THEORY AND PRACTICE OF PAINT MAKING 95 



governing the strength and durability of a paint coating it is 

 necessary to regard the coating as consisting of a series of flat 

 arches, in which the pigment particles of largest characteristic 

 size serve as the piers or supports for the flat arches of which the 

 continuous film is composed. 



Corollary A The strength and durability of a paint coating 

 is determined by the strength and durability of the piers or 

 supports (which consist of the characteristic pigment particles 

 of the largest size). 



Corollary B Owing to their inherent strength and durability 

 the pigment particles of largest characteristic size which serve 

 as supports for the paint coating should consist, in part at least, 

 of chemically inert pigments, such as natural crystalline barium 

 sulphate, calcium carbonate, magnesium silicate, etc. 



Corollary C - - It follows directly that the thickness of a paint 

 coating is determined by the particles of pigments having the 

 largest characteristic size, even if that pigment be present only 

 in moderate percentage. Upon this principle depends the com- 

 paratively great thickness of film and moderate spreading rate 

 of paints composed of such pigments as basic carbonate white 

 lead, red lead, barytes, etc., and the strongly contrasted thin- 

 ness of film and high spreading rate of paints composed of the 

 sublimated pigments such as lamp black, zinc oxide, basic sul- 

 phate white lead, zinc-lead white, leaded zinc, etc. 



In commenting upon the announced laws set forth above, 

 Heckel says: " The recognition of these laws was an exercise 

 of pure deduction. Paint manufacturers before Mr. Perry's 

 announcement were producing paints containing three or more 

 pigments with particles of varying characteristic sizes; but their 

 procedure was based largely on empirical knowledge, the result 

 of accumulated experience, due to a conscientious endeavor to 

 produce the highest type of paints for economic service. In 

 the absence of any law to govern or to limit the use of the rein- 

 forcing pigments, inexperienced manufacturers had brought 

 upon the market paints which were badly proportioned as to 

 the several pigments, or burdened beyond the limits of effective- 

 ness with reinforcing pigments. To all paint manufacturers 

 Perry rendered a substantial service in deducing for them the 

 laws set forth in his address. In the results following a recogni- 

 tion of these laws there was nothing new or startling, but Perry 



