STRUCTURAL PAINT TESTS 223 



with the alcohol a quantity of water, it would be found that 

 the shellac could no longer go into solution, and it would remain 

 in its original condition. In the same way, if there be placed 

 in water a small quantity of material, such as soluble chromates, 

 or an alkaline substance like caustic soda or lime, it will be found 

 that iron will no longer have a tendency to go into solution in 

 this treated water, but will stay bright and clean. These mate- 

 rials which prevent the rusting of iron have been called by 

 Cushman, who first advanced these explanations, " rust inhib- 

 itors," or materials which inhibit rusting. The paint maker, 

 realizing the importance of these rust inhibitors, is incorporating 

 them into paints designed for the protection of iron and steel, 

 and the success which paints of this type have met with from a 

 practical standpoint is a justification of what was first called 

 the " electrolytic theory," which suggested their use. 



By placing small, brightly polished steel plates into a mush 

 of paint pigment and water, a determination may be made of 

 the pigment's effect upon the metal. Some pigments, under 

 such conditions, cause rapid corrosion of the steel plates. Such 

 pigments are stimulators of corrosion, on account of acid 

 impurities which they contain, or because of their effect in 

 stimulating galvanic currents. Many carbonaceous pigments 

 are of this type. Other pigments have the effect of keeping 

 bright the steel plates and preventing rust. Such pigments 

 are of the inhibitive type, and their action is to check or retard 

 the solution pressure of the iron. 



The Effects of Moisture. It might occur to the reader that 

 although paint pigments, when mixed up with water and brought 

 into contact with the surface of steel, might show either an in- 

 hibitive or stimulative action, that it is by no means certain that 

 the same tendency will be exhibited by pigments when they are 

 properly mixed with linseed oil and laid out as a film upon the 

 surface of steel. In answer to this, it may be well to state that 

 almost no material used by mankind is absolutely dry. Linseed 

 oil, as it is pressed from the seed, comes from the cells, carrying 

 with it a certain small definite percentage of water, and it is 

 quite certain that even the best linseed oil that goes into use is 

 not theoretically dry. Everyone knows, of course, that oil and 

 water do not readily mix and are, in fact, more or less repellent 

 to each other. It is, however, true that, in spite of this, oils can 



