In commercial parlance resins are incorrectly termedgums. The true gums (Chapter VIII (Gum, Starch, Dextrin, Honey, And Glycerin).) are either soluble in water or swell up in that liquid, but resins are not acted on by water. The term resin is used throughout the present volume in its proper sense, so that 'copal resin,' 'mastic resin' are spoken of, not 'gum copal,' 'gum mastic.' All the resins used for making vehicles and varnishes are of vegetable origin; they contain besides carbon and hydrogen a not inconsiderable proportion of oxygen. They are related to the hydrocarbons known as terpenes, present in many essential oils, but are of more complex constitution. Some resins, such as gamboge, contain gum and are called gum-resins; others contain a hydrocarbon (or terpene, see Chapter XI (Siccatives Or Dryers).) or an aromatic acid, and are called balsams; others are true resins, but even these rarely, if ever, consist of a single definite compound, but are mixtures of at least two, often of three, four, or five different bodies. Generally these constituents of true resins differ as to their degree of solubility in various liquids, such as alcohol, ether, spirit of turpentine, benzene, petroleum spirit, and heated fixed oils.

They contain carbon, hydrogen, and oxygen, with occasionally a little sulphur, and are usually of an acid character, and are capable of forming soaps, called resinates, with the alkalies. Resins differ much from one another, not only in solubility but also in hardness and in the temperature at which they melt. Those which are least soluble are generally those which are hardest, and which require the highest degree of heat to bring them into fusion. Most true resins contain, besides their proper resinous constituents, small quantities of colouring-matter, of water, of crystalline aromatic acids, and of a volatile hydrocarbon or terpene. All these impurities, save the first, may be removed, generally with advantage, by the following treatment. The powdered resin is thoroughly mixed with a little water and placed in a large glass retort. A current of steam is then passed into the mixture until the terpene and volatile acids present have distilled over. To the contents of the retort carbonate of soda is added (1 part for each 100 of resin). The mixture after agitation is allowed to cool and then filtered through a fine cotton cloth.

The purified resin is then washed on the filter with distilled water, then dried in the air and finally in the water-oven: the air-bath and a temperature of 110° to 120° C. may be used for the desiccation of the harder resins.

It might be thought that the subject of resins would be sufficiently discussed from the painter's standpoint by a description of three kinds - amber, copal, mastic. But it will be shown presently that copal and mastic are names given to several distinct substances, andthat there are some other resins which cannot be excluded from our view.

Amber is the most familiarly known of all the resins on account of its long use in its natural state for ornamental purposes. Amber beads have not infrequently been found in early British graves; on the Continent these and other ornaments of amber have often been obtained from ancient interments. At Naples I was shown some years ago a very large number of antique fibulæ carved out of this substance: they had just been disinterred from Etruscan tombs. Such amber has often become brittle and far more soluble in the usual solvents, especially so far as regards the surface layers; but in other instances the preservation of the properties of this resin has been complete. The chief localities where amber is found are the Prussian shores of the Baltic Sea (particularly between Konigsberg and Memel) and the neighbouring plains; it has been found in veins, and is regularly quarried. Some amber, much of it having a dark colour, is found near Catania, Sicily. Near Lemberg (Galicia in Austria) nodules of amber occur in rock.

It occurs in several places in Denmark, Sweden, Norway, and France. In the British Museum collection of minerals there is a fine mass from Cambridge. Excellent specimens occur in comparative abundance on the seashore at Southwold in Suffolk, and at several other places on the Suffolk, Norfolk, and Essex coasts. The dark fossil resin found in Birma, often in large masses, is not identical with Baltic and English amber. The same observation may be made with respect to the so-called ambers of Travancore in the East Indies, and of the Isle of St. Louis, Senegambia, Africa. In fact, amber, instead of being, as commonly stated, the fossil resin of a single species of tree of Tertiary age, has obviously been derived from no inconsiderable number of different plants. Goppert, so long ago as 1853, satisfied himself that at least eight species of plants besides Pinites succinifer have afforded this fossilized resin: he also enumerated 163 species of plants as represented by remains in amber; many others have been since recognised. Amber has a specific gravity of about 1.07; its hardness is 2 1/2 on the ordinary mineralogical scale. In most of the usual solvents of resins it is either insoluble or but partially soluble.

When heated quickly on a spatula it splits up and then fuses into a viscous liquid, the drops which are formed rebounding as they fall upon a cold surface: this behaviour serves as a distinguishing test between amber and copal. When crushed amber is heated in a retort it fuses at about 280° C. (536° F.), gives off water, succinic acid, marsh gas, a mixture of liquid hydrocarbons (known as oil of amber), and, finally, at a very high temperature, a yellow substance having a wax-like consistence. Sulphuretted hydrogen and other sulphur compounds are also evolved in small quantity, for amber, like several other fossil resins, contains a little sulphur (sometimes 1/2 a part in 100) in organic combination. Amber breaks with a conchoidal fracture. When fragments of amber are being ground or powdered they emit an aromatic odour. On being rubbed amber becomes negatively electric in a high degree.