Part 77. Plant Resins

Part 77. Resins, like elastic gums, are derived from liquids exuded by plants, and serve as a protective covering for wounds. The common resin obtained from the pitch or turpentine of pines is a familiar example. More or less fluid at first, owing to the presence of volatile oil, the resinous sap solidifies on exposure to the air, partly through evaporation of the volatile constituent and partly through its oxidation. Finally it may become hard and brittle. In this condition resins resemble various gums. But true gums, as we have seen, are either soluble in water or absorb it indefinitely; while they are insoluble in ether, alcohol, carbon bisulphid, and oils. Resins, on the contrary, are insoluble in water; but are mostly soluble in the other liquids mentioned, at least when hot. Sometimes a gum and a resin are intimately united, forming what is known as a "gum-resin." Such a material is asafetida, which we have already studied. The name "gum" is also applied commercially to gum-resins, and resins, and even to rubbery materials; it is most convenient, however, to restrict the term as indicated above, except for "elastic gums" from which no confusion is likely to arise. When a considerable amount of volatile oil is associated with a resin the mixture (commonly of a honey-like consistency as in the turpentine of pines and firs) is distinguished as an oleoresin. Resins are always mixtures of several different oxidized hydrocarbons which agree in being poor in oxygen and rich in hydrogen and carbon, very inflammable, and, from their large proportion of carbon, burning with a sooty flame.

More or less resinous material is contained in the great majority of plants, and in many cases it is abundant and valuable for use industrially. Rosin and copal, which are among the most important resins, will serve as typical examples.

Rosin

Rosin is so much the most widely known of resinous materials that it is commonly called "resin" as if it were the only substance to which that name could apply. Chemically it is known as colophony. It is one of the products obtained by distilling turpentine. What is properly called turpentine as already stated is the oleoresin which flows from wounded surfaces of pines and similar cone-bearing trees. When this is distilled the volatile parts that pass over and are condensed form the familiar oil or spirits of turpentine, while the residue is rosin. The largest quantities are produced in our Southern States. Rosin is used as an ingredient in common varnishes, is combined with tallow in cheap candles, and is extensively used in the making of yellow soap, inferior kinds of sealing-wax, and various cements. In shoemaker's wax and certain medicinal plasters and ointments it enters as an important part. Musicians depend upon it to rosin the bows of stringed instruments, tin-men and plumbers use it as a flux in soldering, and it serves many other purposes in the industrial world. Its property of generating, when vigorously rubbed, that sort of frictional electricity called "resinous" has led to the use of rosin in certain forms of electric apparatus for experimental purposes.

Copal

Copal is a name applied rather indefinitely to a large variety of resins without much in common to distinguish them, but as strictly defined it is understood to include only such as occur naturally in hard masses resembling amber in appearance, and like that substance melting and dissolving only at a comparatively high temperature-a process requiring special precautions to prevent the resin and the solvent from catching fire. These resins make the best varnishes, and that is their main use. The botanical origin has long been uncertain, but it is now known that the finest copal of the East is a product of the Zanzibar copal-tree while the best South American copal is from the nearly related courbaril-tree (Fig. 273). The difficulty of tracing the product to its source arose from the fact that the best copal is dug by the natives out of the earth often in tracts of country from which all plants that could have produced it have disappeared. The hard resin, sometimes covered by an accumulation of three or four feet of soil, is all that remains to show the former existence of copal-trees at that place. It is a fossil. The happy accident of finding leaves, flower-buds, and flowers embedded in masses of Zanzibar copal, finally gave the last link in a chain of evidence connecting the resin with the species which produced it. The South American copal is found embedded in the earth at the base of courbaril-trees, and often contains bits of courbaril-bark. When the resin exudes from the tree and solidifies it is still too soft to be of commercial value; only the slow process of time, perhaps centuries, can bring it to that state of almost glassy hardness which renders it indispensable for making the most durable varnish.

Fig. 273.-Courbaril-tree and Zanzibar Copal-tree (Hymencea Courbaril, and Trachylobium Hornemannianum, Pulse Family, Leguminosoe). A, flowering branch of courbaril-tree. B, pistil and base of flower, cut vertically. C, part of fruit. G, part of flower-cluster of Zanzibar copal-tree. H, pistil and base of flower, cut vertically. J, fruit. K, seed. L, same, cut across. (Taubert.)-Courbaril-tree 16-20 m. tall; leaves smooth; flowers white; pod filled with a sweet, mealy pulp. Native of tropical America.-Zanzibar copal-tree of considerable height; leaves with translucent dots; flowers white; pod leathery, not opening. Native home, Tropical Eastern Africa.