CASEIN is the best irreversible glue for painting purposes. It is derived from sour skim milk. It must contain no fat, as this prevents it hardening. It is not heated, but strained through butter muslin for 24 hours until all the whey is removed. The remaining curd is called casein. It swells a great deal in water, but is not soluble. To make it dissolve, an alkali, such as lime in solution, must be added.
A glassful of pure powdery white lime can be obtained from a builder's yard. It is best taken from the top stratum of the pit. It should be added to the curd in pea-sized quantities, one at a time, until a glassy, uniform, and sticky mass is obtained. For 70 gr. of this dissolved casein only a few gr. of lime will be needed; one quart of water is poured on and stirred to make a uniform solution, producing a glue which is probably still too strong, depending on the consistency of the curd. If it is very dry it may need twice the quantity of water.
A test can be made with a cheap pigment: the brush stroke should dry hard in two or three hours, but must not scale off the paper, which should be firmly stretched, even when several coats are laid one on the other. Casein takes about a week to become completely insoluble in water. Paintings done in casein colors can thus be considered weather-proof, but only in so far as the support is weather-proof too!
Naturally, only pigments unaffected by lime can be used with casein. This is true also of the casein powders that can be bought ready prepared, as they too need a lye to make them dissolve, generally borax, potash, or soda. Casein, except in dry powder form, is as liable to attack by bacteria as any glue, and must thus be used immediately after it has been dissolved. Formalin converts it into a stable horny mass (galalith), and if a solution of formalin is sprayed on the finished painting it may cause the paint to come away from the ground. Undiluted casein is a strong emulsifier, making other materials emulsify very quickly. It functions at the same time as a glue.
RESINS and BALSAMS behave in the same way as glues for painting, except that they are soluble not in water but in volatile oils; spirits of turpentine (generally called simply "turpentine") is the most usual one used in painting. This is mainly a distillation from balsam, a natural secretion of conifers. Balsams can be considered as resins which occur in nature in a heavy solution with turpentine.
French turpentine, which is twice distilled, is considered the best. Doubly rectified French turpentine can be bought both in art shops and drugstores. Ordinary turpentine contains unevaporated resinous elements. If it is dropped onto a glass plate it should leave no trace that can be seen or felt as sticky after it has evaporated. Resins and balsams dissolved or thinned with turpentine take much longer to dry than water-soluble glues. When dry they form a hard, glassy skin which is very brittle.
Resins, like reversible glues, remain soluble. If overpainting is done with resin essences there is a danger of taking off the underpainting, at least superficially, and smearing. An addition of beeswax can afford some protection, and this also modifies the brittleness of the resins. Resin, like oil, "burns" slowly over a long period. This makes it crumbly and dark, especially if it has been used without pigment as a finishing varnish. Finishing varnishes play an important part in oil painting, because resins do not yellow and can at any time be softened and removed with turpentine, as long as they do not contain too much wax.
Perished resin varnishes can be regenerated by the famous method developed by Max von Pettenkofer: the picture is exposed to alcohol or petrol vapor in a closed container, thereby giving the varnish back its firmness and transparency.
SHELLAC shows that a small amount of wax can prevent the deterioration of resin. As you already know, the basis of shellac is the resin of tropical trees which is secreted when the trees are punctured by insects. This resin contains animal wax derived from these insects, and this makes it an ideal painting medium, though it is, for some unknown reason, very little used. It is sold in thin sheets and is used mostly as a polish for wood and as an insulator in the electrical industry.
For painting, light colored sheets that have not been artificially bleached are the best because shellac loses some of its solubility from the effects of light. It is dissolved in 95 per cent alcohol under mild heat to melt the wax in it. In a 2 per cent solution it is the most usual fixative for spraying on chalk drawings and pastel paintings. In thin coats it is also very useful as an insulator against absorption on glue grounds. Paints take badly on thick layers of shellac, and glue colors even scale off.
MASTIC is the most important resin used in painting. It is obtained in the tropics and sub-tropics from the mastic bush in the form of pale yellow balls or droplets which still have the earth and bark adhering to them. It is easily soluble in turpentine. A standard essence of one part (weight) resin in two parts of turpentine is the most practical.
It is made in the following way:
A glass vessel which can be hermetically sealed is half filled with spirits of turpentine, and the drops of mastic are hung in a piece of gauze over the liquid without touching it. The vapor from the turpentine, which quickly develops in sunshine or warmth, gradually dissolves the resin, which then drips through the gauze into the turpentine. The bottle is shaken now and again to wet the gauze and accelerate the solution. All impurities will be held by the gauze, and the liquid will be clear and light yellow.
Mastic can be added to the bag until the required concentration is reached. This can be gauged by weighing; the contents of the jar should increase by 40 or 50 per cent, assuming that the turpentine has not evaporated to any marked degree. Mastic dissolves much more quickly if heated with turpentine, but it turns brown and is thus rendered unsuitable for painting. Reaby-made mastic varnishes may contain admixtures of other substances such as poppy oil, which looks just like mastic but delays hardening considerably; a thin application of mastic essence on a glass plate will dry completely to the touch in 24 hours, while poppy oil takes several days.