166 PAINT TECHNOLOGY AND TESTS 



illumination the light from an electric arc lamp was reflected 

 from a mirror directly upon the painted surface of the panel, 

 which in turn was reflected through the camera on to the ground 

 glass. The plate-holder was then put in position and six-second 

 exposures were made, afterward developing and printing. 



Checking and Cracking. What was termed " fine matt 

 checking " at the First Annual Inspection was not visible at 

 the time to certain members of the Inspection Committee, but 

 it is an established fact that the checking was an existing con- 

 dition, as the photomicrographs have shown. This checking 

 has a very peculiar characteristic in that the lines are very narrow 

 and hair-like, being somewhat interlaced and peculiarly forked. 

 That this hair matt checking is a preliminary condition which 

 afterwards develops into matt checking and into marked or heavy 

 checking seems to be indicated. 



It appears from an examination of the photomicrographs of 

 the paint films that a paint coating closely resembles the surface 

 of the earth, and is subject to the same basic laws that have 

 caused the various geodetic changes in the earth's crust. Ob- 

 servation of a dried pond or lake bed will disclose types of fissur- 

 ing and cracking similar to those shown by dried paint coatings 

 in which the oil has been fully oxidized, and especially in the 

 case of paints containing pigments which act upon the oil to 

 produce compounds brittle in nature. 



At Atlantic City the panels were all clean and free from dirt, 

 presenting continuous exposure of the films, and thus main- 

 taining conditions for active checking. At Pittsburg, soon after 

 the panels began to chalk, the large amount of dust and black 

 soot in the atmosphere completely covered the panels with a 

 very thick, resistant coating of carbon, which acted as a seal 

 or protector, preventing disintegration to a great extent. This 

 coating was extremely hard to remove, and photomicrographs, 

 before and after removal of this coating by rubbing with a damp 

 cloth, failed to reveal marked checking on any of the formulas 

 except those made of strictly pure basic carbonate-white lead. 

 The checking, even on these, was not as marked as at Atlantic 

 City. It is presumed that after the chalking had taken place 

 and the chalked pigment had been washed from the panels, the 

 gradually increasing coat of carbon and lead sulphide had pro- 

 tected the panels from checking, or possibly the atmosphere of 



