This is a small jar of copal medium that I made last year. It is hard copal that has been fused, processed further, and then cooked into linseed oil. This is an excellent medium, for a variety of uses.

These are the two materials that I will combine, to create a copal/elemi medium. The copal has a resin content of approximately 25% resin to linseed oil.

The copal is from Kremer, and is a hard copal. This doesn't mean that on Moh's scale it is any harder than a soft resin, such as damar, or manila copal. Rather, it must be fused before it can be made into a medium. Fusing hard resins is difficult, and dangerous, and I would not encourage anyone to try it. The copal is actually quite a bit more difficult than even amber to fuse, and make into a medium, as it is very difficult to effectively combine with linseed oil. Amber is much easier. If not done right, one ends up with an unusable mass that resembles snot. A turbid disaster. There are also serious dangers regarding the heat needed to process this material.

In this picture, I have a small amout of copal medium, on a palette knife. I will place this on the glass plate.

This is the copal medium, placed on the glass plate. I would add that although darker than linseed oil by itself, it doesn't alter the color of oil paints, to any noticeable degree, in my experience.