Traditional drying oil – hard resin varnishes

​There are so many issues and tangents involved in your questions that I will have to wait until tomorrow when I have a chunk of free time.

Hi
I will answer each part within the text of your questions and in red.
Dear experts! I have several questions about traditional sandarac and copal drying oil-resin varnishes.
These are two quite different materials. Sandarac resin is alcohol soluble where as hard copal resins are not soluble in any solvents. Both do require heat to be incorporated into oil. I even hesitate to use the word varnish in this context. Yes, they are varnishes in the sense that Spar and marine varnishes used on wood are varnishes, but it is really a bad idea to use essentially irreversible coatings, that will strongly yellow, as a varnish on fine art. This practice was done in the distant past as spirit varnishes do not appear to have been used in the Renaissance but it should be condemned today when much better products are available. The use of these materials in oil painting mediums is a different subject. One that has been mentioned here before.
1. In various articles I encounter mentions of good preservation of such varnishes after hundreds of years, for example, in case of paintings byCarlo Crivelli orOrazio Gentileschi. Are those cases just coincidental exceptions or drying oil-hard resin varnishes age better than their cousins made with oil and colophony, larch balsam or mastic?
The truth is probably the opposite of what it appears. There are examples of old oil varnishes on artwork because they are irreversible and not because they are better. In all of the known examples, the varnish was exceedingly thin where its yellowing was not as distorting, as would be the case if the coating was substantial. Before modern scientific methods of cleaning paintings and before the distillation of the wide range of organic solvents available today, the only way that “restorers” could remove a darkened oil coating was to use a strong base like lye or to sand the coating away. This may sound extreme and unlikely but I recently saw a presentation/poster on the evolution of instructions for cleaning paintings from the 17-19th centuries and these were the two most common recommendations before the 19th century. This is one of the prime reasons why so many Old Masters paintings have areas of abrasion or skinned surfaces.
Oil and rosin is no better as it still yellow and be difficult to remove and it contains a very inferior resin that would likely degrade as or even quicker than the aforementioned. Larch and mastic are reversible but both yellow for earlier than the suggested synthetic available today. There are some who still swear by mastic and dammar due to their initial appearance. I disagree for the following reason: Every time a work needs to be cleaned, some component is inevitably leached away, even if this component is infinitesimal. Because of this it is our responsibility to make sure that we use the most stable and slow changing coatings as possible to stretch out the time between necessary cleanings.  
2. Is it absolutely necessary to heat sandarac or copal resins in oil to make a good varnish? Or maybe I could use an intermediate solvent to avoid extensive thermal treatment?
It is necessary to heat sandarac and hard copals to get them into an oil solution. Given the above, though, I am not sure that these varnishes are appropriate for your needs
3. Could I use modern zirconium-calcium octoate drier additions instead of traditional lead linoleate? How much worse are such driers than lead-based ones in the long term?
Are you attempting to speed up the drying of an oil varnish? If so, yes, although as implied above, I believe that these should not be used as coatings for artworks.
4. How important is to use fresh Cypress or Juniper sap instead of dried and oxidised resin which is always sold in art supplies stores? In Da Vinci’s recipe, use of fresh sap in spring is recommended.
Younger sap or resin is usually soluble in lower aromatic solvents than older resin (eg mastic is initially soluble in cold turpentine but requires heat after it has oxidized for a while). This has no bearing on oil-resin varnishes and I have made my position clear on those.
5. Could I enhance ageing performance of such varnishes by adding Tinuvin 292 and 1130? Or maybe using other modern additives?
No. These function in a far different manner and would very likely inhibit the proper drying of an oil-resin varnish.
6. I’ve heard that new methods of old varnish removal gain popularity, such as laser ablation. Could I rely on such technologies for future conservation efforts of my paintings or I should care about varnish solubility and use Regalrez 1094 with Tinuvin 292 because they remain soluble despite their not so good appearance, scratch resistance and necessity to wait one year before varnishing?
Please care about your choice of varnishes today. Lasers MAY be the panacea in the distant future, or they may not. Even if they do, there is no guarantee that those who inherit your works will have the access or funds to have your paintings cleaning in such a manner.
I hope that was of some help.

Thank you.
 

​I was under the impression that the exact composition of “Amber Oil of Venice”, the material associated with the Gentileschi studio as mentioned in the De Mayerne manuscript, was unknown and still the subject of speculation. As I understand it, Orazio and Artemisia Gentileschi were reported to have used this varnish in measures of a few drops added to flesh tones and white passages in the early stages, to induce fast initial drying and to render light colors insoluble and impermeable to staining from subsequent glazes. This type of performance is consistent with fossil and semi-fossil tree exudate varnishes, but can easily be obtained today with alkyd-based mediums.

Also, in my opinion, no discussion of heat-polymerizing (“running”) resins for varnish is complete without mentioning the serious danger associated with heating oils near the flash point. The average studio (or home) is not the place to be making this type of product.

Matthew, ​you are right about the dangers of cooking drying oils. That is discussed in a previous thread on this subject.
https://www.artcons.udel.edu/mitra/forums/question?QID=534  

​Brian, I just re-read the linked thread- excellent! I will bookmark it for future reference.

​Historical treatises on varnishes that I have read remark that amber and copals dissolved in ether and alcohol yield a more brittle coating than those made with linseed oil. While I don’t know the chemistry, it’s always been my impression that “running” or heat-polymerizing the resin improved the performance of the resulting varnishes, most of which* were intended as protective coatings for ships and carriages, where flexibility and resistance to fracture were essential. 

*edit* I mean to say that most of the recipes I am familiar with are for coach varnishes and other utility coatings, because most of the historical books I have read where these materials are discussed in detail were intended for skilled tradesmen like house painters. Just for sake of specificity…