I have been warned about the dangers of using Resins in mediums. My understanding is that the issue is twofold: on is in cracking of the paint layer itself ( possibly yellowing as well) and then also in conservation as the removal of the resinous varnish layer would also affect/remove a portion of the paint layer.
I want to be very specific to the Resin and meidum that I use becuase the answer may not apply to all resins and mediums equally. I use 2 parts sun-thickened linseed oil, 1 part turpentine and 1 part Canada Balsam.
I have a number of questions. 1. With respect to yellowing, would that not be mitigated by the varnish layer which would protect the paint. If the varnish yellows, it can be removed and a fresh varnish put on, so that yellowing is not an issue for the paint layers themselves?
2. To what degree is the cracking attributed to the rigidity of the painting surface and to what degree the resin in the medium? Are the caveats for using Canada Balsam in the medium significantly reduced is the painting surface is a stable one? Many thanks.
Certainly using a rigid support is preferrable if you are going to use any additives that will increase the brittleness. As for exactly HOW much you can add….well that has yet to be extensively studied. No matter what please consider recording your materials (even the proportions used) on the reverse of the painting. You might be interested in reading this previous threadhere and reading a bit more about some of these additives in our “Mediums and Additives for Painting” document in the Resources section.
This question is essentially answered in theMyths, FAQs and Common Misconceptions document, but since you’ve asked: 1: Color change (darkening) due to excessive use of Canada Balsam in the medium will not be prevented by a top coat of picture varnish. I think 1 part resin to 2 parts oil sounds like an awful lot of resin, BTW. Artists going back at least to the mid-19th c have remarked about problems with bloom and darkening when CB is used as a varnish, so this isn’t a recently identified issue. 2: It usually takes a trained conservation specialist to know for sure why a painting has cracked. (I’m not one, BTW.) A panel reduces planar distortion, but there are other stresses between paint, ground and support. If using a rigid support could prevent brittle paint from cracking, we wouldn’t see so many cracked paintings on panel.
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