Vegetable oils used in painting.

Scientific American 9, 27.2.1869

There are two kinds of oils found in plants, called respectively volatile, or essential oils, and fixed oils. The former are those of which essences and extracts are made, and are called volatile because when exposed to air they will, like ether or alcohol, entirely evaporate. The fixed oils, on the contrary will not evaporate, hence their name. The latter are divided into two classes, unctuous, or greasy oils, and siccative, or dryingoils. The drying oils are of great value in the arts, their principal application being in the art of painting. They are the vehicles for the distribution of colors over the surfaces of materials which it is desirable to ornament or to protect from the chemical action of external substances. Thus used they perform a two-fold office, as beside enabling the colors to be uniformly spread upon any surface, they form of themselves a protective coat owing to their siccative properties.

The sources of the siccative oils are numerous. They exist in the seeds of the order of plants, called by botanists Linaceae, commonly known as the flares. Of these a species is grown in the East Indies, and large quantities of the seed are imported to this country from that source. The plant is also largely cultivated in Ireland, Holland, America, and other places, not only for its fiber, but the seed. The oil obtained from flaxseed, commonly known as linseed oil, is an important and valuable article of commerce, and is sold in two states, called raw and boiled.

Beside the flares numerous other plants produce seeds containing siccative oils. Of these the hemp, poppy, sun-flower, and many nut-bearing trees may be mentioned. Indeed good nut-oil, according to some authorities, possesses the siccative property to a greater extent than any other.

The fixed vegetable oils are either cold or hot expressed. The former are the best oils, but the latter are much used, as a better yield can be obtained by the use of heat, and conse-quently they are cheaper; while if too high a degree of heat is not used, their quality is not very seriously impaired.

In extracting these oils, the seeds are ground under heavy stone rollers, revolving upon an axis which passes through an upright shaft. As the outside of the rollers must travel faster than the sides nearer the upright shaft, a rubbing as well as crushing effect is obtained. The meal thus produced is subjected to enormous pressure, and the oil is squeezed out. This is the raw oil of commerce. The siccative property of this oil, as of all other drying oils, depends upon the effects of oxygen upon it. When exposed to the air, it absorbs oxygen and becomes resinous in its character. This is drying in one sense, but not, as is often supposed, drying by evaporation. The latter takes place when any substance parts with its liquid portions, or that which holds its solid ingredients in solution. Oils, on the contrary, dry by absorbing oxygen and combining with it to form resinous substances nearly allied to the well-known resin obtained from pine. Cold solidifies linseed oil, and most other drying oils. They therefore spread better in in a warm temperature. The siccative property of linseed oil is increased by heating it with litharge. It was formerly thought that the increased drying property of linseed oil, when heated with litharge, depended solely upon its combination with the oxygen contained in that substance, and it would dry quicker when exposed to atmospheric action. But, according to Liebig, the principal use of the lead oxide is to precipitate the mucilaginous and albuminous matters contained in oils, which, when present, interfere with the action of oxygen.

Linseed oil is used not only in painting but in the manu-facture of printers' ink, varnishes, oilcloths, etc. When adul-terated with fish oil, the presence of the latter may be detect-ed by robbing a small quantity in the palm of the hand; the smell of the fish oil can then bo detected. It is also used in the manufacture of linoleum, which is a combination of the oxidized oil with resinous gums and other substances, possess-ing the appearance and many properties of india-rubber. This substance can be vulcanized like rubber, and is applicable to very many purposes in the arts.

Many painters suppose that it is necessary to use "dryers" in paint, as litharge, dissolved usually in linseed oil by the aid of heat. It has, however, been demonstrated by Chevreul that these substances are not essential to make paint dry. He performed the following experiments:

Four oak strips were painted, each on one side, with a paint composed of white lead and linseed oil, and on the other side with a paint composed of white zinc and linseed oil. The strip No. 1 was exposed to the air to dry; No. 2 was put into a bottle of the capacity of two liters (3.52 pints) and closed; No. 3 was put into a similar bottle, containing dry oxygen gas; No. 4 was put into a similar bottle, containing dry car-bonic acid gas. The results as to drying were examined after twenty-four hours, and again after 72 hours:

After twenty-four hours the lead paint on No. 1 was almost dry; the zinc paint had set, but was not dry. On No. 2, the lead paint was almost dry; the zinc paint had set, but was not dry. On No. 3, both the lead and the zinc paints were perfectly dry. On No. 4, both paints were still wet and fresh, and had undergone no change.

After seventy-two hours the paints on Nos. 1 and 2 were perfectly dry. The lead paint on No. 4 had almost set, but it had no adhesion to the wood, and could be easily removed by friction; the zinc paint had undergone no change, but stuck to the finger like fresh paint.

These paints contained none of the so-called dryers, yet when they came in contact with free oxygen they dried perfectly. But while it is thus shown that dryers are not absolutely essential, it is none the less true that their use greatly facilitates :the setting and drying of paint, a very desirable thing under many circumstances.

Any admixture of non-drying, or unctuous oils, in the oils used for painting renders them " tacky " when spread upon any surface. A good test of their presence is, therefore, their behavior in this respect when their layers are exposed to the atmosphere or oxygen in a closed vessel.

It is the affinity which such oils possess for oxygen that renders them liable to talre fire spontaneously when spread over the fibers of wool or cotton waste, by the heat resulting from the slow combustion which takes place uncles such circumstances. Even animal oils, similarly treated, are liable to pontaneous combustion.