Mastic

There is one more resin which requires mention. This is mastic. The best and most important sort of mastic is produced by a small tree (Pistacia Lentiscus), belonging to the cashew-nut order or Anacardiaceæ. This tree occurs in Scio and other islands of the Greek Archipelago. Mastic exudes in the form of tears from incisions made in the bark. It occurs in small pea-like masses, and presents when fresh a very pale straw-colour. It is so fragile that it may be crushed to powder between the fingers. It has an aromatic odour, and dissolves completely in boiling alcohol and in spirits of turpentine. Its melting-point is low, on an average about 110° C. It contains, besides its resinous constituents, a small quantity of a volatile essential oil (a terpene) and of moisture. It yields a tender but glossy varnish, largely employed for the final protection of pictures in oil. This varnish yellows with age, and becomes fragile and fissured.

Resins, sometimes called mastics, are produced by other trees of the same genus. These resins, which are of no value for artistic purposes, are:

Indian mastic from Pistacia cabulica. Bombay mastic from P. Khinjak. Pistachio mastic from P. Terebinthus.

In the following table, compiled from the results of Böttler, are comprised some particulars concerning twelve of the resins commonly called 'copals.' In the first column the names are given, in the second the melting-points, in the third the relative hardness (12 being the hardest), and in the fourth and last column the degree of solubility (xii being the least soluble). The specific gravity of these copals depends so much upon the relative freedom of the several kinds from cavities and bubbles that the figures for this character are not included in the table. It may, however, be mentioned that all these resins are rather heavier than water, their specific gravities ranging from 1.035 to 1.07.

All these numbers are approximative only; and the valuation of these resins for varnish-making ought to take into account other properties not here recognised, such as toughness after as well as before heating and solution, liability to darkening in colour by lapse of time, and durability.