LABORATORY STUDY OF TEST PANELS 173 



centages of barytes was also of a distinct nature, being generally 

 forked in appearance and of no definite striation. 



Surface Condition of Fume Pigment Paints. The panels 

 painted with basic sulphate-white lead (sublimed white lead) 

 showed complete absence of checking. This was also true of 

 the panels painted with zinc lead. These are both fume prod- 

 ucts and are extremely fine in their physical size, which may 

 account for this condition. Although zinc oxide is made in a 

 similar manner, it gives a much harder paint coating than either 

 of the afore-mentioned pigments, and presents a surface which 

 develops considerable checking, generally of a medium order. 

 The past theories regarding zinc oxide, in which it has been main- 

 tained that zinc oxide gives the maximum checking, are evidently 

 incorrect, as the checking found on the zinc oxide panels was 

 not as marked or deep as the checking on the basic carbonate- 

 white lead panels; in fact, the checking might be more in the 

 line of a cracking, possibly due to the brittle nature of the coating 

 composed of straight zinc. This is especially true of zinc paints 

 containing insufficient oil. 



The Importance of the Physical Nature of Pigments. It 

 appears that very fine grinding of materials, chosen for their 

 characteristic fineness, with the absence of any unfavorable 

 physical condition or chemical sensitiveness, are important 

 factors in the making of a paint to resist cracking or checking. 

 The purity of the essential materials, as well as the scientific 

 compounding of these materials, with due regard to the law of 

 minimum voids, are great factors which enhance the qualities 

 of paints, greater, perhaps, than the variation of percentages of 

 the various pigments which go to make up a paint. 



