The drawbacks to the use of the oil medium for wall-painting are several. The most evident is the condensation of vapour upon the surface. The more impervious the surface is to damp the more liable it is to be affected by damp from behind, which will, unless special precautions are taken, detach portions of the painted surface from the wall or burst through it in patches of efflorescence. The history of Leonardo da Vinci's "Last Supper" at Milan shows how fatal its use is, even in the hands of a master in art and science, and it is a matter for lasting regret that he did not trust to the older fresco medium with which the great " Crucifixion" • at the other end of the refectory, still in excellent preservation, was painted. Another disadvantage is the darkening to which colours ground in oil are liable and the shininess of the surface in parts, which is very likely to occur notwithstanding careful surfacing with wax and turpentine. It is, however, the favourite medium of the French, and since the application of "marouflage" to the fixing of large canvases on walls it is rare to hear of paintings being executed on the wall in France. This process makes it possible for the painting to be executed entirely in the studio and even exhibited in a picture-gallery (with doubtful benefit, since it cannot be in harmony with its surroundings there) before being placed in the position for which it was designed. Professor Max Koch gives his opinion very clearly as to the advantages of painting on plaster in situ and the disadvantages of the oil medium and • marouflage" thus: - The oil medium so much loved by the French is quite the least suitable technique for wall-painting. At first great surfaces shine so brightly that the beholder can only see a part of the picture clearly and then they darken with time surprisingly. Water colours give a much clearer, lighter tone of colour, do not shine, do not crack, and do not darken, and if properly handled are more lasting than oil." « For interior work one may either paint on the wall or on stretched canvas. I have made many studies in my various journeys, and have come to the conclusion that plaster painting is far the best for every reason. To me it is imperative to paint in the given place. In the studio one is never in the position to think of the relative spacing. I remember how a colleague (who was an easel painter) painted a nine-foot figure for the ceiling of an opera house on canvas; when it was up, sixty or seventy feet away, he had to look with an opera-glass for the beauties which were invisible, though they had looked so pretty in the studio. He had to paint his figure twice on account of this miscalculation. The French generally paint their decorative pictures on canvas, while the Italians always worked on the wall, much to the benefit of decorative art, to my thinking".
The operation of "marouflage" has been developed from the " lining " of pictures by picture restorers. It is thus described in La Orande EncyclopMie: " Application to a canvas, panel, or ceiling, of an oil painting on canvas. 'Maroufle' is used for this purpose; a kind of glue, very concentrated by cooking, partly formed of the remains of brush washings, the tenacity of which is extreme." M£rime£ advises for the "lining" of pictures for damp situations the use of linseed oil thickened by long boiling, with which white lead and a little very fine red lead is to be ground. Let this get half dry and then lay the canvas down very carefully, pressing from the centre towards the edges with broad palette knives or a roller.
Professor Church says that the paintings in the Eoyal Exchange are " maroufi£ " on slate slabs slightly inclined forward at the top and with a ventilated airspace behind. The composition used was a thick paste of white lead, oil, and copal varnish spread on the slate and the back of the picture at the same tima " It may be affirmed that paintings so secured are free from all risk of injury from the back," but of course the exposed surface requires protection either by some unaltering substance, such as paraffin wax, or by a material which can be removed when dirty and replaced without the surface of the paint being affected. Whatever the medium employed the air-space behind the surface on which the paint is applied very much increases the likelihood of its permanency, and if back and front are both protected and the colours used so as to avoid chemical changes, one is inclined to think that there can be no deterioration. If slate is used as a ground for spirit fresco or for oil painting, it must be free from crystals of iron pyrites (of a brass yellow colour). The firm adhesion of the priming may be secured by slowly warming the slate in a water oven, and then immediately covering it with a very thin coat of oil copal varnish largely diluted with turpentine and applied warm. When this is hard the painting may be carried on, or a priming of flake white in oil mixed with copal varnish and turpentine may be applied. Terra-cotta and stone may be treated in the same way, Professor Church says, and slate may be prepared for using Keim's process upon it by warming as before and then rubbing with paraffin wax. A second heating is required, after which the slate is to be rubbed with a cloth so as to remove any excess.
The most usual mode of securing the matt surface so desirable in wall-paintings, when oil is the medium which has been employed, is turpentine or benzine. These substances both act by causing a certain amount of the oil in which the colours have been ground to evaporate, thus lessening the chance of permanence of effect It is possible to obtain the same effect by finishing with a mixture of wax and turpentine, like Parris's marble medium, but if this is used the application of it must be the final process, as the film of wax will prevent any subsequent touches of colour adhering to the subjacent layers. A process which was used a few years ago in the decoration of a semi dome (now destroyed) was to press canvas on to the wet plaster so that it came through the pores, thus forming a rough granulated surface, upon which the painting was executed with oil colours diluted with turpentine to a considerable extent, so that the colour sank into the plaster, the absorbency of which was not destroyed by the first coat of colour. This had the advantage of a matt surface and a plastery texture, which was pleasant, but was nearly as difficult to manage as fresco, without its advantages.
M<Srim<5e gives the following instructions for preparing a wall for receiving painting in oil. When the plaster is quite dry give the surface several coats of boiling linseed oil, and cover it afterwards with white lead or any colour preferred. To guard against the negligence of workmen in the preparation of the plaster an oily cement is preferable, which may be made of quicklime, linseed oil thickened by boiling or exposure, white lead and fine sand. If the oil is fresh the cement is so fluid that it runs down, though it becomes hard in a few days. A better preparation for the preliminary soaking is a mixture of drying oil and wax, as used in the Panth&m, Paris (by MM. Arcet and TWnard), warming the surface till the mixture no longer sinks in. On this preparation he says that the colours do not sink in.