Resins, a class of natural vegetable products composed of carbon, hydrogen, and oxygen. They are closely allied to the essential oils, all of which, when exposed to the air, absorb oxygen, and finally become converted into substances having the characters of resin; and in most cases they are obtained from the plants which yield them mixed with and dissolved in a corresponding essential oil. Like the natural oils, the natural resins are usually mixtures of two or more distinct resins, which admit of separation by their unequal solubility in different fluids.
The following are the general characters of this class of compounds. At ordinary temperatures they are solid, translucent, and for the most part coloured, although some are colourless and transparent. Some are devoid of odour, while others give off an aromatic fragrance from the admixture of an essential oil. In their crude state they never crystallise, but are amorphous and brittle, breaking with a conchoidal fracture; when pure several of them may, however, be obtained in the crystalline form. They are readily melted by the action of heat, and are inflammable, burning with a white smoky flame. They are usually described as non-volatile, but it has been shown that common resin may be distilled in a current of superheated steam. They are insoluble in water, but dissolve in alcohol, ether, and the essential and fixed oils. They are insulators or non-conductors of electricity, and become negatively electric by friction. Many of them possess acid properties, in which case their alcoholic solutions redden litmus. These resins combine with the alkalies, and form frothy soap-like solutions in alkaline lyes. The resinous soaps thus formed differ from ordinary soap in not being precipitated by chloride of sodium.
The resins are divisible into the hard resins, the soft resins, and the gum-resins. The hard resins are at ordinary temperatures solid and brittle; they are easily pulverised, and contain little or no essential oil. Under this head are included copal, the varieties of lac, mastic, and sandarach, and the resins of benzoin (commonly called gum-benzoin), jalap, guaiacum, &c. The soft resins admit of being moulded by the hand, and some of them are viscous and semi-fluid, in which case they are termed balsams. They consist essentially of solutions of hard resins in essential oils, or admixtures of the two. They become oxidised and hardened by exposure to the air into the first class of resins. Under this head are placed turpentine, storax, balsam of copaiba, and the balsams of Canada, Peru, and Tolu. The gum-resins are the milky juices of certain plants solidified by exposure to air. For these, see GUM.
The resins are very widely diffused throughout the vegetable kingdom. They are generally obtained by making incisions into the wood of the trees which produce them; sometimes, however, they exude spontaneously, and in other cases they require to be extracted from the wood by boiling alcohol. The crude resins are separated from the essential oils with which they are usually mixed by distillation with water, the resin remaining while the oil and water pass off; and from the gummy and mucilaginous matters by alcohol, which dissolves out the pure resins, which can be precipitated from their alcoholic solution by the addition of water. The resins are extensively employed in medicine and the arts.
Various fossil resins are known, of which the most important is Amber (q.v.). Some chemists place bitumen and asphalt amongst this class; and amongst the fossil resins described by mineralogists may be mentioned Fichtelite, Hartite, Idrialite, Ozokerite, Scheererite, Xyloretin, &c.
The common resin, or rosin, of commerce exudes in a semi-fluid state from several species of pine, especially Pinus taeda, P. mitis, P. palustris, and P. rigida of North America, P. pinaster, P. pinea, and P. Laricio of southern Europe, and P. sylvestris of northern Europe. The crude article, consisting of turpentine and resin proper, is subjected to distillation, when the resin alone remains behind. The resin thus procured is used very extensively in the manufacture of common yellow soap, also for sizing paper and various other purposes, including the preparation of ointments and plasters in pharmacy.
The other resins most generally known and used in Europe, and here all treated in separate articles, are Anime, Copal, Dammar, Mastic, Sandarach, Frankincense, Lac, and Kanri Gum.