This is hard copal, and is not soluble in gum spirits of turpentine. The material must be fused, in order to be cooked into a drying oil. The image above is of copal in its unproecessed state (right). On the left, and much darker in appearance is the same copal that has been fused. The one on the left is ready for further processing, in order to combine with linseed oil.
Hard copals are really no harder than soft or or recent copals, when using Moh's hardness scale. The difference is in the level of polymerization, and the resulting necessity of various methods of processing. Hard copals require a greater degree of processing, in order to make them into viable art materials. Much of what is presently available (copal mediums) are made from soft copals, and I don't think that these materials are the same as those used by artists in previous centuries. Hard copals are exceptionally difficult to process, and then even more difficult to subsequently combine with a drying oil, to form varnishes, and or mediums. Taubes method is very useful, yet without an added step, not mentioned in his book, it remains a very difficult proposition. If not completed just right, one will end up with a turbid mass, or a pot of ash.
I personally would not use any copal mediums, in my artwork that are light, and made with recent copal resins. Due to several reasons, the best varieties of hard copals are no longer available, and any of the resulting mediums and varnishes that I would buy, are no longer being produced.