When it comes to oil painting mediums, one of the best, for special purpose, is copal. Traditional copal oil painting mediums were made from several hard copals found in Africa, and thus were unknown in European painting, prior to the time that Africa was colonized. Many of the former resources have dried up (having been used extensively in a variety of products), and or the areas were these materials were found, have presently become far to politically unstable to visit.
One of the very best copal mediums formerly available, was made by Frederick Taubes, and sold through Permanent Pigments out of Cincinnati, Ohio. That company later became Liquitex, and subsequently discontinued oil painting materials. Taubes used a very hard copal, which came from the Congo region of Africa, and produced a very light varnish. Again, these products are no longer easy to find, and thus difficult to hold as a general material, in the tool box.
Presently there are soft copal mediums available, but in my opinion, having made them, and having used them, I do not think that they perform the same as hard copal versions. When we say hard, vs. soft copal, does this imply that one version is actually harder than another? Well, yes it does seem to imply this. However, when we look at these materials in terms of actual hardness, using a tool such as Moh's hardness scale, we find that the hard versions of copal are no harder than the softer versions. Therefore, I don't think that "hard" or "soft" is a good descriptive measure. Rather, we should look at copals in terms of polymerization. What we consider to be hard copal, is generally polymerized to a much greater degree. These materials may be centuries, if not millennium in age. Of course, the copals are no where near the state of the several amber materials (which are tens of millions of years old), but what we commonly call hard copal is polymerized to a much greater degree, than the so called soft copals we see today. In addition, what we know as hard copal, comes from entirely different botanical sources, and regions of the earth. Copal is such a generic word, and connotes a particular type of resin. However, in reality it is akin to saying the word "vehicle." What is meant by this? Boat, car, airplane? In fact, copals are as different from one another as a boat is from a bicycle. They are of different ages, from different regions, different botanical origin, different chemical makeup, and of course, have vastly different properties from one another.
The use of mediums which contain highly polymerized copals of African origin, does permit special effects that no other medium will provide. It allows one to lay a brush stroke down, and have that stroke remain just as it was laid down, without any fusion of edges, or migration. It also creates a stringiness to paint, when used in just the right concentrations. Some people have come to use mediums which contain damar, as a substitute, and in fact, even Taubes did this, but these mediums do not create the same effect, and it is my belief that they are not as durable. Old copals of African origin will not dissolve in turpentine, but damar will. The recent copals will also often readily dissolve in some solvents. I have also observed multiple oil paint films, which contained damar as a medium, to crack in a relatively short time.
When it comes to making copal mediums, as stated earlier, some copals will readily dissolve in solvent, and can then be added to a drying oil. However, those old highly polymerized copals of African origin were not dissolved in solvent, but rather heat fused with drying oil. That process was formerly completed in industrial production facilities, but when undertaken by the individual, can be a very difficult and dangerous task. Not to be the bearer of bad news, but it is my belief that there is no product available today, which will produce the same results as the old copal mediums, from the past. Of course, this is my opinion, and as with everything else, it is a debatable subject. This is not to say that artists can not paint without copal, or that there are no good oil painting mediums. There are many. However, in my opinion, there is nothing around that will presently provide the same results. It is also my opinion, that artists should spend their time painting, and experimenting with mediums that are available, for the purposes they seek, rather than undertake the dangerous task of making highly polymerized copal, into a usable medium. Considering the cost of materials such as Congo copal, or copal from Sierra Leone, most people will destroy a great deal of material in the attempt to produce a usable medium, and there simply is not enough material presently available to support the learning curve (of most people), necessary to drive this material into a drying oil.