When it is borne in mind that the varied tints and colours employed by the old masters (and many of which are of doubtful origin at the present day) require to be faithfully matched, it will be understood that only an artist of great skill and experience, possessing an extensive knowledge of the productions of the old painters, should be entrusted with the delicate operation of renovating, without spoiling, works of olden time. The process called "stippling" is adopted for matching the various colours and tints, very small brushes being employed, and each brush being reserved for its special use with great care, in order to avoid even the most trifling risk of mismatching any required tint. When the stippling has thus been done by an artist possessing knowledge and experience, as well as natural ability (the two first-named attributes being the most essential), the picture, when "restored," and subsequently varnished, presents the appearance of a perfect picture, the touches of the restorer being imperceptible. Before the picture is varnished, strips of white paper about 1 1/2 in. wide are neatly pasted round the edge of the frame, and overlapping the picture about \ in., so as to leave a neat but scarcely perceptible margin.
The varnishing of oil paintings is more properly effected by skill than by rule of thumb. The operation should be conducted in a warm room, perfectly free from dust. The picture should be laid flat on a level bench, and a small quantity of varnish poured on its centre; a flat soft brush is then taken, and with this the varnish is brushed over the surface, care being taken to avoid "brush-marks." Th-picture is then allowed to remain in its horizontal position until the varnish is thoroughly dry.
(4) Radlkofer, of Munich, proved some time since, by microscopic observation, that the deterioration of the works of art hung in the Pinacotheca of Munich and in the galleries of Schleissheim was not due, as had been suspected, to any organized matter.
Pettenkofer succeeded in tracing the cause of the mischief; and Goppelsroder, Director of the Municipal School of Industrial Chemistry at Mulhouse, made a report on the subject to the Industrial Society of that town, declaring Pettenkofer 8 theory and mode of proceeding fully confirmed. The following are Pettenkofer's theory and modes of operation: - Linoleine (the lin-oxine of Mulder) is the principle of the greater portion of the oils used by artists, but, unfortunately, this principle cannot be prepared in a pure state, and painters are compelled to employ either linseed oil, which contains 80 per cent, of linoleine, or poppy oil, which only contains 75 per cent. Linoleine, which, when pure, is liquid, solidifies by oxidation, on contact with the air, without decrease in volume, but with an increase of 10 per cent, in weight. It is because linoleine acquires an unvari-able consistency in any temperature that colours, after a picture is dry, are not affected by moderate pressure, by fatty or ethereal oils, nor by varnishes. Paintings absorb moisture from the atmosphere, and afterwards allow it to evaporate.
After a longer or shorter period when these successive absorptions and evaporations of moisture have been pretty often repeated, the colour laid on by the artist generally has lost its primitive aspect, and ceases to produce the same optical effect.
As to the means employed previous to the discoveries of Pettenkofer for the regeneration of the physical condition of the collours, it must be remembered that the artist himself varnishes his dry picture to fill up the pores, which during the work contained oil, but which after the picture is dry contain only air and varnish. He employs resinous oil, solutions of resin in essence of turpentine or in fatty and drying oils. These last are very dangerous. After a certain time the varnish perishes and no longer allows the light to pass through it; new varnish is applied, and the operation is repeated unfortunately until all brilliancy is destroyed. To repair the evil, there are no other means but the removal of the varnish, the nourishing of the colour with a fresh coat of oil, and, after drying, to apply a new coat of varnish, to say nothing of brushwork. When the restoration is made by moistening the varnish with water, the effect after drying is a white spot wherever the water has been applied.
Pettenkofer has shown that paintings are constantly liable to those successive condensations and evaporations mentioned above, which cause loss of cohesion of the varnish. He has, moreover, succeeded in re-establishing the molecular cohesion by means of the vapour of alcohol mixed with the air; at the end of 48 hours the resin takes up and condenses 80 to 100 per cent, of its own weight of alcohol, which, however, it loses again after a short time. The resin, thus softened, becomes absorbed by the painting, and by the same act the cohesion of the resin and the colour is re-established. Softened resin has less effect on the colours of a painting than varnish applied with a brush, for the friction of the latter may cause displacement of the colouring bodies.
Pettenkofer's plan is simple; in the first place he makes a small experiment on the painting to be restored, by means of a small round box made of cardboard, the inside of which is dressed with glue and the bottom lined with flannel moistened with alcohol at 80°; the picture is freed from dust, and the box turned down upon a part of it. The spot thus restored serves as a guide for the general restoration of the work, which is done by fixing the picture to the lid of a box, the bottoms and sides of which are lined with flannel moistened with pure alcohol, as above described, and shutting very closely, so that a small quantity of alcohol serves for a series of pictures.
A second method, indicated by Pettenkofer, consists in the use of the balsam of copaiba, which dries very slowly, and which resembles in constitution the varnishes composed of dammar or mastic dissolved in essence of turpentine. The copaiba should have the consistency of unboiled oil, but must not contain oil, resin, or essence of turpentine. The essential oil of the balsam of copaiba is less volatile in ordinary temperatures than the essence of turpentine. The balsam of copaiba fulfils well the optical conditions of the ordinary resinous varnishes, and may be applied to certain parts only of a picture without being perceptible; it fills up the pores which have been produced in the coloured parts, and sometimes this object may even be effected by applying the balsam to the back of the canvas. The application of copaiba and the vapours of alcohol has in many cases to be repeated several times, and they may cause the appearance of cracks previously invisible, in which case it is only necessary to rub them with a small quantity of the balsam, and expose them to the vapour of alcohol.
If there be an excess of resin, and above all, if the pictures become too yellow in tone, it is absolutely necessary, unfortunately, to remove that excess, but without injuring the primitive character of the colour, before commencing the restoration proper. The varnish, however, can never be entirely removed without some slight deterioration of colour, because the resin is not only superposed but incorporated with the colour.
To remove the excess of resin, either rub with the finger dipped in powder of colophony, or dissolve it with essence of turpentine; and, on the other hand, to fill the pores of the picture with resin, first wash with water, and then with essence of turpentine, and having nourished it, as it were, with balsam of copaiba, the part is made to swell by the application of vapour of alcohol.
If the picture contain both resinous and oil varnishes, the former alone takes up alcohol, becomes softened and retires into the colours, while the latter remains on the surface, and renders it dull and even rough. In this case only the balsam of copaiba is used, and smoothness of surface is obtained by pressure.
A painting regenerated by means of balsam of copaiba resists for a long time the influence of the condensation and evaporation of humidity.
(5) A correspondent of the Philadelphia Evening Bulletin has taken the pains to find out how the galleries and the pictures in the Louvre are kept clean. On Mondays the palace is closed; it is then that the weekly cleaning takes place. The first thing done is to cover the floor with damp sawdust to the depth of an inch or so. Oak sawdust is used for the boards, and elm dust for the marbles. This is allowed to remain some time, and is then removed, and with it goes every particle of dust or dirt which may have adhered to the floor. Then the men buckle on to their feet large stiff brushes, and, armed with a stout stick, to one end of which is fastened a great piece of prepared beeswax, they first rub the floor with wax, then skate over it with their brushes, and finally give it the finishing polish with a great woollen cloth made expressly for this purpose. The same cloth is passed daily over the floor before the opening of the museum, which is all that is required until the following Monday. In this way no dust arises, and the pictures need rarely to be cleaned. When this becomes necessary, which happens about once in 4 or 5 years, the museum is closed for several days. No one is allowed to touch a picture unless the "Conservateur du Musle" be present.
The pictures are taken down, and it is the " Conservateur " himself who places a thick sheet of clean wadding over the painting, pressing it down gently in such a way that every particle of dust adheres to the wadding. "After this is done, a thin coat of oil or some mixture which replaces it is rubbed on, and the picture is not again touched until the next general house cleaning.
Immerse the parchment in a solution of acetic acid, and gently rub the stained parts while wet on a flat board with lump pumice, then bleach it with chloride of lime. This process was recommended in the English Mechanic, It is not very successful, but it makes it white enough for bookbinding. It has, however, the objectionable qualities of not making the parchment flexible, and when dried it is as hard as a board, and it has no gloss like the virgin parchment. On no account must the parchment be washed in very hot water, or held before a fire, as it will shrivel up in a most provoking manner.