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
1. In various articles I encounter mentions of good preservation of such varnishes
after hundreds of years, for example, in case of paintings by Carlo
Crivelli or Orazio
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
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
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.