This is usually prepared by dissolving mastic in spirit of turpentine, although other volatile oils and even absolute alcohol may be employed. In order to prevent the mastic from agglutinating together, warm powdered glass, or warm fine white quartz sand, may be added to the resin before it is mixed with the solvent. The spirit of turpentine should be absolutely free from moisture, the mastic may be in tears, or, preferably, have been purified and dried as before directed. The materials are introduced into a capacious glass flask fitted with a cork, tube and condenser so arranged that, when the flask is heated in a water-bath, the vapours given off from the solvent may be condensed and return to the vessel. The temperature of the water-bath may be 100° C. if oil of turpentine be used, but should not be allowed to rise beyond 78º C. if absolute alcohol or 96 per cent. alcohol (specific gravity .806) be substituted for the oil of turpentine. The following receipt gives a varnish which contains nearly 25 per cent. of its weight of mastic, but the proportion may easily be increased or diminished:
14 ounces of mastic, 44 „ „ spirit of turpentine, 6 ,, ,, powdered glass, or fine sand.
When the mastic has dissolved the varnish is allowed to cool, and then poured off into a closed glass vessel, in which it is allowed to rest until perfectly clear. Or it may be clarified by filtration through a plug of dried carded cotton fitted into a funnel. The funnel should be closely covered with a ground-glass plate, but a specially contrived filtering apparatus has been designed for the purpose of preventing any escape of vapour during the process of filtration.
The varnish prepared according to this receipt is nearly colourless, and leaves a brilliant glassy film when it evaporates on a smooth surface. But this film is very brittle, and easily abraded by gentle friction even with the finger, in fact it consists of little more than the original mastic resin, the fragility of which is well known. To obviate this brittleness many plans have been devised. Sometimes Venice turpentine, Canada balsam, or Elemi resin is introduced in small quantity, not exceeding one-seventh in weight of the mastic used. In consequence of such admixture of a natural soft turpentine the varnish produced dries more slowly, and leaves a less brittle, tougher, more adhesive, and more elastic film on evaporation. Ultimately, however, these balsams become brittle like mastic itself. This remedy is, therefore, of a temporary character, but, at the same time, these additions do not interfere with the ease with which the varnish, when old and discoloured, can be removed from a painting by means of solvents or of friction, without injuring the glazing pigments which may lie immediately below it: they also render the varnish more easy of application.
The other classes of substances added to toughen the resinous film left by the drying of a spirit varnish, are fixed oils, and those liquid paraffins which boil at temperatures above 170º C. A very small proportion of 'manganese' linseed oil is, perhaps, the more effective and safer toughener of the two, but its introduction involves the disadvantage just named. In many French mastic varnishes camphor is introduced for the same purpose to the extent of 5 to 8 parts for each 100 of mastic. The camphor, however, gradually escapes by volatilization, the varnish losing its fine lustre and becoming brittle and fissured. It should be mentioned here that the more easily resinified varieties of oil of turpentine, when used as solvents for mastic, also toughen the resinous film left on the drying up of the varnish, although the effect is not permanent. If alcohol, benzene, light petroleum ether, or other non-oxidizable solvents be substituted for any kind of essence of turpentine in making mastic varnish, there is no doubt that the brilliant films they yield are more brittle and less adhesive.
Sandarac and the various kinds of soft pale dammar may be substituted wholly or in part for the mastic mentioned in the receipt for spirit varnish above given. But if these dammars be used great care must be taken that they are themselves free from moisture, and that the oil of turpentine or other solvent be also perfectly dry. It has been recommended to employ oil of spike lavender instead of oil of turpentine in making mastic varnish. The spike oil in this case must be free from water, and freshly distilled: mastic varnish thus prepared has less tendency to 'bloom' than the ordinary kind, but if pictures are varnished in a perfectly dry atmosphere and kept therein till the surface has hardened, the formation of bloom is minimized if not prevented.
A copal spirit varnish may be made by the use of acetone, or of ether (both water-free), or of absolute alcohol, light petroleum-ether, or benzene. The copal to be dissolved may be either Sierra Leone copal, Zanzibar copal, or Demerara copal, the first two yielding the harder varnish, but the last-named being easier of solution, or, rather, dissolving less incompletely. The powdered copal, prepared as directed previously (by exposure to the air, and heating), or first fused, or at least heated till it has lost from 10 to 20 per cent. of its weight, is kept in contact with four times its bulk of the solvent until it is nearly dissolved. Three measures of dry oil of turpentine are then added, and the mixture submitted to distillation from a water-bath until three measures of the acetone or other original solvent have been drawn over: an efficient condenser must be used. If it be desired to prepare a mixed varnish (partly oil or fat varnish), 1 measure of 'manganese' oil, and 2 measures of oil of turpentine may be used in lieu of the quantity of turpentine above mentioned, the distillation being then proceeded with as before.
In another method of preparing copal (and amber) spirit varnishes the resins duly prepared and powdered are heated with the selected solvent under pressure - that is, at a temperature above that at which the particular solvent used boils under ordinary conditions. With purified oil of amber, oil of copal, oil of turpentine, oil of spike, or the heavier petroleum spirit, and on a small scale, glass tubes hermetically sealed and heated to 200° C. may be used, but if a higher temperature or more volatile solvents be employed, copper tubes with screw stoppers are necessary. But operations of this order can be carried out safely and successfully only in a well-equipped laboratory or factory by skilled operators, and it is therefore unnecessary to furnish further particulars in a work like the present.