CHEMICAL PKOPEKTIES OF FATS AND OILS 301 



oxygen. Not all oils are capable of this change; only those which possess a 

 large proportion of unsaturated acids can undergo this reaction. The 

 ability of a fat to act as a drying oil is therefore roughly proportional to the 

 iodine number, i.e., the higher the iodine value the more effectively an oil 

 can serve in such a capacity. However, the latter constant is not an ab- 

 solute index, and one is forced to resort also to the following practical test. 

 A so-called "drying oil," when spread in a thin layer on a glass plate, will 

 dry to an acetone-insoluble film in 2 to 6 days. The "semidrying" oil will 

 become somewhat sticky after a week, while a "nondrying" oil will still be 

 fluid after exposure to air for 18 to 20 days. The boundaries between these 

 classes of fats are naturally somewhat arbitrary, but the following grouping 

 is fairly satisfactory: 



Hilditch^^^ has used a formula based upon total polyethenoid acids and 

 the linolenic acid content to ascertain the so-called "quick-drying" index. 

 The formula employed is as follows : 



Quick-drying index = P X Len X 10 ~^ 

 P = Lin (linoleic acid) + Len (linolenic acid) 



The percentage of P is an index of drying power and this should not fall 

 below 65-70 in a satisfactory drying oil. The "quick-drying" index based 

 on P multiplied by linolenic acid content is considered as a relative but not 

 as a quantitative measure of the property of this fat. An oil with proper- 

 ties comparable to those of linseed oil should have an index "about 25 (or not 

 much less)." These calculations of Hilditch are summarized in Table 45. 



The mechanism by which this transformation takes place is not thor- 

 oughly understood. The free fatty acids of linseed and tung oils give com- 

 pletely negative results in film formation, as do their esters with mon- 

 atomic alcohols. ^^^-^^^ Likewise, the mono- and even the diglycerides of the 

 linseed oil fatty acids will not undergo the drying reaction, although the 

 diglycerides may acquire this propert}^ on heating. Also the mono- and 

 diglycol esters of these fatty acids are completely inactive until subjected to 

 heat treatment. ^^^ 



"" T. P. Hilditch, J. Oil Colour Chem. Assoc, 31, 1-24 (1948). 

 S66 T. F. Bradlev, Irui. Enq. Chem., 2D, 440-^45 (1937). 

 «6 T. F. Bradlev, lad. Eng. Chem., 29, 579-584 (1937). 



