is sprayed with hydrofluo-silicic acid to dissolve away the crystalline
skin of carbonate of lime formed on the surface and to "open the
pores " of the plaster. The surface of the painting ground, which
is left with a decided " tooth " upon it, is then well soaked with
the solution, and when dry will be found " hard but perfectly
absorbent and ready for painting."
The pigments consist in the usual ochres and earths; chrome reds, greens, and yellows; Naples yellow (antimoniate of lead); cobalt blue and green; and artificial ultramarine; terre verte, &c., with zinc white or baryta white.
It is important however to note, that the pigments (which can be supplied by Messrs Schirmer, late Faulstich, of Munich, and many other firms) are mixed with various substances so as to render uniform the action upon them of the fixing solution and neutralize the action of its alkalies. The operations of painting, in which only distilled water is used with the colours, are easy and admit of considerable freedom. " Every variety of treatment is possible, and the method adapts itself to any individual style of painting." The work can be left and resumed at will. After the painting is dry there comes the all-important final process of fixing with the water-glass solution. This is sprayed on in a hot state by means of a special apparatus, and the process is repeated till the wall can absorb no more, the idea being that the substance will penetrate right through to the wall, and when set will bind pigments, intonaco, roi'gh plastering and wall into one hard mass of silicate that will be impervious to moisture or any injurious agencies. The last paragraph of the official account of the Keim process issued in 1883 for the guidance of those contemplating mural work runs as follows: "The fixing of the picture is accomplished by means of a hot solution of potash water-glass, thrown against the surface by means of a spray-producing machine in the form of a very fine spray. This fixing done, by several repetitions of the process, a solution of carbonate of ammonia is finally applied to the surface. The carbonate of potash, which is thus quickly formed, is removed by repeated washings with distilled water. Then the picture is dried by a moderate artificial heat. Finally a solution of paraffin in benzene may be used to enrich the colours and further preserve the painting from adverse influences."
§ 38. Spirit Fresco or the " Gambicr Parry " Process, with modifications by Professor Church. — (See Spirit Fresco Painting: an Account of the Process, by T. Gambier Parry, London, 1S83; Church, Chemistry of Paints and Painting, 288 seq.).
This process is also one of quite modern origin, but in Great Britain, at any rate, it is now very popular. Mr Gambier Parry, who invented and first put it into practice, claims for it that it " is not the mere addition of one or more medium to the many already known, but a system, complete from the first preparation of a wall to the last touch of the artist, " and that the advantages it offers are " (i) durability (the principal materials being all but imperishable); (2) power to resist external damp and changes of temperature; (3) luminous effect; (4) a dead surface; (5) freedom from all chemical action on colours."
The theory of the process is much the same as that of stereochromy, the drenching of the ground with a solution that forms at the same time the medium of the pigments, so that the whole forms when dry a homogeneous mass. The solution or medium is however not a mineral one, but a combination of oils, varnishes and wax, the use of which makes the process nearly akin to that of oil painting. The objection to the use of oil painting proper on walls is the shininess of effect characteristic of that system, which is in mural work especially to be avoided, and " spirit fresco " aims at the elimination of the oleaginous element and the substitution of wax which gives the "matt " surface desired.
Mr Gambier Parry directs a carefully laid intonaco of ordinary plaster suitable for fresco on a dry backing, " the one primary necessity " being that the intonaco " should be left with its natural surface, its porous quality being absolutely essential. All smoothing process or ' floating ' with plaster of Paris destroys this quality. All cements must be avoided." When dry the surface of the wall must be well saturated with the medium, for which the following is the recipe: pure white wax 4'oz. by weight; elemi resin 2 oz. by weight dissolved in 2 oz. of rectified turpentine; oil of spike lavender 8 oz. by measure; copal varnish about 20 oz. by measure. These ingredients are melted and boiled together by a process described in his paper, and when used for the wall the medium is diluted in one and a half its bulk of good turpentine. With this diluted solution the wall is well soaked, and the directions continue, " after a few days left for evaporation, mix equal quantities of pure white lead in powder and of gilder's whitening in the medium slightly diluted with about a third of turpentine, and paint the surface thickly, and when sufficiently evaporated to bear a second coat, add it as thickly as a brush can lay it. This when dry, for
which two or three weeks may be required, produces a perfect surface " both white and absorbent.
The pigments, which are practically the same as those used in oil painting, must be ground in dry powder in the undiluted medium, and when prepared can be kept in tubes like oil colours. Solid painting with a good deal of body is recommended and pure oil of spike is freely used as painting medium. Pure spike oil may also be washed over the ground before painting " to melt the surface (hence the name Spirit Fresco) and prepare it to incorporate the colours painted into it." The spike oil is " the one common solvent of all the materials; . . . the moment the painter's brush touches the surface (already softened, if necessary, for the day's work) it opens to receive the colours, and on the rapid evaporation of the spike oil it closes them in, and thus the work is done." The oil of spike lavender, it may be noticed, is an essential oil prepared from Lavandula spica.
Professor Church has suggested improvement in the composition of the medium by eliminating the " doubtful constituents " elemi resin and bees'-wax and substituting paraffin wax, one of the safest of materials, dissolved in non-resinifiable oil of turpentine. This is mi.xed as before with copal varnish and used in the same way and with the same or better results as Mr Gambier Parry's medium.
§ 39. Oil Processes of Wall Painting. — The use of the oil medium for painting on plaster in medieval days opens up a much debated subject on which a word will be said in connexion with oil painting in general. In the later Renaissance period in Italy it came into limited use, and Leonardo essayed it in an imperfect form and with disastrous result in his " Last Supper " at Milan. Other artists, notably Sebastiano del Piombo, were more successful, and Vasari, who experimented in the technique, gives his readers recipes for the preparation of the plaster ground. This with Cennino (ch. go) had consisted in a coat of size or diluted egg-tempera mixed with milk of fig-shoots, but later on there was substituted for this several coats of hot boiled linseed oil. This was still in common use in the i6th century, but Vasari himself had evolved a better recipe which he gives us in the 8th chapter of his " Introduction " to Painting. Over undercoatings of ordinary plaster he lays a stucco composed of equal parts of lime, pounded brick, and scales of iron mixed with white of egg and linseed oil. This is then grounded with white oil toned down with a mixture of red and yellow easily drying pigments, and on this the painting is executed.
In Edinburgh and other places Mrs Traquair has recently carried out wall paintings on dry plaster with oil colours much thinned with turpentine. The ground is prepared with several coats of white oil paint, and the finished work is finally varnished with the best copal carriage varnish.
In most cases oil painting intended for mural decoration has been executed on canvas, to be afterwards attached to the wall. This is the case more especially in France, and also in America at the Boston public library and other places. The effort here is to get rid of the shiny effect of oil painting proper by eliminating as far as practicable the oil. As this however serves as the binding material of the pigments the procedure is a risky one. To suppress the oil and to secure a " matt " surface Mr E. A. Abbey employed at Boston and elsewhere, as a medium for painting with ordinary oil colours, wax dissolved in spike oil and turpentine. In France Puvis de Chavannes used some preparation to secure a matt effect in his fine decorative oil painting on canvas.
§ 40. Tempera Painting on Walls. — This is a very ancient and widely diffused technique, but the processes of it do not differ in principle from those of panel painting in the same method. It is accordingly dealt with under tempera painting in general (§ 43).
§ 41. Encaustic Painting on Walls. — (See Schultze-Naumburg, Die Technik dcr Malerei, p. 122 seq.; Paillot de Montabert, Traite complct de la peinture, vol. ix.).
It has been already mentioned that wax is employed in modern mural painting in order to secure a matt surface. Many pictures have been carried out within the last century on walls in a regular wax medium that may or may not represent an ancient process. Hippolyte Flandrin executed his series of mural pictures in St Vincent de Paul and St Germain des Pres in Paris in a process worked out by Paillot de Montabert. Wax dissolved in turpentine or oil of spike is the main constituent of the medium with which the wall is saturated and the colours ground. Heat is used to drive the wax into the plaster.
A German recipe prepared by Andreas Müller in Düsseldorf has been used for mural paintings in the National Gallery', Berlin.
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