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Moderator Answer
(koo schadler)
Hi Brian,
I did a series of test panels applying various isolators over egg tempera (cured from one to 15 days); among the products were Golden GAC 500 medium and Golden Acrylic Gel medium. I was skeptical they would go on without lifting or smearing the paint (being water-based, like tempera) but no problem – and almost five years later they look perfect, with no efflorescence.
Schadler, Isolator Test Panel.jpeg
I know, five years isn't much in the scheme of things, but as a solitary artist in my studio (not a research lab or paint company) my research/testing capacities are limited.
The techs at Golden Paint are very smart and helpful, and I trust their input. On the other hand they don't do much (if any?) testing on egg tempera. This is often the challenge with our medium: there's neither a large pool of artists working in it (never mind innovating) nor many companies doing extensive research and testing, and it can be hard to find answers. As an experienced tempera painter and varnisher, I don't see, off hand, a problem with using a synthetic polymer-based varnish atop tempera - but I'm not a trained chemist or conservator, and hopefully some will chime in.
A Golden technical expert said to use the gloss first to seal it, then use the matte over top for the final sheen.
I want to underscore this. Matte/satin varnishes are gloss varnishes with matting agents (small, crystal-like particles) added to create a microscopically irregular surface that scatters light, akin to frosted glass (versus perfectly smooth glass, like a gloss varnish). If a matte or satin varnish is applied directly to a high PVC, absorbent surface such as egg tempera, the liquid varnish sinks in while the matting agents remain on top, resulting in a cloudy or frosted appearance. Hence an initial isolating layer (the gloss varnish, in your case) is requisite when applying a matte or satin varnish to egg tempera (or any high PVC surface).
…Krylon Crystal Clear as an isolation coat. I'm a little worried about orange peel, to be honest, but I think that's worth looking into as well.
Krylon Crystal Clear is, last time I checked, B-72. I have tried applying B-72 both via Krylon spray cans and making it myself and using a sponge brush – so far I have found it a bit finicky, and have gotten orange peel a few times. The last time I used it I first practiced on 4 test panels, all with success, before applying it to a finished piece, which promptly erupted into orange peel. Whether B-72 is truly tricky to apply, or just tricky to apply on a high PVC or recently cured ET surface, or I'm not very good at it, I can't say. I know Dr. Joyce Stoner uses it on Wyeth temperas with success, always via a spray booth. If she, Brian or Kristin have more insights on B-72 on egg tempera (how widely it's done, if there's been testing or research on it relative to tempera) it would be interesting to hear.
Also in another thread was the concern about the alkalinity of this varnish. Would that negatively interact with the egg tempera?
With my usual disclaimer that I'm not a chemist/conservator, I'd say alkalinity shouldn't be a problem; after all, true gesso is very alkaline and doesn't affect tempera (and actually servers as a buffer to the acidity of a wood-based panel, although that's a different topic).
The expert at Golden said they do not recommend the Polymer Varnish for oil paint. The binder in tempera is a drying oil.
I wouldn't say egg oil is generally considered a "drying" oil – it's more often categorized as a "non-drying" oil. But in actuality, it's not quite either: many of the lipids in egg yolk are not polymerizing while others may polymerize to a degree...it's not certain (and one of the mysteries of egg tempera). What is certain is that, over time (about 3 months, I estimate) the egg oil + proteins within the paint cure enough to form a durable paint film, but don't fully cure; the egg oil retains some degree of "mobility", which is why it can travel and lipids effloresce onto the paint surface (i.e. Fatty Acid Migration). So tempera is like oil paint and not at all like oil paint. How this affects varnishes used on top of tempera is hard to say (due, again, to limited group experience and testing/research).
Varnishing, already a challenge in many mediums, is definitely a bit complicated in tempera (due to its high PVC, propensity toward FAM, rarity, etc.). But there always have been and will be ET artists who want and/or need to varnish their paintings. If you go ahead with the Golden Polymer Varnish, please let us know how it works out. It's very helpful to have the practical experience of ET artists contributing to the pool of knowledge.
Koo
