Various Other Varnishes

Bell's medium contains no resin, but consists of thickened linseed oil dissolved in oil of spike. To prepare it, pure linseed oil may be oxidized by having a current of warm, moist air passed through it until it has acquired the consistence of fresh honey: this change occurs slowly when a layer of oil is exposed to the air in a large flask, the mouth of which is lightly plugged with carded cotton. The flask should be shaken occasionally to mix its contents, and to prevent the formation of a pellicle on the surface of the oil.

Paintings executed with this thickened oil medium do not acquire the hardness and solidity of those carried out with a vehicle containing a hard resin.

Roberson's medium has now been in use for something like seventy years. It has been and is a favourite vehicle with many distinguished artists. A substitute for it is sometimes prepared in the studio by mixing and warming together strong copal varnish, poppy oil or linseed oil and a trace of white wax.

Of megilp - a mixture of linseed oil and mastic varnish - it is only necessary to say this: that however agreeable as a medium with which to work, it contains a poor and weak resin, which becomes in course of time yellow and brittle, and is liable to be injuriously affected when a picture, in which it has been used freely, is cleaned.

For painting in oil on plaster, slate, or stone, a perfectly sound and convenient medium is made by warming 12 ounces of oil of spike, or of non-resinifiable oil of turpentine in a glass flask plunged in water heated to the boiling-point, and then pouring into it in a slender stream 4 ounces by weight of paraffin-wax (melting-point about 58º to 62°C), or of ceresin, or of a mixture of these materials. The mixture becomes perfectly clear if it be thoroughly agitated and maintained at a temperature of 80° C. Then 20 measured ounces of 'picture'-copal varnish, or 16 ounces of oil-copal varnish, are slowly added, with constant shaking, in the same way. It is of the highest importance that the copal varnish used should contain a sufficiency of oil. If a thin varnish with much terpene be employed in preparing this medium the pigments may lack coherence. I have found this defect counteracted by using pigments ground in inspissated or blown linseed oil, rather than in the medium itself. The 'paraffin-copal' medium thus obtained may be diluted with oil of turpentine exactly to the same extent as recommended by the late Mr. Gambier-Parry, in the case of his 'spirit-fresco' medium, and may be used in the same way and for the same purpose.

Paintings executed with this medium present a perfectly dead or matt surface without the least shine. This medium is superior to that used in spirit-fresco, for it contains neither elemi-resin nor wax, the two doubtful constituents of the latter preparation, but it is not so pleasant in use. Moreover, if a painting executed with this medium on canvas be rolled up, the paint seems to have some tendency to crack. This accident may be obviated if the rolling up and the unrolling afterwards be carried out in a warm room.

Mr. Gambier-Parry's medium, to which reference has just been made, is prepared with five ingredients. The original instructions are unnecessarily complicated, and may be simplified while keeping to the original proportions, and without modifying the nature of the product in the slightest degree. Eight ounces of oil of spike are warmed in a glass flask to 80° C., then 2 ounces by weight of elemi are added, the mixture being warmed and shaken till the elemi has dissolved. Some dirt and woody fragments are sure to be introduced with the elemi, and so the solution (still warm) must be filtered. Upon the filter, when all the liquid has run through, 2 ounces by measure of oil of turpentine, heated to 80° C, are now poured, and the united filtrates are thoroughly mixed. The liquid is then introduced into a flask, and heated to 80° C.; then 4 ounces by weight of pure white wax (previously melted) are poured in a thin stream into the solution of elemi and thoroughly shaken. When the commixture is complete, 20 ounces by measure of 'picture'-copal varnish, or 16 ounces of oil-copal varnish, are gradually introduced with constant agitation. The water surrounding the flask is now made to boil, and kept boiling for five minutes. The flask is withdrawn, wiped dry, and allowed to cool.

As the cooling proceeds the flask is gently agitated from time to time. When the mixture begins to get treacly in consistence it is at once poured into the bottles (bottles with wide mouths, holding 4 ounces apiece, are convenient) in which it is intended to preserve the medium for use. The dilution of this medium and the mode of using it are described in the section Painting Methods.