Church 1873/V

[Arthur Herbert] Church, Chemistry of the Fine Arts V. Water-Colour and Tempera Painting – Fresco – Siliceous Painting, in: The Technical Educator. An Encyclopædia of Technical Education III, London – Paris – New York [Cassell – Petter – Galpin] 1873, pp. 165–167.

pp. 165–167

CHEMISTRY OF THE FINE ARTS. – V.

By Professor Church, Royal Agricultural College, Cirencester.

WATER-COLOUR AND TEMPERA PAINTING – FRESCO – SILICEOUS PAINTING.

AFTER the full details already given as to the nature of the materials employed in painting, it will not be necessary to treat the chemistry of the various processes in which they are used at any very great length.

Beginning with that method of painting which involves the least complication in the process of carrying it out, we may say a few words on water-colours. Here the finely-ground pigments mixed with small quantities of gum, sugar, etc., are merely spread over the surface of prepared paper or ivory. The binding material mixed with the colours prevents any particle from being subsequently removed by friction from the painted surface, but of course water destroys any work executed by this method, the binding material being dissolved, and no part of the dried pigment and paint as such have sunk into the fibre of the painted surface, or actually soaked into and stained it. In like manner water-colour drawings do not resist the injurious influences of sulphuretted hydrogen and of other gases. The minute protective film of gum which surrounds each particle of pigment is so far from being repellent of moisture that it actually absorbs it freely, charged as it may be with all sorts of dissolved impurities. Water-colour painting, then, possesses little chemical or mechanical advantage, so far as the preservation of the pigments against injurious influences is concerned. Nor must we omit to notice another source of weakness in this process – namely, the quality of the paper, vellum, or other material used as a surface for the reception of the colour. This material must be pure and strong, and free from the china-clay, pipe-clay, whiting, or plaster of Paris so largely used now to give weight and body to inferior papers. The less ash it yields when a trial piece is burnt, the better; many of the best sorts do not yield five parts in a thousand of ash. Nor should there be any specks of metallic iron from the machinery used in the manufacture, for these fragments will rust and discolour the paper round them with iron-mould stains. But the worst impurity in paper is a residual trace of the chloride of lime employed to bleach the pulp. This will destroy or injure most of the lighter tones of the pigments used. The purer the pulp, and the higher the quality of the rags used, the better will the paper be as to strength and as to freedom from excessive hygroscopic changes. Paintings executed in the older system of water-colour painting, as practised after the middle of the last century, were, for a long time at all events, merely tinted or stained paper. But with the progressive improvement of the water-colour method and its associated materials, there came also a wonderful development of the quality and richness of the pigments placed at the disposal of water-colourists. And within the last twenty years or so the practice of this branch of art has further changed by the gradually increasing use of opaque or body-colours. These are employed as freely as in oil-painting, though it is a question how far their use should be carried. For after all you cannot glaze over opaque white with clear tones in water-colour as you can do with such success in oil; so that the two methods of painting cannot really be assimilated in this important particular, while the loading of opaque colour in water-colour painting renders the work more liable to injury. It must not be supposed, from what we have said above, that the use of water-colours is of recent date. English artists painted with transparent colours and gum on linen cloth, at least as early as the fourteenth century; while the use of honey as a medium for the colours can also be traced back to mediæval times. But as we have said, in speaking of honey, its use is dangerous, owing to its absorbent power for water, and liability to change.

Before dismissing the subject of painting in water-colours, we may properly refer to the plans in use for preventing the greasiness of surface which paper and other materials so often present. Ox-gall, the liquid contents of the gall-bladder, is its colour less decided. Solutions of carbonate of soda, of borax, and of ammonia have been employed with a similar intention. The last-named compound seems to present some advantages over the others.

Tempera painting

Tempera painting, or distemper painting, is by no means so simple a process, from a chemical point of view, as that just described. Not only does it demand the use of a prepared ground, but it involves rapid work, and sometimes requires subsequent treatment of the painted surface. Although in its widest sense tempera meant any more or less fluid medium with which pigments might be mixed, the term was generally employed in connection with the use of yolk and white of egg and parchment size. The process is a very ancient one, being mentioned even by Pliny, but in a damp climate and impure atmosphere cannot be regarded as satisfactory. The egg preparations were sometimes mixed with the milky juice (latex) of the fig-tree, sometimes with vinegar, wine, or honey. When size was employed, as in inferior works, it was prepared from washed parchment cuttings. The grounds were prepared with plaster of Paris, or size and whiting; upon these grounds the tempera colours were laid, while the finished painting was often finally saturated, in some parts at least, with boiled oil, or other kinds of varnish. Various contrivances were likewise adopted, especially in mural works, to protect the paintings from injuries arising from dampness or cracks in the walls or panels which were thus decorated. Linen, parchment, leather, and even leaves were often used for this purpose, being glued to the wall, and then covered with a coat of plaster or gesso, which was then saturated with warm parchment size, and thus made ready for painting. The generally unprotected condition of the pigments in true tempera pictures, the perishable nature of the egg and size binders, and the presence of whiting and organic substances, are the chief drawbacks in the use of this process.

Fresco

Fresco. – This method of painting demands experience, and great technical as well as artistic skill. The particles of pigment are, it would seem, bound to the surface on which they are placed by a fine film of calcium carbonate (carbonate of lime), which forms upon them. A dry wall, pure sand and lime, both free from soluble saline matters, rapid work upon the fresh plaster, as well as a pure atmosphere, are among the chief conditions of permanence of works executed in fresco. There is, indeed, an inferior kind of fresco, called fresco secco, as distinguished from buon fresco, in which the wall plaster is allowed to dry, then rubbed down with pumice-stone, and moistened with lime-water just before commencing work. All the colours must be mixed with a little lime, prepared as described in Lesson IV. This method is adapted for ornamental or decorative rather than pictorial work, and under favourable conditions possesses some degree of permanence. Yet, compared with some of the other methods which remain to be considered, fresco secco cannot be regarded as of much importance.

In true fresco, or buon fresco, the following are the steps pursued: – Clean-washed quartz sand, as white, sharp, and uniform in grain as possible, is mixed with the old lime-putty described in our former lesson. The proportions of sand and lime differ; but the surface coat consists of two parts sand and one of lime. No plaster of Paris must be used in the preparation of this ground or intonaco. It must be laid upon a surface of tooled stone, brickwork, rough dry mortar, or roughened slate, previously moistened. An air-space behind the wall to be decorated is, whose possible, a desirable precaution. It has been thought that sand may be omitted from the intonaco, if its place be supplied by crushed and sifted white marble, or even pumice. This substitution, however, may render the production of an even surface, to which an excess of lime has not been brought up by the float, somewhat difficult. Several coats of plaster are sometimes employed on the wall before the final one, which is prepared from day to day to receive portions of the design. The earliest and roughest coat may have small flint pebbles, etc., introduced into it without disadvantage. When the artist is prepared to begin work, the plasterer lays on a fine coat of intonaco, about one-eighth of an inch in thickness, upon that part only of the wall intended to be at once painted. Upon the wet, soft plaster the cartoon is laid, and the outlines and important details pounced in, or else indented into the plaster by means of a bone point. The artist then begins to paint, using hog’s bristle and other hair brushes, bone or ivory palette knives, and palettes of sheet-zinc. The various colours should be kept ready mixed, and may be preserved in earthenware pots, their exact tone when dry being ascertained by trying a little of each on a lump of dry umber or clay. It is impossible to describe in writing all those minute details of the practice of fresco which it is necessary to recognise in working. Success can only be secured by experience. Still it may be as well to note that the absorption of the moisture of the coloured tints by the ground has to be carefully watched. When this suction is too rapid the colours may very probably cease to become firmly adherent; a fine spray of lime-water, directed on to the part which has become too dry, has been found successful in remedying this accident. Before noting the pigments which may be safely employed in fresco, a word or two on the chemistry of the process may not be unadvisable. The gradual absorption of carbon dioxide or carbonic acid gas by part of the lime in the ground which has not before been carbonated, seems to be the only chemical action which takes place in a fresco painting. But the assertion has been often made and repeated that a calcium silicate (silicate of lime) is formed in the course of time by the reaction of the lime and the silica of the plaster, and that the hardness of old mortars and similar preparations is mainly due to this cause. This idea is scarcely supported by the intense hardness which mortars made of lime and non-siliceous substances often acquire, nor indeed by the analysis of the old Roman and mediæval mortar themselves. It is possible that traces of such a silicate may be found when slaked lime and sand containing soluble silica remain long in contact. But the absence of mutual decomposition in the case of mortars made of dissolved water-glass and calcium carbonate renders extremely improbable any distinct union of lime and silica in the case of the ordinary mixtures containing sand used in fresco. Mr. Barff, however, maintains* that “lime-water, in the presence of water, acts upon the sand, dissolves and unites with some of the silica of which the sand is composed, and so a silicate of lime is formed; and this silicate of lime forms, I believe, the binding power in mortar.”

* Journal Soc. Arts, xix., p. 160.

In speaking of the colours which may be safely used in fresco, we may cite one simple test to which it is easy to submit them. Let a mixture of any doubtful pigment and some strong lime-white be allowed to grow gradually dry on a piece of earthenware. Then moisten the coloured ground and see if its colour remains as at first. Some inadmissible pigments may stand this test, but it is clear that all colours which do not stand it should be at once rejected. Ochres, cadmium yellow, cobalt yellow, some varieties of vermilion, and all the iron reds, chromium oxides, Guignet’s green, some cobalts and ultramarine blues, with burnt sienna, raw and burnt umber, and browns and blacks, may be safely employed in fresco. Vegetable and animal colours, with the exception of one or two combinations of organic acids with lime or magnesia, such as the euxanthates and picrates of these bases, cannot be safely used.

Siliceous painting

Siliceous painting. – The only kind of siliceous painting which can be regarded as of real practical importance, is that invented by the late Prof. Fuchs, and largely used by other German artists. Stereochromy, which is the name applied to this art, has undergone several modifications in details since its first introduction. In giving an account of this most valuable method of painting, we shall describe the various steps of that form of the process which has proved to be the most successful.

A dry ground, free from sulphates, whether soluble or insoluble, and from all soluble matters of other sorts, is a necessary preliminary to a stereochromic painting. Some cements, such as Portland and Roman, which contain sulphates, have been, however, recommended as ingredients in the mortar ground. Rain does not injure a picture painted by this method, but damp from behind or soluble saline efflorescences will do so. The ground may be a mortared wall, a surface of terracotta, a slab of stone or slate, or such a surface as would be suitable for fresco. Such of these grounds as exhibit joints or irregularities may be rendered available for use by means of a kind of cement or intonaco, which is made by mixing precisely such sand and lime as are best adapted for securing the surface desired, which should not be too smooth or too dense. Three parts of fine sand to one of lime is found to be the best proportion generally for the intonaco, which should be one-eighth to one-tenth in thickness, and which may be laid upon a mortar or undercoat of coarser quality: two undercoats are often used, the first containing more sand, half fire and half coarse, has been recommended to give good results. Or crushed marble or limestone, or, better still, dolomite, or indeed any rocky material adapted to take the place of the sand, may be used instead of it in this cement, provided it be reduced to a proper and uniform degree of fineness in grain. The water-glass used in preparing this cement is that variety known as double water-glass, containing both potash and soda. Its preparation has been given in the preceding lesson. Unlike the water-glass which has to be subsequently employed for fixing the finished picture, the water-glass used in the mortar need not be rendered alkaline by the addition of caustic potash or soda. It will be none the worse for the presence of an excess of silica, introduced by shaking up with the solution some gelatinous silica recently precipitated. If the ground thus prepared, or any of the other grounds just mentioned, are not absorbent enough, they may be improved in this respect by, burning a little spirits of wine upon them, a very useful expedient, in dealing with damp or non-absorbent walls, in many processes of mural decoration. Another way of curing a too dense or non-absorbent surface is by washing it over with a weak solution of phosphoric acid, or of monocalcic phosphate (biphosphate of lime). The plan, however, of saturating either the wall itself, or any of the mortars or other grounds just named, with soluble silicates or water-glass, though formerly recommended, is not now practised by any of the great German authorities on this process. It is still more important that the final coat of mortar, rich in fine sand but containing very little lime, and perhaps about ten or fifteen per cent. of zino-white, should not be saturated with water-glass before commencing actual work. We are aware that Professor Barff gives (1870) entirely different directions, stating that both the under-ground and that which has to receive the painting must be saturated with soluble silicate. This, however, is a plan which the best stereochromic artists have long ago abandoned.

When the surface is dry it is prepared for work by moistening it with pure distilled water, or with lime-water, if the mortar has been spread some time, otherwise a solution of ammonium carbonate may be used. Then the colours, mixed either with distilled water, lime-water, or baryta-water, and containing, whenever possible, a little zinc-white, are laid on in the manner of fresco painting. Evenness of distribution is a great point here; the colours should not be loaded or laid on with an impasto. In the case of white draperies, these must be painted in zinc-white, the only white admissible, or the ground may be left. Cross-hatchings are allowable, yet all the work should be done as much as possible before any part of the picture is submitted to the fixing process. The use of lime-water with the colours, and in wetting the wall, enables touches to be put in one upon the other without disturbing the paint already laid on. The fixed surface, when dry, must not shine. The fixing is done with an apparatus in which a spray is produced by the violent commingling of air and water-glass solution. We have found that the little instrument called “l’odorateur,” or “la bouffée,” is admirably adapted to the work of distributing the fixing liquid upon the painted surface without disturbing the colours. It should be connected by an india-rubber tube with the nozzle of a pair of bellows or a blowing machine, the tube of the instrument dipping into a vessel of the fixing liquid kept warm. The fixing liquid is the double water-glass before referred to, and described in Lesson IV., or else pure potash silicate; but before it is used for our present purpose it should be alkalised by adding to fourteen parts of it, of specific gravity 1.2, one part of pure caustic potash solution, of specific gravity 1.33. Previous to use this liquid may be diluted with from one-half to twice its bulk of distilled water. Repeated applications of a weak water-glass are better than fewer applications of a more concentrated liquor. The fixed surface, when dry, must not shine. The success of the fixing process may be ascertained by passing a dry white cloth, and then the same material wetted, over the painting. If a little colour comes off from the rough points, it is of no consequence; but if anything like a smear occurs, it shows that the fixing process must be repeated. It is an advantage to the finished work to wash it several times with water, to remove any soluble salts which may effloresce on the surface being thus removed. There is, however, a siliceous bloom which sometimes appears on stereochromic pictures, and this no mechanical or chemical process can remove. It occurs when over-silicated water-glass is used. This hard white opaque film may, however, be rendered transparent by a process which the author of these papers used ten years ago for this purpose, namely, the application of a warm solution of copal and paraffin to the clouded surface.

The colours used in stereochromy are used in fresco. Other colours also have been employed, both by English and German artists; a few pigments even of vegetable and animal origin having been found to possess an unexpected permanence.