PAINTING. 265 



of combination. Against the use of varnishes upon metallic 

 surfaces, it has long been pointed out that, on account of the 

 volatile properties of the medium, either turpentine or ben- 

 zine, its rapid evaporation causes a fall of temperature, 

 causing a deposition of moisture upon the surface, which 

 acts deleteriously upon the resin or gum of the varnish, while 

 preventing the proper adhesion of the film to the metal, and 

 possibly causing the commencement of the corrosive action 

 of moisture upon the metallic surface. 



The cost of a well-prepared asphaltic varnish, of pure ma- 

 terials, will be about $1.50 per gallon, which will cover about 

 40 sq. yds. of surface, one coat. 



Application. It is generally conceded that two coats of 

 good paint will last at least three times as long as one coat, 

 and that the first, or priming coat, is of especial importance. 



In the prize essay of Prof. Spennrath, Director of the 

 Technical Schopl at Aix-la-Chapelle, upon " Protective Cover- 

 ings for Iron," his conclusions are that: " It is therefore 

 advisable, in putting on iron coatings, to prime with a paint as 

 heavy as possible and have the upper coat rich in oil." The 

 specific gravity of red lead being shown to be about 9.0, it 

 is the heaviest known pigment in use in the preparation of 

 paints. 



In a number of exhaustive tests, Prof. Spennrath distinctly 

 traces the bad experiences with red-lead coatings to the action 

 of heat, under which conditions the metal expands, the paint- 

 skin remaining hard and brittle, a severe stretching takes place, 

 cracks and rents develop in the paint-coating, and as a conse- 

 quence rust appears. Where the atmosphere contains hydric 

 sulphide, the red lead is changed to the sulphide of lead, ac- 

 cording to Prof. Spennrath, to which he attributes the sole 

 specific weakness of red lead as a pigment. 



To sum up, in favor of the use of red lead and oil is its 

 well-known high specific gravity and its peculiar chemical 



