The superiority of the highly siccative oils prepared with borate of manganese (or the oxalate, resinate, or linoleate) over those in the manufacture of which lead compounds are used, is so decided that all description of the older and less satisfactory methods will be omitted. But there are two other ways of rendering linseed oil more siccative, which deserve a passing notice. Into a clear-glass quart-bottle an ounce of distilled water and an ounce of clean iron brads are first placed, and then one pint of raw linseed oil, agitation being avoided. The next day, the bottle, placed in as strong a light as possible, is to be shaken frequently, the shaking being repeated every day, until a drop of the oil, when tested, shows a sufficient degree of drying character. Finally, the liquid part of the mixture in the bottle is poured into a separating-funnel, and the aqueous part allowed to run away. The oil may require drying and filtration. In another similar process green vitriol is substituted for the metallic iron, the other directions being identical.

The most important property of linseed oil, and some methods for the further development of this property having been discussed, we may now describe the remaining characters of this oil. The cold-pressed oil is very pale straw-coloured, or pale yellow, with occasionally a faint greenish hue; the hot-pressed oil is a darker yellow or brown. The cold-pressed oil, when considerably cooled, remains clear long after the hot-pressed oil has become turbid. The fluidity of the oil is less than that of water in the ratio of 1 : 10. The hot-pressed oil has a much stronger taste and odour than the cold-pressed oil.

The adulteration of linseed oil with other oils may be recognised with more or less precision by means of several different tests. Most of these tests (oil of vitriol test, nitric acid test, etc.) produce reactions in which the oil and the acid acquire varied colours characteristic of different oils. These tests must be applied under exactly similar conditions of temperature, agitation, lapse of time, strength of acid, etc.; and even then, unless the experimenter is well-versed in the work, the indications obtained are sometimes perplexing and difficult to interpret. The amount of iodine absorbed by a given weight of linseed oil is also a measure of its drying power as shown in its capacity for absorbing oxygen. This 'iodine-value,' as it is called, is the amount of iodine absorbed from chloride of iodine in the presence of glacial acetic acid, by 100 grammes of oil. The iodine-value for linseed oil is somewhere near 200; the figures for walnut oil and poppy oil are always lower, while the semi-drying and the non-drying oils may not show half this value, and these are the oils likely to be used as adulterants. But such quantitative determinations can be properly performed only, by the skilled chemist.

There is another test known as Valenta's acetic acid test, which has been used for the detection of non-drying oils in linseed oil. It is based upon the less solubility of the former oils in glacial acetic acid. It is, however, not easy to secure constant results with this test. Determinations of volatile matters, of unsaponifiable substances, and of insoluble bromine derivatives, afford valuable indications as to the purity of samples of this oil.

The specific gravity of linseed oil also affords a valuable means of testing its purity. At 15.6° C. (60° F.) it is denser than most other vegetable oils:

The four oils marked with an asterisk are practically non-drying.