Pastel, Charcoal, Plumbago, Silver-Point

The common characteristic of all the processes which form our sixth group is the absence of any vehicle or binding material. The usual ground on which drawings in the above-named substances are executed is paper (Chapter I.); but as the hold of coloured chalks and of charcoal is very precarious, the paper is generally mounted on some comparatively rigid backing, such as millboard, cardboard, copper, or panel. If a chalk or charcoal drawing be carried out on paper which has first received a wash of gum-water or of dextrin-solution, it is easy to effect a partial fixation of the powdery pigment by subsequently steaming the finished work, although it is usual to employ a fixing solution in the form of very fine spray to the finished drawing. For pastel work a specially prepared paper is now generally employed. This has a surface of finely-powdered pumice, which affords an efficient tooth, and helps in securing the coloured chalks or clays. This result is further aided by the plan of working in and mingling the pigments by means of rubbing with the fingers and the palm of the artist's hand. Pastel-paper is often made of inferior pulp, and lacks strength.

It should be less sized than paper intended for water-colours. Pastel colours are generally made with a basis of purified chalk or pipe-clay mingled with the usual pigments in powder, a slight degree of cohesion being secured by making up the crayons with starch-paste or gum-tragacanth.

¶ For fixing pastel-drawings it is convenient to use the following medium: Pound 15 grams dry casein and 3 grams of borax together, and then shake the powder with constant stirring into 100 cubic centimetres of distilled water. After some hours a syrupy mass will have been formed. Dilute this with more water so as to make the liquid up to 750 cubic centimetres; then add 250 cubic centimetres of spirits of wine. After a time a white precipitate may form; pour off the somewhat opalescent liquid from this sediment. This fixative is to be sprayed on to the face of the pastel, care being taken to prevent the liquid from gathering in actual drops upon any part of the drawing. When the surface looks moist and shiny, it shows that it has been sufficiently dosed with the fixative. The more completely the ground is protected by the colour laid on, the less risk there is of the fixation affecting the appearance of the picture. If, however, the effect of the work has been obtained by a mere whiff of the powdery pigment, it is wiser to omit the fixing procedure, for the delicacy of such very fine layers of colour would thus be impaired.

In any case, the artist who is concerned for the permanence of his work will always try to obtain his effects by building up as solid a layer of colour as possible.

¶ Pastels, as already mentioned, containing no binding material, or next to none, drawings made with them are exempt from the drawbacks inseparable from the use of vehicles. Consequently there is no fear of the surface cracking, darkening, blooming, becoming brown, or otherwise altering. When we further consider that pastel-drawings, unlike water-colours, depend for their effect on the presence of a fairly solid layer of pigment, and that many colours which are unstable when employed in other methods of painting, have proved to be durable in pastel, we are bound to admit that this beautiful technique is not only simple in method but expressive in the effects which it commands; but is only capable of producing drawings which last better than most others, provided they are protected by glass, and are not exposed to damp.

¶ Since the pastel crayons of the shops bear usually no indication of the pigments employed to colour them, and frequently contain unstable coal-tar dyes, special care must be taken to test their permanence when exposed to light. To do this it will suffice to expose to direct sunshine in bright weather one half of a strip on which clear tones of the set of coloured pastels which we wish to use have been spread and fixed. It is possible, if the sunshine be strong, to detect the more alterable pastels after a few days' exposure. For serious work the artist should use only such pastels as have stood the test. It is fortunate that, owing to the absence of any medium, chemical interaction between pastel-pigments when mixed together in the process of painting is virtually non-existent.

Details concerning the making at home of pastel-crayons will be found in W. Ostwald's 'Letters to a Painter,' English edition, pp. 22-27. Here we need add only the following memoranda: Excellent pastel-grounds may be prepared by laying, on Bristol board or stout drawing paper, a thin and even coat of powdered pumice mixed with liquefied starch (see p. 95). Or the same coat may be spread upon a surface of a fine fabric, such as thin calico, linen, or silk, previously secured to the board or paper by means of starch-paste.

In plumbago (lead-pencil) and silver-point work, the mechanical adhesion of the coloured particles is naturally less imperfect than in pastel, a portion of the plumbago or silver becoming, in fact, incorporated with the fibres of the paper-ground. This is particularly the case with silver-point, in which method the ground receives a particular preliminary preparation. One of the best materials for this purpose is Chinese white (oxide of zinc). An even wash of this pigment in the form of 'moist' water-colour is first spread over the paper. As a silver-point drawing is often heightened with touches of Chinese white, it is desirable to bring these into prominence by tinting the ground. For this purpose a small quantity of some permanent pigment is mixed with the wash of Chinese white. Yellow ochre, raw umber, green oxide of chromium, Mars violet, ultramarine with a little ivory-black, may be used. The 'tooth' of surface which increases the attrition of the silver-point is, however, furnished by the presence of the Chinese white. It should be added that the silver used should be free from any alloy of copper, which hardens the metal, but may advantageously contain a few per cents. of metallic lead; an alloy of 2 parts of lead with 1 part of tin was sometimes used instead of silver.

The silver in silver-point drawings is liable to become brown from the sulphur compounds in impure air. The blackening of the high lights in old silver-point drawings is due to the tarnishing of the lead white employed; it may be got rid of by keeping the drawing for some time in an atmosphere of moist ozone, or by a careful treatment with a solution of hydrogen peroxide in ether.

A pointed pencil of pure gold is occasionally used instead of one of silver or graphite. It produces on a prepared paper surface the same grey line, and is theoretically a perfect material for drawing purposes. But, strange to relate, gold-point drawings have been observed to suffer change, becoming nearly invisible, not by reason of any chemical action on the metal, but in consequence of a rearrangement of the metallic particles whereby their grey hue disappears and the original yellow lustre of the gold is resumed. Drawings in platinum-point are not susceptible of this change. The platinum employed must be pure, otherwise this metal is too hard for comfortable manipulation.

Pastel or coloured chalk drawings frequently show a higher degree of preservation, so far as certain hues are concerned, than contemporary works executed in oil. One can easily account for the pure and fresh air of old pastel drawings, knowing that they have been carefully mounted and framed, and that there has been no oil or resin to yellow and darken the pigments. But how can the remarkable state of preservation in which the 'carnations' are found in so many examples be explained? Has the intimate commixture of chalk or of clay with crimson lake preserved the latter from the destructive action brought about by light? If there have been such a preservative action, has it been physical rather than chemical? Answers to such questions must be reserved until the chemistry of coloured pastels has been thoroughly studied. It should, however, be recollected that the white basis of coloured pastels is not always the same. In the eighteenth century it seems to have been invariably purified chalk, that is, 'whitening' or 'whiting,' which is essentially calcium carbonate. But, on examining lately a well-known make of French pastels, a considerable percentage of calcium sulphate was recognised, in addition to chalk.

Further experiments seem to show that the colouring matter used is first ground up with a mixture of chalk and plaster-of-Paris, and that, in consequence, the subsequent addition of water causes the whole to set into a mass of just sufficient tenacity to hold together, though very soft and fragile. In this way the use of starch-water as a binding material is obviated. In other pastels pipe-clay or china-clay has been employed as the basis for the colouring matter.

Paintings and drawings executed in fresco, in tempera, and in water-colours, may be protected from the hostile attacks of impure air and moisture by applying to the finished work a coating of pure hard paraffin-wax. If such an after-treatment is contemplated in the case of a work executed in water-colour the amount of vehicle employed (gum, etc.) should be reduced to the necessary minimum. The mode of applying the paraffin-wax is described further on at the end of Chapter XXV (Conservation Of Pictures And Drawings)., pages 356-357.