Linseed Oil, of all vehicles used to form, with pigment, paint, linseed oil is the most important, and the painter cannot know too much about this substance, of which from 30 to 35 million gallons are produced and consumed annually in this country alone, mainly in the manufacture of paints, varnishes, oil cloths, and linoleums.

There are, of course, many other drying oils more or less adapted for paint, but with few exceptions, little used in general practice. We shall consider the exceptions a little later.

Linseed oil is obtained from the seed of the flax plant, cultivated sometimes for its fibre, otherwise for its seed, from which to produce oil. Flax is largely grown for the latter purpose in the region about the Baltic Sea, in Russia, in Egypt, India and North and South America. The domestic supply comes mostly from the southwest, middle west and northwestern States. Seed of average quality contains about 39 per cent. of oil, the residue being organic matter; cellulose, phosphates, etc. The quality and yield varies considerably, depending on the nature of the seed, the presence or absence of other seed, such as dotter and wild mustard, the conditions of ripening and the locality in which it is grown.

The process of manufacture is as follows: The seed is ground to a meal and pressed cold or hot, or the oil is extracted from the meal by the use of a suitable solvent, such as benzine or carbon bisulphide. The solvent process is used in producing the so-called new process oil; old process oil being simply meal hot or cold placed in bags and the oil obtained by hydraulic pressure. When heat is used, from 30 to 35 per cent. of oil is extracted, while by the cold process barely 25 per cent. of oil can be obtained. Cold pressed oil is, without question, superior to hot pressed oil for paints and varnishes and may account in part for the better wearing qualities of the paint applied years ago, when cold pressed oil was in general use. There is now practically none in the market. Cold pressed oil has a pale yellow color and a pleasant sweet taste, while hot pressed oil is darker and has a stronger, somewhat biting taste. Evidently more mucilage and substances of no value, of perhaps, some detriment, are produced when heat is used to increase the yield.

Linseed oil is soluble in turpentine and benzine in any proportion, but very slightly soluble in alcohol. It boils at about 446 degrees Fahrenheit. On boiling several hours, it becomes a thick syrup; with dryers added, and moderate boiling, it becomes true boiled oil as met with commercially. It forms an emulsion with water on the introduction of a weak alkali and saponifies in stronger alkalies. The drying property is due to its power of absorbing oxygen from the air which causes the oil, when spread in thin layers, to oxidize and become a hard, neutral substance. Its specific gravity at 60 degrees Fahrenheit is 0.932. For ordinary paint it is used raw with the admixture of liquid dryers and is more durable than boiled oil, because the oil in being boiled has already been partly oxidized and when oxidization of oil is complete and the oil becomes hard, it tends to brittle-ness and eventually perishes, leaving the pigment to chalk off. Raw oil, likewise, works easier under the brush and is less likely to blister. Furthermore, boiled oil is likely to be adulterated with rosin, rosin oil, and oils of mineral origin.

Boiled oil is commonly produced by adding cheap heavy bodied rosin or oil dryer to oil cold, or slightly heated, so that you seldom get real boiled oil anyway, except from a varnish factory.

Raw oil is adulterated with cotton seed oil, mineral, rosin, and occasionally fish oil. Its low price, however, precludes much adulteration with other vegetable or animal oils. It takes fire at between 600 degrees and 700 degrees Fahrenheit. If mineral or rosin oil is present, it flashes at a lower temperature.

Pure linseed oil may vary in composition. It may contain a large percentage of foots, mucilage and other foreign and non-drying substances, and it may contain an excess of water, which hinders drying also. To be at its best, it should be tanked and well settled for a few months. Linseed oil is refined or decolorized by agitation with a small percentage of sulphuric acid, by mixing it with fuller's earth and filtration, and with the use of peroxide of hydrogen, bichromate of potash and in various other ways. Refined oils should not be used for outside painting, being adapted only for use in varnishes and similar products or for use in the arts, where paleness and freedom from mechanical and other substances is necessary.

Tests For Purity Of Linseed Oil

Place one drop of strong sulphuric acid on ten drops of oil. If pure, the oil will pass in color from orange through red to brown. Rosin oil, if present, gives a brown color at once turning to black. If it contains fish oil, the first color is violet.

Rosin oil may be detected by the increase in density of the oil and by the odor and general appearance.

If a sample of linseed oil containing mineral oil be spread on glass and allowed to dry, if much mineral oil is present, it will sweat out of the drying layer in drops or form a wet greasy surface on the drying oil and tend to turn the oil yellow.

Another test for mineral oil is to dissolve a small portion of the sample in glacial acetic acid. Linseed oil will dissolve, but mineral oil will not in this acid.

Much oil from Calcutta seed was formerly used in this country. The better grades of domestic oil as now produced have forced it from the market. For practical purposes it is not worth the difference in cost.