Mediums

I will let Brian Baade answer more specifically regarding copal gels as he is far more knowledgable than I about these mediums. However, I can address the general pros and cons of fossilized resins to a certain extent: 
Soft and Hard Copal Resins – This class encompasses a wide range of natural resins that are typically defined by their place of origin (e.g. Manila Copal). Soft copals, usually dissolved in alcohols, are simply collected directly from a range of living trees. Soft copals can differ in quality from one grade to the next and are not recommended as final varnishes. These resins not only suffer from the same degradation process as mastic and dammar but can also become more difficult to remove over time. Furthermore, they can render a paint film more sensitive to solvents should your painting need to be restored in the future. On the other hand, hard copals are fossilized and semi-fossilized resins collected from the ground where they were deposited from ancient and even extinct trees. Hard copals need to be cooked at high temperature to incorporate them into oils or solvents. They are not reversible and should never be used as final surface coatings for fine art​; however, small additions of these resins to one’s primary paint medium can yield intriguing and beautiful effects. While there is some concern about potential darkening and cracking associated with these particular types of resins, there has yet to be a scientific study on this subject. Certainly adding too much of these resins can create a more brittle paint layer, one that will in turn be more prone to cracking. But if used in reserved amounts (in conjunction with a solid support) this may be less of an issue. Further study is still needed to look into potential yellowing/darkening associated with small to large additions of hard copal resin.

I am not exactly positive which mediums you are speaking about but will comment on the subject in general. Kristin hit on many of the issues surrounding preservation and the use of copal mediums. As to how the mediums handle when mixed with oil paint, this is quite varied depending how the resin was processed (including the temperature used), which oil it was incorporated into, and many other variables. In general, oil mediums containing copal create a paint that “sets” rather quickly allowing for early blending. It also allows for early overpainting which may or may not be a virtue. As to heat modification. With the exception of a few complicated recipes using chemicals that are not recommended for use in art materials, all hard copals require thermal processing to incorporate them into an oil, and generally even more heat processing to incorporate them into a solvent alone. In the past when copals were important for industrial coatings they were separated into a huge number of categories based on their geographic origin, botanical origin, color, solubility, etc. Some were considered far superior than others for certain purposes. It required great experience to cook these resins into oil with and without dries to produce a quality product, especially to create a medium that was as clear and light in value as possible. Handbooks on industrial oil varnish making in the early 20th century often mentioned the difficulty of making a quality product in very small amounts. This has been collaborated in my own experiments with making sandarac, amber, and Congo copal varnish in my research. There has been much writing both for and against copals in fine art oil painting mediums. Neither side really had scientific studies to back up their claims. Truly more research needs to be done on this subject. Most 19th-20th century writers that approved of the use of copals in oil paint mediums suggested the harder and less soluble resins like Congo copal and Zanzibar copal. The issue is practically mute today. There is little or no trade in these resins today and little quality control for those that can be obtained. There are no large scale manufacturers with a long history of perfecting the creation of natural oil-resin varnishes. The few producers of true copal painting mediums are almost exclusively cottage industry companies that make very small batches. I would expect that the gel copal that you mention is produced by just such a company. Without a full knowledge of the ingredients and processing, I can’t even comment on how it would perform in the long term. Taken as a whole, unless there is a very concrete and important effect that only copal provides, it is probably far better to use a more universally approved medium like one of the alkyd oil mediums produced specifically for artistic use which can provide some of the effects that you desire without the potential embrittlement and/or darkening that many associate with copals,

Thanks Mathew Yes, it is very dangerous to make these varnishes and the result is seldom satisfactory. I have made many types of oil-resin varnishes in a laboratory setting and one of them did catch fire. This is in no way recommended for the home tinkerer. I have always wondered how many fires were started after reading some of Taubes’ instructions for making Congo copal mediums. Also, I did not mean to say that no “real copals” are available, although the availability of the hard resins that were recommended in paint mediums is very limited. Certainly, L & B does offer a Copal Picture Varnish which is very thin and rather dark. It is sold as a final varnish and not a painting additive. We do not recommend such a practice as it is irreversible (please see our downloadble pdf on VARNISHES in the “resources” section for more info as well as the mediums section referenced by Kristin above}. There are a number of other cottage industry copal mediums out there. Each would really need to be tested before anything could be stated about its performance.

Mathew, no problem. More info is always better than less.

I figured you were speaking about products from James Groves. He does disclose the source of the copal that he uses in his very voluminous pages on mediums. He has done a lot of research on varnish and medium making. I did speak to him on the phone once and asked him about his specific processes but he was wary to disclose them as they are a product of his own research and are proprietary. Without more information I can make no comments on the long term stability of his products.

You are right about the Flemish Siccative Medium. I had forgotton about that one even though I have a jar of it in my collection. Incidentally, I seem to remember that that contains a metallic drier as well as hard copal. My collection is in my lab/studio at the University but I will check the packaging when I am there in the next few days. I also have a container of Windsor and Newton’s Lt Amber Varnish from around 1910 which is still fluid (although quite thick) and quite a number of other oil mediums ( eg Taubes Permanent Pigments, etc). When we have secured some additional funding, I hope to have these analyzed both determine what is in them and to compare the results as well as the sensitivity of our analytical tools against oil-resin varnish/medium samples that Kristin and I have made from known resins, oils, and recipes.