This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
If more action is requisite than the spirituous mixture affords, the more active essential oils, and those of naphtha and coal-oil rectified, or kreosote, may be employed, or the pure alcohol, with the addition of sulphuric ether in extreme cases; and if their action be too strong, the turpentine alone may be employed, or Unseed oil added to the mixture.
We have succeeded in this operation with excellent effect and perfect security, by using, instead of the more active spirits and essences, a simple spirit varnish. That which we employed is the lac, but any other may answer the same purpose, which being applied, with gentle friction, will at once remove the varnish and dirt from a picture, from which it may be immediately wiped by a clean piece of silk or rag, and a minute after the place will become dry and the design secure; while all that has been done remains visible. In this manner the whole picture may be gone over, and revised and retouched to remove spots, after which a coat of varnish may be applied as a finishing.
Many other methods of cleaning have been recommended and employed, and in particular instances, for sufficient chemical reasons, with success; some of which we will recount, because, in an art so uncertain, it is good to be rich in resources, although the legitimate doctor may deem them empirical.
In an instance of difficulty, where much care was required, we succeeded upon a picture entirely obscured by various foulness, by varnishing over the whole, and when thoroughly dried, removing the varnish by the above means, bringing off with it the entire foulness and original varnish of the picture, with which, in this instance, the new varnish had combined. Strong solution of gum or glue will sometimes effect the removing a foul surface by the same method, but requires care.
A thick coat of wet fuller's earth may be employed with safety, and, after remaining on the picture a sufficient time to soften the extraneous surface, may be removed by washing, and leave the picture pure, - and an architect of the author's acquaintance has succeeded in a similar way in restoring both paintings and gilding to their original beauty by coating them with wet clay.
An eminent artist and friend of the author passed oxgall over a very dirty old picture, which resisted washing with soap, repeating the application of the oxgall during several days, but without washing it off, till the last day, when a sponge and water easily removed the oxgall and dirt together, leaving the picture beautifully fresh and clean; the efficacy of this very safe method is due to the animal alkali contained in the gall.
Another friend, known to the public as an eminent engraver, was equally felicitous in restoring the purity of an excellent picture, by carefully washing it, gradually and in parts, with some of dilute aqua-fortis, and cautiously sponging with water as he proceeded.
He found the acid equally efficacious in cleaning the gilding of frames.
The principle of safety in this case is, that acids, when not excessively powerful, do not act on the resinous varnishes and oils used in painting: and that nitrous acid does not act upon gold; but there is danger if the picture is cracked or abraded, both for the colouring and the canvass, and it can be employed with safety on oil-gilding only.
This method is the opposite of the alkaline process, and they may be employed together alternately in some cases to remove spots, in doing which all manner of agency must occasionally be resorted to.
Among other ingenious means of cleaning, we have it on the authority of a talented and experienced friend, that by damping the face of a picture and exposing it to the action of a frosty night, all foulness will be effectually loosened, and may be removed by the subsequent use of a sponge.
Another very efficacious mode of cleaning is by means of a raw potato, which being grated to a surface upon a tin grater, or rasp, is then employed as a rubber, and, by a mixed chemical and mechanical action, removes the foulness from the face of a picture. From time to time, as the operation proceeds, the rubbing surface of the potato is to be renewed by the grater, and in the end the pulp and dirt are to be washed by a sponge or soft leather and water, leaving the picture in general perfectly clean: this process, has however, the disadvantage common to most others of obscuring the work during the operation.
In every method of cleaning there is great danger of removing the glazings and otherwise injuring the colouring of a picture, which require great skill and judgment to restore.
In filling cracks and replacing portions of the ground, putty formed of white-lead tempered with lac vehicle and oil of turpentine, or of whitening, varnish, and drying oil, tinted somewhat lighter than the local colours require, may be employed: as plaster of Paris may also in some cases; and, in restoring colours accidentally removed, it should be done by stippling with a vehicle of simple varnish, because of the change of tint which takes place after drying in oil: so much is necessary, but in no case is gratuitous painting on an original picture of merit to be justified.
There is a state of declining health which occurs to every picture in the course of time, arising from the natural oil that clothes its colours and forms a semi-opaque skin, or thin surface, which, after being removed, and the picture lined, if requisite, and varnished, conduces greatly to its perfect state and preservation. This operation, which gives freshness without the crudeness that belongs to pictures which have not been ameliorated by time, is necessary to every work deserving reputation.
We have thus recounted various occasions, and described a variety of methods, for the cleaning and preserving of pictures; nevertheless, we earnestly recommend that no inexperienced person should attempt to clean a valuable picture by any more powerful means than is afforded by soft water and a sponge.