

 of lampblack, which shows a gravity of 1.8. It is used in 

 much the same way and for the same purposes as lampblack. 

 In physical appearance it shows great similarity to the particles 

 of lampblack. 



Lampblack. This pigment, made from the combustion of 

 oils, consists very often of more than 99% carbon. It has 

 wonderful tinting value. The particles show a fine, fibrous 

 structure with a tendency toward agglomeration. They differ 

 greatly in physical appearance from those of either graphite or 

 bone black, being exceedingly more uniform than the latter-. 



Graphite. Graphite, both in the natural and artificial form, 

 contains impurities such as silica, iron oxide and alumina, but 

 the natural form has a much greater percentage of these foreign 



