This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The gum must first be melted alone till it is quite fluid, and then the clarified oil is poured in very slowly. The mixture must be kept over a strong fire until a drop pinched between the finger and thumb will, on separating them, draw out into filaments. The pot is then put upon a bed of hot ashes and left for 15 or 20 minutes, after which the turpentine is poured in, being carefully stirred near the surface. The mixture is finally strained into jars and left to settle. Copal varnishes should be made at least 3 months before use; the longer they are kept, the better they become. When it is necessary to use the varnishes before they are of sufficient age, they should be left thicker than usual. The more thoroughly the gum is fused, the stronger the varnish and the greater the quantity. The longer and more regular the boiling, the more fluid the varnish. If brought to the stringy state too quickly, more turpentine will be required, which makes the varnish less durable.