26 PAINT TECHNOLOGY AND TESTS 



be 0.02%. This critical percentage, as it may be termed, 

 should not be exceeded, and any added amount of man- 

 ganese has the effect of making the film much more brittle 

 and causes the so-called " burning up" of the paint. The 

 loading of paint with drier and the bad result therefrom may be 

 explained to some extent from the above results. 



In the same way with lead driers, excessive amounts of lead 

 oxide seem to have no beneficial effects on the drying of an 

 oil, and when the percentage which seems to be the most bene- 

 ficial, namely 0.5% lead oxide, is exceeded, the film is apt to 

 become brittle. 



Oils containing lead oxide driers are less influenced in their 

 drying tendencies by conditions of moisture in the atmosphere 

 than oils containing manganese, but frequently, however, the 

 former dry much better in a dry atmosphere. As a general 

 rule, varnishes rich in manganese dry more quickly in a dry 

 atmosphere, while those containing small quantities dry more 

 quickly in a damp atmosphere. 



Volatile Products Formed. It was furthermore noticed in 

 these tests that sulphuric acid, placed in dishes on the bottom of 

 the large box in which the samples of oil were drying, as dis- 

 colored and turned brown after several days, showing that the 

 acid had taken up some material of a volatile nature that was a 

 product of the oxidation. 



Another curious feature of these tests was the development 

 of a peculiar aromatic odor which was given off by the oils 

 upon drying in dry air. When the oils were dried in moist air, 

 a rank odor resembling propionic acid was observed, and this 

 led the observer to believe that a reaction was effected by the 

 absorbed oxygen, that caused the glycerin combined with the 

 linoleic acid as linolein to split up into evil-smelling compounds. 

 It has been suggested that the oxygen first attacks the glycerin, 

 transforming it into carbonic acid, water, and other volatile 

 compounds, which are eliminated before the oil is dried to 

 linoxyn. Toch, 1 however, has shown that the drying of linseed 

 oil gives off only very small percentages of carbon dioxide. 

 Mulder has observed that in the process of linseed oil being 

 oxidized, glycerin is set free, which becomes oxidized to formic, 



ir Toch: The Chem. and Tech. of Mixed Paints, p. 89. D. Van Vostand 

 Co., N. Y. 



