242 PAINT TECHNOLOGY AND TESTS 



the life of such coatings is due rather to the property of these 

 pigments, of taking up large quantities of oil, than to their effect 

 upon the slow drying of oil. Excessive oil carrying, however, 

 should be avoided, as shown by the early failure and pitting of 

 those carbon black and lampblack paints ground with very 

 large quantities of oil, as is the usual practice. When these car- 

 bon and lampblack pigments were ground with barytes (which 

 is a heavy pigment and requires only about 9 pounds of oil to 

 100 pounds of pigment, as against 175 pounds of oil to 100 

 pounds of lampblack), it was found that the lampblack and car- 

 bon black paints were reinforced and made more suitable for 

 actual practice. The stimulative nature of these black pig- 

 ments, however, asserted itself in both cases, and large pittings 

 and eruptions were evident at the end of a year. Carbon black, 

 lampblack, graphite, or any other good conductor of electricity 

 should never be placed next to the surface of iron. They are 

 good as top-coatings, but not as prime-coaters. Some pigments 

 are stimulators of corrosion, because they contain water-soluble 

 impurities that hasten the rusting, while others, like graphite, 

 hasten it simply because, being good conductors, they stimulate 

 surface electrolysis. 



Panel No. 20 Willow Charcoal. In excellent condition 

 throughout. Presence of small quantities of potash may be 

 responsible for the inhibitive nature of this black pigment. 



Panel No. 24 Ochre. While the film seems intact, it has a 

 very mottled appearance and examination shows eruptions of 

 rust through the film, in several places. 



Panel No. 27 Natural Barytes. Within a year the film 

 became pin-holed, and corrosion was apparent. At the end of 

 three years very little of the pigment was left upon the plate, 

 having chalked and scaled off. Barytes has proved its useful- 

 ness as a constituent of a combination type of paint, but it 

 should not be used alone. 



Panel No. 28 Blanc Fixe. In the same condition as Panel 

 No. 27, but slightly more chalking and disintegration was 

 shown. 



Panel No. 29 Whiting. Plates coated with calcium car- 

 bonate or whiting in oil presented a very fair appearance at the 

 start of the test, but they soon began to chalk and disintegrate. 

 It is well known that whiting, being alkaline, has the property 



