Whatever materials and processes may go to their making, all paintings are essentially the same in physical construction. Each is composed of a series of layers which, in the most elaborate form and moving from back to front, consist of (I) the support, (2) the ground, (3) the priming, (4) the paint layer, (5) a protective coating.
This construction is illustrated in Plate VIa, a drawing made from a photomicrograph of a fifteenth-century painting on top of which a modern forgery had been made. The support (not shown) was of old wood. On top of this was a thick ground of old gesso (plaster of Paris and size), on which in turn was a layer of paint, covered with a thin layer of varnish. The forger began work with another layer of gesso, on which he put his paint layer.
Over this he had put a thick varnish, not illustrated here since it had been removed to permit examination. The character of each layer in a painting and the way they are combined one with another, affects in varying degrees both the appearance of the painting and its permanence under different conditions. Since the number of layers may vary, and since each layer can be composed of a wide range of substances, it is easy to see that the number of possible combinations is very large, with a corresponding possibility of differences among paintings when completed. Some knowledge, therefore, of the layers of which a painting may be made up, is important for understanding why the final result of a painter’s work is what it is.
The Support
As its name indicates, this is the material that mainly holds a painting together. The only limits (and these are elastic) set to what that material may be, are (I) that it is strong enough to serve its principal purpose; (2) that it is suitable for the ground or for the paint layer which it is to carry — a metal support, for example, would probably not be chosen for a pastel; (3) that it suits the purpose for which the painting is being made.
Thus, sheets of vellum sewn together would hardly be a practical support for a 6-foot portrait. So supports have included paper and millboard in various forms; vellum; leather; all kinds of textiles, among them canvas, linen, and silk; various metals, notably copper and zinc; all kinds of wood; glass; stone of different types; walls, including those of stone, brick, and plaster; while in recent years have been added composition boards of various kinds. Sometimes two or more materials may be united to form a support. The most obvious example is the case of brick or stone walls, whose surface may be so irregular that a facing of some other substance such as wood, canvas, or plaster is necessary before the ground can be applied.
By changing the nature of the material on which the ground is laid, the influence of the original support on the appearance of the painting may be negatived, but it may still affect the permanence of the picture, and so must be taken into account in any estimate of why changes have occurred in a painting. For example, canvas applied to a wall may be affected by movements in the wall, or by damp coming through the wall.
Again, a support may be strengthened by being backed with another material. A painting on paper, vellum, or a textile may be mounted on a wooden panel; and canvas is sometimes fastened to millboard, to make what is called by artists’ colourmen ‘canvas board’. In such cases, the reinforcing material may well become a cause of changes in the painting. A panel may warp or split, and tear the material mounted upon it; millboard may decay or discolour, and stain the canvas attached to its surface.
The Ground
Sometimes the surface of the support is such that the paint layer cannot conveniently be applied to it direct. It may be so rough that delicacy and precision become impossible, and irregularities cause disturbing shadows on the paint surface; it may be so smooth or so dense in substance that the paint layer will not adhere satisfactorily; it may be so absorbent that it is difficult to lay the paint upon it; or it may contain substances such that undesirable reactions may take place between it and the paint layer. It is the business of the ground to adhere firmly to the support, to act as a buffer between it and the paint layer, and to provide a surface and consistency suitable for the reception of the paint layer.
It follows that grounds vary widely, according to the nature of the support and the kind of medium with which the pigment is mixed to form the paint. Since various types of ground are described in the chapters devoted to different processes of painting, there is no need to describe them in detail here. It should be realized, however, that a ground is not necessarily uniform, but may be composed of a series of layers of somewhat different composition. Also, the ground is not an indispensable element in a painting, since in some cases the paint can be applied direct to the support.
The Priming
This is sometimes confused with the ground. In fact, it is properly a thin layer applied to the ground to make it more suitable to receive the paint layer. For example, a ground may be too absorbent for the painter’s purpose, and the priming be used to make it less so. In such cases, the medium with which the pigment is mixed is often employed. The term priming is often used of a similar thin layer applied to the support, to prepare it for the paint layer when it is proposed to paint directly upon this. In this case, however, it is more in the nature of a ground, and serves the same purpose.
The Paint Layer
This almost invariably consists of a pigment, in more or less fine states of subdivision, held in suspension in a medium. Occasionally cases are known of dyestuffs being dissolved in a medium, and being used to stain a surface. Usually, however, dyes are precipitated on to some inert substance (aluminium hydrate is often used), the product then being ground up and incorporated in a medium. Some of the most famous colours used by painters, such as crimson lake and rose madder, are prepared in this way.
The hue and brightness of colour in a painting depends on the extent to which it absorbs some of the component elements of white light and reflects others back to the eye. Thus if it absorbs all the reds and yellows, but reflects the blues, it will appear blue. The size and the shape of the grains of pigment are among the factors that determine the amount and character of the absorption and reflection, and some pigments (e.g. azurite, a blue copper carbonate), if ground too small, lose their colour because too much white light is reflected.
Pigments used in painting may be classified in several ways, which vary according to the purpose the classification is to serve. Groupings which in varying ways and in varying degrees concern the painter and the student of painting are those based on chemical composition; the source from which the pigment is derived; its use in history; and its colour.
Chemical Composition. Here, an important distinction is that between organic and inorganic pigments. The organic are those composed of carbon with various other elements, and include the madders, carmine, crimson lake, gamboge, indigo, and the various colours derived from coal tar. The inorganic pigments include those derived from the combination of various substances with metals to form oxides, sulphides, and so on; examples being red and yellow ochre, true ultramarine blue (made from lapis lazuli), terre verte (the French form of the Italian terra verde = green earth), vermilion, and green oxide of chromium (generally called viridian). The importance of chemical composition is that it helps to determine the permanence of a pigment when used in painting.
The ideal pigment for painting should be both stable (i.e. should not change when exposed to influences which are potentially disintegrating) and inert (i.e. if mixed with other substances, chemical action between the two should not be set up). Stability and inertness, however, only have to be considered in relation to the treatment a painting is likely to receive. Extreme heat, or contact with strong acids or alkalis is not a normal risk. On the other hand, light, air, and moisture, to all of which a painting is ordinarily exposed, can either singly or in combination change the character of a pigment.
For example, very few of the organic pigments are stable, and light alone may cause them to fade, while light and moisture together are still more potent. The oxygen in the air may itself combine with certain elements in a pigment, and cause discoloration. Air, however, is also likely to contain moisture, as well as impurities such as sulphur dioxide and sulphide of hydrogen; and some of the pigments based on chromium (such as chrome yellow), red lead, and Prussian blue are then peculiarly susceptible to its action. Attention, too, has to be paid to the mixture of pigments.
Thus, sulphide pigments (e.g. cadmium yellow) may interact with copper and lead pigments (such as emerald green or red lead) to produce a black sulphide. Similarly, the chromates (e.g. chrome yellow) may lose their oxygen to various organic pigments with which they might be mixed (e.g. rose madder, crimson lake), causing both to lose their colour. Lastly, the nature of the pigment itself has to be taken into account. Asphaltum (or bitumen, as it is sometimes called), which is a tarry organic brown pigment, much used in English eighteenth-century paintings, never completely hardens, so that it develops broad cracks and in its movement is likely also to injure paint layers above or below it.
It should, however, be noted, as will be seen later, that the composition of the pigment is not the only factor in permanence, which also depends on the character of the medium used and on its ability to protect the pigment. Clearly, however, some knowledge of chemical composition will help the painter to safeguard his work against change, and will help the student of a painting to discover why a colour today, or some area of paint, does not look today as the painter intended it to look.
The Protective Coating
The medium of the paint layer is sometimes sufficiently hard and tough adequately to resist the effects of moisture, deleterious gases in the atmosphere, or of rubbing, vibration, and even blows. More usually, a protective layer has to be used. Ideally, this should be completely transparent, contain no substance likely to damage the paint layer, and be easily removable so that if damaged it can be taken off and replaced.
Sometimes the protective coating serves as more than a protection and penetrates into the paint layer so that it serves in part as a medium. An example is in oil painting, where only a little medium has been used and much diluent. When the medium has hardened, the pigment may be incompletely enclosed, and the paint surface be dull; a coat of varnish will partly penetrate the paint layer, and help to bind it more firmly together, and also make it more transparent and darker in colour. It is because of this penetration of the paint layer that in the case of certain processes, such as pastel and watercolour, a protective coating cannot be applied, without affecting the reflecting power of the pigment particles by making them more translucent and so darkening the colour.
The protective coating, like other layers, does not necessarily consist of one material only. A layer of one may be superimposed on another, as when a coating of wax is applied to the surface varnish of a painting. The structure described above of five layers, each one again perhaps composed of further layers, is that of the most elaborate type of painting.
The only layers that are indispensable, however, are the support and the paint layer. Some paintings consist of these only, others of three or four, instead of the full five layers. This fact provides a very convenient basis for classifying and describing various processes of painting. Those concerned with the simplest types naturally involve the least number of operations; and consideration of these leads on to that of the more complicated types and of how they are constructed.
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