Hardening of Linseed Oil

The most important chemical property of linseed oil, from a painter's standpoint, is its behaviour with oxygen. Under certain circumstances, it absorbs oxygen to the extent of 13 or even 14 percent of its weight, becoming converted into a mixture of substances for which it is convenient to retain the old name linoxine. Linoxine is solid, and not liquid; it is far less soluble than linseed oil in any solvent, and in many liquids it is insoluble. Linoxine is, moreover, denser than the original oil; 100 grains of linseed oil produce about 109 or 110 grains of linoxine. Notwithstanding the greater density of linoxine, when compared with the original oil, its formation is attended by a considerable expansion. In consequence, a layer of raw linseed oil spread upon glass becomes wrinkled during the drying and oxidizing process. During the oxidation of linseed oil, the small quantity of olein it contains remains unoxidized - its presence confers elasticity upon the product.

The incidents associated with the hardening or solidifying of drying oils have always been an attractive study, but it is only through the chemico-physical researches of recent years that their true nature has been made clear. The most important points will now be briefly explained. This hardening depends, as before stated, on a process of oxidation - that is, on the absorption of free oxygen from the atmosphere. During this process carbon dioxide and other volatile organic compounds are formed and given off, while simultaneously there are produced solid, nonvolatile bodies which constitute the dried and hardened oil. These solids then, in their turn, by a further and very slow oxidation, yield other volatile products. While in the first stage, the gain in weight of the oil, due to the absorption of oxygen, far more than compensates for the loss which arises from the escape of volatile matters, in the second stage there is a distinct diminution in bulk and in weight, while the residue acquires a deepening brown hue.

This last phenomenon, however, does not seem to be inevitable, for it occurs when light is excluded; and this yellowing or embrowning of the hardened oil may often be remedied by subsequent exposure to light, although it does occur in many pictures which hang on well-lighted walls. As this change does not take place always, it seems reasonable to conclude that, putting aside discoloration through the deposition of dirt and sooty matters, it must arise by an alteration in something present which is not oil. Indeed, it seems to be traceable to the presence of lead introduced into the oil used as a siccative or dryer. As equally efficient dryers may be prepared without the employment of lead compounds, it seems desirable that these should be rejected.

If a thin layer of linseed oil be put into a bottle full of air and provided with a contrivance for observing the absorption of the oxygen, it will be seen that at first the oxidation proceeds very slowly - this is Period I. The absorption then becomes more and more rapid till it reaches a maximum - this constitutes Period II. Once more the absorption becomes slow - this is Period III. It is only during this last period that the oil loses its fluidity, becoming first viscous and finally solid. Oil examined at the close of the second period or stage dries quickly, and is often called varnish.

The processes just described, if carried out in the dark, occupy some weeks, but they may be hastened by means of several different agents. For instance, by heat - that is, by raising the temperature. It is a general law that the rate of chemical action increases proportionately with rise in temperature. Thus the higher the temperature at which the linseed oil absorbs oxygen, the more rapid does the action become. It is on this fact that the very old process of varnish-making by boiling raw linseed oil rests. The oil, heated in open vessels up to a fairly high temperature (200° C. or more) absorbs oxygen rapidly, while at the same time, principally through the overheating of the sides of the vessels used, some brown decomposition products, which give the varnish a dark colour, are formed. If the heating be stopped when the oil is in its second stage of oxidation, a product is obtained which, though still fluid, has acquired the property of absorbing with great rapidity the small amount of oxygen still needed to render it solid. Oil thus treated dries, therefore, much more quickly than raw oil, and is used when quick drying is of importance; it is known as linseed oil varnish. However, along with the advantage of quick drying, its dark colour constitutes a drawback.

But this discoloration, being due to excessive and unequal heating, can be avoided by conducting the operation of limited oxidizing at a moderately high temperature (say 100° to 150° C). It may, too, be hastened by passing a stream of air through the heated oil. In this way, not only is discoloration avoided, but the oil is actually bleached, and a nearly colourless varnish produced. If the treatment be stopped at the right moment, a sufficiently fluid varnish is obtained, while if the process is carried farther, viscous and almost solid products are formed. Naturally this mode of preparing the varnish takes more time, owing to the lower temperature employed, than the old boiling process, but the product is much better.

The second agent which may be employed to hasten the oxidation of oil is light, which acts energetically. On this fact depends the process of transforming linseed oil into varnish by exposing it to sunshine in shallow vessels, so as to facilitate the access of light, care being taken to exclude dust, while the oil is occasionally stirred in order to prevent a skin being formed on the surface through a superficial oxidation. If this skin be produced, it hinders the access of oxygen to the oil beneath, while the product is not homogeneous. It is on this action of light that a common practice of artists is based, the placing a picture in the sun that it may dry quickly. The converse practice, however, of keeping paintings soft and moist during the time when the artist is not working upon them, by excluding light from them, is less known. The best means of securing this object consists in arranging a sheet of aluminium in front of the picture so as not to touch the surface; it is light in weight, and rigid.