Nut Oil

This oil is obtained from the kernels of the common walnut, Juglans regia. Leonardo da Vinci directs it to be made from the peeled kernels in order to avoid the chance of darkening its colour, and also causing the subsequent alteration of the tone of the pictures painted with it. The kernels were to be soaked in water first, before being peeled and pressed. The introduction of nut oil into painting followed that of linseed oil, and preceded that of poppy. Cold-pressed nut oil is much paler in colour, and has much less taste and smell than the hot-pressed oil; it also differs in composition much in the same way that cold-pressed differs from hot-pressed linseed oil. The constituent glycerides of nut oil are the same in kind as those of linseed oil, but a larger proportion of linolein is present. Nut oil closely resembles linseed oil in its physical characters; its specific gravity, .929, is intermediate between that of linseed and poppy oil: cold pressed oil from the Black walnut (Juglans nigra) has the specific gravity .922, and is quite as good for painting purposes as oil from the common walnut. Besides the three drying oils already described we may name that expressed from niger-seed, Guizotea oleifera.

It is occasionally employed in grinding artists' colours as a substitute for linseed and poppy oil. Tea-seed and camellia-seed oils, and the oils extracted in Japan from the seeds of Perilla ocimoides and from the kernels of Torreya nucifera, are not of sufficient importance to demand description. There is, however, one remarkable drying oil of recent introduction which ought to be named here. This is Chinese Wood oil or Tung oil obtained from the seeds of Aleurites cordata. It is distinguished from the oils we have been discussing by the change which it undergoes when heated to about 282° to 285° C. After having been maintained at this temperature for a few minutes the oil becomes a gelatinous mass, firm and free from stickiness. Tung oil is heavier than any of the oils hitherto described. It is probable that it may find certain applications in artistic painting, for it has been shown to yield a durable film when oxidized.

A few observations may now be offered as to (1) the action of certain pigments on oils; (2) the different amounts of oil needed for grinding with different pigments.