So far linseed oil has not found an equal in paint making, although the subject has been one of deep study, and while other fixed oils have been discovered, that for certain purposes have been expected to take its place, it has yet to be demonstrated, that such is really the case in long practice. China wood or tung oil, while superior to linseed oil in certain directions, especially in its resisting power to water, has not shown itself adapted to replace linseed oil in making oil paints, as we know and desire them. As we go along in the consideration of other paint oils, we will show the reasons for this. To go into the technical study of linseed oil and its chemistry would be a waste of time and space here, because it would simply be a useless repetition of what has been written on the subject by eminent students in its chemistry. To those, who wish to go further into the characteristics and chemical composition of linseed oil, the writer would refer to the following works as being standard on the subject: "Linseed and Other Seed Oils," by William D. Ennis, M.E. a very complete American treatise; "Drying Oils, Boiled Oil and Solid and Liquid Driers," an English translation from the German of Louis Edgar Andes.
The practical paint maker and color grinder, however, need not consider linseed oil for any other purpose but that of getting the best and purest as well as that, which is of a quality giving the results he is striving for.
If he deals with a reputable crusher of linseed oil and carefully checks up and examines deliveries, he need have no anxiety about the effect of the oil upon his products, but it is also up to him to exercise the utmost care and watchfulness to have his storage tanks cleaned out at regular intervals, because even the purest and most well settled oils will, in time, deposit a slimy substance known as oil foots and when this is permitted to accumulate, it will cloud the oil, when the contents of the tank become agitated from any cause. Many a batch of color or paint has gone wrong and the cause could not be traced to the proper source, because of the stereotyped excuse, that the formula was followed to the letter and the oil was the same as had always been used. Yet the trouble of improper drying or inferior binding properties of the material might be directly traced to oil foots. In the case of boiled oil this is even worse, as it stands to reason, that the litharge and manganese, after having given up their oxygen to the body of the oil, are still in fine division and on settling drop to the bottom of the storage tanks as a useless slimy mass, carrying with it some of the glycerides, that are broken by the boiling process. This refers to the kettle or fire boiled oil. But in the present day process of boiling oil, where the oil is only heated to a high enough temperature to expel moisture and a cheap lead or manganese resinate drier is introduced in liquid form, it is still more necessary to be on the lookout for precipitated matter.
As to the purity of linseed oil, the best precaution is to purchase supplies from reliable crushers or their agents only, who will not sell sophisticated linseed oil in any form. As to detection of adulteration in linseed oil, there are a few very simple tests. In the first place, a standard gallon of 231 cubic inches must not weigh less than 7 3/4 pounds, when oil shows a temperature of 60° F. Ordinary boiled linseed oil should weigh 7 lbs. 13 oz. to 7 lbs. 14 oz., while heavy bodied linseed oil (oxidized oil) may weigh anywhere from 7 lbs. 15 oz. to 8 lbs. 2 oz. per gallon. When linseed oil is placed on a strip of glass, that has been painted jet black, and shows a bloom or iridescence, it is doctored either with mineral oil or rosin oil, which can be determined by the characteristic odor. Admixtures of linseed oil and corn oil or linseed oil and cottonseed oil can be detected, by placing some of the oil between the palms of the hands, rubbing briskly and noting the odor thus emitted. Fish oil in admixture with linseed oil is readily detected by its odor, which cannot be disguised. The presence of soya bean oil, however, cannot well be ascertained by simple tests and a chemical analysis is necessary and sometimes misleading at that.