The low cost of mineral oils, including coal oil and petroleum, has caused them to become highly regarded as linseed oil adulterating mediums. Mineral oils more unfavorably affect the drying property of paint than its working and spreading property.
Cottonseed oil belongs to the non-drying class of oils, but since recent processes have made possible the elimination of the pronounced acrid taste, its presence in linseed oil by the sense of taste is not easy to expose.
Hempseed oil is a mean tasting, mean smelling, but good drying oil, and only because of its rapid color changes, wearing finally to a dull brown, is its employment in linseed oil restricted to narrow limits.
In testing for linseed oil adulteration, ammonia is often effectively used, equal parts of the ammonia and oil being employed. Cottonseed oil under the ammonia treatment shows an opaque brown. When it is present in linseed oil the liquid goes to an opaque yellow. Fish oil under the effects of ammonia goes white. Rosin oil will disclose its presence in linseed oil if confined in a bottle, with alcohol added in the proportion of five parts of alcohol to one part of oil, and smartly shaken, the alcohol afterwards being poured off. A clear sugar-of-lead solution is added to the oil, and should rosin oil be an ingredient a cloudy precipitate will manifest itself. A practical and simple test often used in the carriage paint shop consists in taking a couple of test tubes and putting a quantity of linseed oil of known purity in one tube and a quantity of suspected oil in the other, then immersing the tubes in warm water for, say, 1/4 of an hour, and immediately upon removal from the water pouring the pure oil into the tube of suspected oil. If any impurity exists, different colors will form in layers. And it may be here proper to say, in passing, that in making tests and comparisons of materials, an article of established purity and quality should be used as a standard. Some time ago a well-known paint firm issued a card giving some easy and practical tests for the detection of linseed oil adulterants, and knowing their value to the vehicle painter, the writer herewith appends three tests:
No 1.—Shake equal parts of oil and strong nitric acid in a small white glass vial or bottle, and allow to stand from fifteen minutes to two hours.
| UPPER STRATUM | LOWER STRATUM | ||
| Pure Linseed oil | } | Muddy olive green | Almost colorless |
| Presence of Fish oil | } | Decided deep red brown | Deep red or cherry color |
No. 2.—Shake with concentrated solution of potash or soda, and then add warm water and shake again. Allow to stand half an hour, and if any petroleum (paraffine oil) is present it will separate from the soap.
No. 3.—Put samples of oil in tubes and place them in a freezing mixture (2 parts ice or snow, 1 part salt). If the oils solidify at 0° or 10° to 13° F., then cottonseed oil is probably present. (Pure linseed oil solidifies at 17° F.)
The hydrometer should be among the possessions of every well-regulated paint shop. It is an inexpensive little instrument, and for testing turpentine it is unsurpassed, while for the detection of cottonseed and mineral oil in linseed oil it is a quick and active agent. First test a brand of linseed oil of absolute purity; and such an oil, bear in mind, should not vary 1/2 degree from 20° to 60° Fahr. In the case of a 20% addition of mineral oil to linseed oil (the same temperature being maintained in testing both the pure and the suspected samples) the specific gravity will be 1 1/2° less than the pure oil. A 25% addition of cottonseed oil will be 1° lower. Fish oil being of about, if not quite, the same specific gravity as pure linseed oil, the adulterator can beat the hydrometer.