Pigments, Paint and Painting: A practical book for practical men

by George Terry

3. Boiled oil, containing lead, is coloured brown by exposure to air, owing to the presence of minute quantities of sulphuretted hydrogen, which causes the formation of lead sulphide.

The remedy is obvious: no oil should be used which has been boiled with dryers containing lead. In other words, oil should be boiled with pure manganese compounds only.

In cases where it is desirable to have information of the presence or absence of lead in a boiled oil, the following test will be found most useful:—A mixture is made of 4 oz. of glycerine with 1 oz. of ammonium sulphide, the liquid being kept in a stoppered bottle. Or glycerine is mixed with an equal volume of water, and saturated with sulphuretted hydrogen. Half an ounce of the oil to be tested is placed in a white basin, with the addition of two or three drops of the glycerine solution. The two liquids are thoroughly incorporated, by stirring with a strip of glass. A brown or black colour, which gradually appears, indicates the presence of lead. A pure manganese oil simply becomes slightly yellow. It is true that, if iron is present, a black colour might appear, but iron is also an undesirable impurity. Should it be required to ascertain that the coloration is or is not caused by iron, two or three drops of glacial acetic acid may be stirred into the oil, when, if the black colour remains, it is certainly not caused by iron.

Under the old process of oil-boiling at a high temperature, the brown colour of the oil was, to some extent, an indication that the oil had been sufficiently heated—that is to say, properly boiled; but in the modern processes, so largely used, in which oxidation is aided by a blast of air, this coloration is no indication whatever of the excellence of the oil; it may be, in fact, the very reverse.

This fact appears to be unknown, or, at any rate, is not a matter of common knowledge among practical men in this country, who, being uninformed as to the methods of preparing the oils, consider that a brown colour is desirable, if not essential.

When oil-boilers were compelled to adopt some expedient to give a reddish-brown colour to the oil, they added a small amount of litharge, the introduction of which actually spoils the oil, and makes it unsuitable for many purposes to which it is otherwise applicable. Of late years, pale boiled oils have been more largely manufactured for special purposes. It is obvious that, for decorative house painting, in which delicate tints are a leading feature, they may be advantageously employed.

Notwithstanding that some of the brown oils, when mixed with white lead, do not entirely retain the brownish tint, but, to some extent, lose it upon drying, yet they never preserve the whiteness of white lead. It follows, therefore, that a pale colour in the oil, provided it is not the yellow colour of raw oil, is greatly to be preferred. Moreover, when paints are mixed with zinc white, no trace of lead should be contained in the oil, otherwise, one of the valuable properties of zinc white pigments is destroyed, namely, its power to retain its whiteness in the atmosphere of a town, because its colour is not affected by sulphuretted hydrogen.

Very generally, zinc white and white lead paints are not mixed with drying oils, but with refined linseed or bleached oil. This, at any rate, is the practice on the Continent. That is to say, the pigments are mixed with an oil from which the impurities, and the natural yellow and red colouring matter, have been removed, so that the colour of the paint is white. If ordinary oil be used, the paint is more or less yellow. In order to render such paint quick drying, a certain amount of dryers, in a solid or liquid form, is added. These dryers almost invariably contain lead, so that zinc white paint is contaminated by lead in another way, which may not be suspected, or which is overlooked.

Now as to the chemical action of dryers on oils. Raw oil contains water and mucilage; the former can be absorbed by anhydrous zinc salts and by dried alum, and solutions of the salts and the salts themselves are capable of precipitating mucilage from the oil; hence these substances cause the impurities to become insoluble, so that they are carried down as “foots.” Heat greatly facilitates this action, particularly by causing the oil to become more fluid; and by the action of the anhydrous salts water is withdrawn from the oil. On the “drying” or oxidation of the oil, they exert no chemical action whatever.

Zinc linoleate and lead linoleate do not act as dryers when simply added to the oil. Though the former is soluble in hot oil it is insoluble in cold oil, and it therefore separates from the oil as it cools. The latter is very soluble in linseed oil, but only adds to its drying power when heated therewith.