PAINTING. 271 



softening of paint on bridges by burning and lubricating oils 

 from passing trains. 



" It is finally recommended that the coating be of such a char- 

 acter as to successfully resist the mechanical injury due to sand, 

 cinders, and other material carried by the wind. 



" FEASIBILITY OF RECOATING. There can be no question 

 that a satisfactory coating must permit recoating when needed 

 without additional labor for cleaning and removing old coat. 



" COST. Upon this point it is only necessary to say that the 

 other valuable requirements being obtained, that coating is best 

 which can be furnished and applied at minimum cost. 



" SUB-COMMITTEE ON TESTS TO DETERMINE EFFICIENCY OF 



COATING. 



"It is the opinion of your committee that it is useless to pre- 

 scribe the same tests to all classes of protective coverings. An 

 efficient coating in the dry atmosphere of the Western States 

 may fail to withstand the moist saline air of the coasts. A coat- 

 ing which is perfect for structural steel under a static load may 

 fail entirely when subjected to vibratory shock imposed on bridge 

 members and steel cars. In short, tests must be in harmony 

 with conditions imposed in service. 



" The general cause of failure of coatings to protect is the same 

 as the corrosion of the metal itself, i.e., moist air and carbon 

 dioxide. Dilute acids, as a rule, have far less action on paint 

 films than alkaline solutions. A paint made from some inert 

 pigment and linseed, oil will show no sign of disintegration when 

 immersed for days in a dilute sulphuric- acid solution which would 

 rapidly dissolve the metal it protected, and the same paint would 

 go to pieces in a few hours when exposed to the action of a corre- 

 spondingly strong solution of ammonia or carbonate or caustic 

 alkalies. Strong acid solutions rapidly destroy the coating, 



