Resins, Waxes, And Solid Paraffins

In commercial parlance resins are incorrectly termedgums. The true gums 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 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.