by, December 9, 2011 at 04:11 PM (980 Views)
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, 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 in world trade, but there are many other varieties of copal resins that are suitable for use in varnishes as well. Congo copal was a hard fossil resin that originated in the former Belgian Congo (now 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.
Congo copal was derived mainly from Guibourtia demeusei (syn. Copaifera demeusei) but 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 pieces of resin by prodding the ground to depths of 15 cm with iron tipped sticks. In addition to collection from the ground, fresh resin was collected from living trees where it had accumulated following natural wounding. Resin collected from the ground has often been called semifossilized 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 taken from trees and was considered superior for use in varnishes. Congo from Guibourtia demeusei was second in hardness only to Zanzibar copal (Hymenaea verrucosa).
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.) in 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 originating from tree species in southeast Asia. These copal resins, known as Manila, Pontianak, East India, etc., are from various species of Agathis (Araucariaceae), the conifer with the most tropical distribution. Although the resins are generally called copal, those of some species have been called damar. They are softer resins than the African copals and are more soluble in solvents and oil.