When a painting is described as being in encaustic, pastel, watercolour, or fresco the idea is conveyed not only of certain definite materials being used, but of fairly well-standardized methods of using them; while there comes to mind some tolerably clear notion of the appearance of the painting. This holds good even for tempera, despite the confusion between egg tempera, size painting, and the difference in appearance that a coat of varnish may produce. But when a picture is said to be ‘painted in oil’ the case is very different.
In the long history of oil painting, the range of materials has been so wide and these have been used in so many different ways, that results widely different in structure and appearance have been produced. A spectator confronted by examples of, say, the fifteenth-century Flemish School, the sixteenth-century Venetian School, and the French Impressionist School, and being told that they are all ‘oil paintings’, might well wonder whether the term has any meaning. In fact, the only justification for grouping such diversities under one technical heading, is that oil, in some form or in some way, has played a considerable part in their making.
Before, however, discussing the different uses of oil at different periods of oil painting, a few definitions are desirable. The oils used in painting are the ‘drying oils’. The term ‘drying’ has not the same significance as when used in domestic washing; but means that the oil when exposed to the air is changed into a hard substance as a result of oxidization combined with certain obscure molecular changes. The contrast is with the ‘non-drying’ oils, such as castor and olive oil, which do not harden in this way.
Best known and most widely used by painters is linseed oil. Walnut and poppy-seed and other oils have been and are still used, but involve no substantial difference in methods or results. If a drying oil is mixed with certain metallic salts (e.g. those of lead), the process of oxidization is hastened, and these salts are therefore known as driers, and since the mixing is usually effected by heat the product is known as boiled oil. Artist’s oils when first prepared are yellowish, and are therefore sometimes bleached by exposure to sunlight or by chemical means.
One important point is that oils, especially linseed oil, may become yellow with lapse of time, though it is sometimes alleged that the oil can be so treated as to prevent this. In consequence, any painting in which oil has played a considerable part, is apt to become yellow or brown in tone as the oil changes colour. This helps to account for the ‘old master’ tone of many old pictures, which used to be so much admired that it: was often simulated by applying a layer of transparent yellow paint.
An oil may sometimes be too thick for convenient manipulation, in which case it may be mixed with a volatile fluid such as turpentine or benzene, known as a diluent, to make it flow more easily, the diluent, of course, evaporating in time. This practice became more common with the increased use of ready-prepared paints, since when artists prepared their own oil and ground their own colours, the viscosity of the paint was largely under their own control.
Another important constituent in many oil paintings is varnish, used either as a medium, either by itself or mixed with oil, or as a protective coating to a painting. Varnish consists of a resin dissolved either in a spirit such as alcohol, turpentine, or naptha, and known as a spirit varnish; or in a drying oil, such as linseed, and known as an oil varnish.
The resins used in the past cannot always be identified. They include, however, soft resins and balsams from living trees such as mastic, damar, sandarac, Canada balsam, and Venice turpentine; and hard fossil resins such as copal and amber. The extent to which these are soluble in oil or spirit varies, and on this, the type of varnish depends. Apparently spirit varnishes did not come into use until the sixteenth or seventeenth centuries; and before that, oil varnishes were the rule.
The early Italian painters, it may be noted, used to call the resin from which the varnish was prepared vernix, and the varnish itself vernice liquida; a distinction to be borne in mind in reading early. recipes. In addition to the natural varnishes, as they may be called, are those prepared from synthetic resins, well-known examples being the vinyl resins. These have their chief employment in industry, but are today being used to some extent by painters, and to an increasing extent in the conservation of works of art. Ordinarily, however, the student of the history of painting is not concerned with them.
A third term which it is convenient to define here is emulsion. This is a combination of two liquids which in the ordinary sense cannot be mixed, such as oil and water. If these are stirred together, drops of the one will be suspended in the other; but if nothing more is done, the drops will come together, and the two liquids will separate. To prevent this an emulsifying agent is introduced into the mixture, which has the effect of surrounding the drops and preventing their coalescing. A familiar example of an emulsion is egg yolk in which particles of oil are suspended in albumen.
Three other terms concerned not with materials but with manipulation, which will often be used in the following pages, are glaze, scumble, and impasto. A glaze is a layer of transparent paint, through which the light passing to the surface beneath is reflected back, so that the colour of the glaze modifies that of the underlying colour. The essential characteristic of a glaze is its transparency, and not (as is sometimes said) the thinness of the layer.
In glazing, therefore, the pigments used are mainly those which are themselves transparent. A scumble is also a layer of paint used to modify the colour of the surface to which it is applied. But the pigments used are opaque, so that this surface is partly concealed; and the main characteristic of a scumble is, therefore, that it should be thin enough for some light to penetrate it, and be reflected back from the surface beneath.
Impasto (derived from the Italian impastare, to make a paste), is the term applied to a thick, opaque, area of paint which stands up above the surface to which it has been applied. For example, ‘a heavy impasto in the lights’ means that the high lights in a painting have been put in with a solid mass of paint, which contrasts with the comparative smoothness of the rest of the surface.
One thing to be kept very clearly in mind is that ‘oil painting’ was not invented by any one person, despite what Vasari says about it in his life of Antonello da Messina in the Lives of the Painters. This account is worth quoting, since it is a useful starting-point for discussion. After mentioning the disadvantages of tempera, and unsuccessful attempts made to overcome them, he proceeds:
It happened, therefore, when matters stood at this pass, that Giovanni da Bruggia 1 set himself to try different sorts of colours; and being a man who delighted in alchemy, he laboured much in the preparation of various oils for varnishes and other things, as in the manner of men of inventive minds such as he was.
Now, it happened upon a time, that after having given extreme labour to the completion of a certain picture, and with great diligence brought it to a successful issue, he gave it the varnish and set it to dry in the sun, as is the custom. But, whether because the heat was too violent, or that the wood was badly joined, or insufficiently seasoned, the picture gave way at the joinings, opening in a very deplorable manner.
Thereupon, Giovanni, perceiving the mischief done to his work by the heat of the sun, determined to proceed in such a manner that the same thing should never again injure his work in like manner. And as he was no less embarrassed by his varnishes than by the process of tempera painting, he turned his thoughts to the discovery of some sort of varnish that would dry in the shadow, to the end that he need not expose his pictures to the sun. Accordingly, after having made many experiments on substances, pure and mixed, he finally discovered that linseed oil and oil of nuts dried more readily than any others of all that he had tried.
Having boiled these oils therefore with other mixtures, he thus obtained the varnish which he, or rather all the painters of the world, had so long desired. He made experiments with many other substances, but finally decided that mixing the colours with these oils, gave a degree of firmness to the work which not only secured it against all injury from water when once dried, but also imparted so much life to the colours, that they exhibited a sufficient lustre in themselves without the aid of varnish, and what appeared to him more extraordinary than all besides was, that the colours thus treated were much more easily united and blent than when in tempera.
Hits: 82