Copal

Copal is a name given to a number of hard resins which vary not only in their degree of hardness, but also in their degree of solubility: they are the produce of many different species, and even genera of trees, while the origin of several of the kinds still remains unknown. One of the hardest, palest, and best of all is known as Sierra Leone copal, from the port of collection and shipment. It has been identified as the resin produced by a tree, Copaifera Guibourtiana, which belongs to the sub-order Cæsalpineæ of the order Leguminosæ. It is probable that the hard West African pebble copal is the resin of the same tree, but it occurs in rolled pebbles with an abraded surface, and is at least semi-fossil: it is collected from the beds of streams. Pebble copal has more colour than Sierra Leone copal, but yields as strong a varnish. The latter resin occurs in irregular rounded lumps or masses, generally varying in size from that of a hazel-nut to that of a walnut. It is hard and elastic. It consists of at least two resins, one of which, present to the extent of 33 per cent., is soluble in absolute alcohol and in spirits of turpentine.

The other resin constitutes nearly the whole of the remaining part of the copal, and becomes soluble in most of the usual solvents, as well as in hot linseed oil, when it has been previously heated to its melting-point or to a temperature of 180° to 221° C. (360° to 430° F.). Another process for rendering this and other kinds of copal soluble is reduction to a fine powder in the presence of water and the subsequent exposure of this powder to the air for several months, or even a whole year. The time requisite for this change may be shortened by keeping the powdered copal at a temperature higher than that of the ordinary atmosphere. More will be said as to this and other methods of increasing the solubility of copal in the chapter on varnishes.

Other species of the genus Copaifera yield similar but inferior resins to that produced by C. Guibourtiana, but C. Gorskiana is the source of Inhambane (near Mozambique) copal; Benguela copal, Angola copal, and Gaboon copal are other sorts, varying in hue from straw-colour to a dull reddish-orange, produced in all probability by different species of Copaifera. Much of the so-called Manilla copal is the produce of Agathis loranthifolia.

Zanzibar copal is another hard and valuable resin of African origin: it is often called anime. It is produced by another leguminous tree, Trachylobium Hornemannia-num, which belongs to the same sub-order, Cæsalpineæ, as Copaifera. Most of this Zanzibar copal occurs in a fossil or semi-fossil state in the earth near the roots of the trees, or in places where the trees have formerly stood. This fossilized resin is covered when dug up with a semi-opaque, rough, and dull-brown crust; when this powdery coat is removed the remainder of the mass appears of a transparent yellow colour, with a surface covered with small rounded elevations like those on the rind of an orange: this is spoken of as 'goose-skin.' Many of the pieces are flat and tabular, with a thickness of a quarter of an inch or more. The same resin, when occurring on the bark of the living trees of the same species of Trachylobium, presents a smooth and glossy surface; it is not so hard as the fossil variety. Zanzibar copal melts at a higher temperature than Sierra Leone copal, and is very hard. In order to render it soluble it may be treated in the same manner as the Sierra Leone copal. Its chemical nature requires further study.

The varnish made with Zanzibar copal, though darker in colour, must be regarded as at least equal in strength and durability to that prepared with Sierra Leone copal.

A third resin, sometimes designated as copal, sometimes as anime, is produced by another leguminous tree, Hymenæa courbaril, a native of Brazil and other countries of South America. It is rather softer and more soluble than Zanzibar copal. The copal of Madagascar comes from another species of the same genus, H. verrucosa. A Mexican copal is probably the resin of an allied species. The resin from H. courbaril is generally known as West Indian copal; fine specimens have been received from Demerara.

The Bungo tree of Sierra Leone, Daniellia thurifera, affords a resin of inferior quality. It is probable that the same leguminous tree is the source of some of the Niger and Sudan copals.

A rather hard resin of comparatively recent introduction is Kauri or Cowdi copal, produced by the Cowdi pine of New Zealand, Dammara australis. This is a coniferous tree belonging to the tribe Araucariæ. The largest masses, some of them occasionally over 100 pounds in weight, are found in the earth in many places far from those in which the trees now grow. Kauri resin usually becomes more transparent and yellower by keeping. It is generally somewhat whitish, or streaked with opaque bands, when first found. It is cleaned and scraped and then sorted into several qualities. Great quantities were imported into England for some years, and for a time it was largely employed as the basis of most of the so-called copal varnishes, on account of its abundance, its low price, and its easy manipulation. But the varnish which it yields, or of which it constitutes the chief resinous component, is inferior in hardness, toughness, and durability to that made from Sierra Leone copal or Zanzibar copal.