Congo Copal in Painting

Congo copal is no longer available commercially (at least as it was available in commerce formerly) because the suppliers have long stopped trading due to strife in Africa since the last quarter of the 20th century. Trade of Congo copal was once controlled by the Belgian government, but as this region of Africa gained independence and the demand for copal resins diminished during the last half of the 20th century, the commerce of Congo copal has all but ceased.

There are many types of copal; the word is a generic term applied to resins from a wide variety of tree species. Congo copal was the most important resin in world trade, but many other varieties of copal resins are also suitable for use in varnishes. Congo copal was a hard fossil resin that originated in the former Belgian Congo (now the Democratic Republic of Congo) and entered commerce from the grading centers in Antwerp. It was collected by the indigenous people of the former colony in Africa, which they traded for food and supplies, and then brought to Antwerp, where it was graded and exported to locations throughout the world.

According to botanical observations in the references, the legume tree Guibourtia (such as Guibourtia demeusei with species formerly assigned to the genus Copaifera) is considered to have produced most of the Congo and other West and Central African copals. But some Congo copal was also collected from several species of Tessmannia, all leguminous tree genera in tropical Africa. The flooded forests along the rivers were the best collecting areas. Native collectors located resin pieces by prodding the ground to depths of 15 cm with iron-tipped sticks. In addition to the collection from the ground, the fresh resin was collected from living trees where it had accumulated following natural wounding. Resin collected from the ground is often called semi-fossilized, although it probably did not attain the age (5,000 to 40,000 years) considered subfossil. Copal dug from the soil was much harder than the fresh resin from trees and was considered superior for varnish use. Congo from Guibourtia demeusei was second in hardness only to Zanzibar copal (Hymenaea verrucosa).

Copal and amber from different locations and tree species.

African rosewood is the common name for the species of the legume tree Guibourtia. Pictured is Guibourtia coleosperma.

Congo copal is insoluble in practically all organic solvents and vegetable oils. Hence it must be 'run' or thermally processed to enable its solution in oil and solvents. Congo copal running is a step-like operation during which the resin softens with the breaking down of some of its components and the formation of spongy masses that pass into a liquid soluble in oil. Congo copal is commonly heated to 330–340° C. (625–650° F.) for about one and a half hours and held at that temperature until a clear solution is obtained.

Most manufacturers of copal varnishes and mediums today use copal resins from tree species in Southeast Asia. These copal resins, known as Manila, Pontianak, East India, etc., are from various Agathis (Araucariaceae) species, the conifer with the most tropical distribution. Although the resins are generally called copal, those of some species have been called dammar or damar. They are softer resins than the African copals and are more soluble in solvents and oil.