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Moderator Answer
(koo schadler)
Hi Zarina,
I know we've discussed this varnish before, but for readers who don't know the specifics…The Lascaux UV Varnish spray is, I believe, B-72, a synthetic resin recommended by conservators. The solvent, according to the Materials Safety Data Sheet, is N-butyl Acetate. I don't know or have experience with this solvent except that it's flammable, evaporates quickly and is noxious to breath (particularly since it's aerosolized) - so wear a mask when spraying.
For a resin to be sprayable, it must have a high percentage of solvent, which makes for a thin application; this minimizes the shine, which I know is your preference. Applying a very thin coating of B-72 is essentially what Dr. Stoner does to Andrew Wyeth's paintings (per Wyeth's request); Wyeth too felt this was minimally disruptive to the natural finish of tempera - so you're in good company! (Although, to be clear, Dr. Stoner is varnishing atop cured Wyeth paintings - not quite the same scenario we're discussing).
Two things to address:
1. How long does it take for egg tempera to polymerize? I haven't found a consensus. It's common to see 6 to 12 months suggested (there's variability depending on drying conditions, layers, etc.) but I think this time frame may be transposed from oil paint. Tempera paint has less binder than oil paint. The surface is porous and thus more open to oxygen, light, and heat (the factors that denature the proteins in yolk and initiate the crosslinking that makes a paint film). Tempera layers are also a fraction of the depth of most oil paintings. For these reasons it seems unlikely that a tempera painting needs a cure time equal to oil. That logic, combined with many years practical experience, leads me to believe that tempera paint polymerizes within 1 to 3 months. However I can't confirm that number and often see longer cure times given (yet don't know what those numbers are based on). So, amidst this uncertainty, I suggest 3 to 6 months (maybe less) as sufficient time for an egg tempera painting to cure.
2. What are the consequences of varnishing before curing?
a. The surface is more absorbent, so coatings tend to sink in more and dry uneven. The high percentage of solvent (along with the nature of the resin itself) makes the Lascaux fast drying; and fast-drying coatings have less opportunity to sink into egg tempera's inherently porous, absorbent surface. Perhaps this is why you're not getting sinking in. Regardless of the reason, it isn't an issue for you – so you're fine in this regard.
b. Varnish crosslinks with the paint film as it cures. If the proteins in the yolk are in the process of denaturing and crosslinking, the varnish becomes entangled, so to speak, in this process, and thus is very difficult to remove in the future. Removal of a coating from egg tempera, even on a cured surface, is already problematic due to the inherent porosity and irregularity of a high PVC paint like egg tempera. (Given that artists are visual, I've attached an approximate diagram of an ET paint film, so you can visualize this KS, ET Paint Film.png). The challenge of varnish removal is, to a degree, inevitable in ET - but the problem is undoubtedly exacerbated by varnishing early.
c. Curing of Underlying Paint layers? While a varnish layer can limit the factors that create polymerization (and thus may slow it to some degree) they don't fully stop it. So I don't think B72 interferes in a detrimental way with the curing of the underlying paint; after all, if it did I doubt it'd be recommended as a varnish! But I'm not sure about this – Brian, do you know? You're applying it as a very thin coating, so really, I don't think this is a concern.
I've been varnishing student work literally minutes after the last brush stroke was applied (by request; they want to go home with a finished painting) and haven't seen/heard of problems. And, under deadlines myself, I have often varnished within a few days of completing a painting, no problems. There are benefits to letting the paint film cure before varnsihing...but beyond what I've mentioned above, I don't see other issues with varnishing as soon as the paint is touch dry, no residual water in the paint layers or gesso (which would be indicated if the surface were cool to the touch). But frankly, I don't know this for certain, and don't know if anyone else knows.
Gloss or Matte? Matte and Semi-matte varnishes contain matting crystals that can settle out atop the microscopically irregular surface of an egg tempera painting, and cause cloudiness - so, I presume you're using the Gloss, yes? I just want readers to understand that tempera must first be isolated with a gloss varnish before spraying on a matte.
Onto your final question: Is it OK to frame under glass and fully seal the back, or can one allow breathability & air circulation by not taping up?
To be clear, the back of a painting panel doesn't need to “breathe"; in fact, if the back of a panel is absorbent, this creates an avenue for moisture to enter in, which can cause movement, mold and other problems in the panel that can travel through to the paint layers. So, as you know, I recommend painting the back of a panel with some sort of sealing housepaint.
As for the backing on a frame (which is what I think you're asking about): backing paper commonly is porous (tho', perhaps, your framer is applying Tyvek, I don't know); the frame itself may be porous (i.e. wood); and even though there's less air flow with glass, there's still air between the painting and glass (unless it's vacuum sealed, which is unlikely).
In other words, most framers don't hermetically seal a painting within a frame; so most framing does not entirely cut off a painting from the factors that help it polymerize. Still, a painting directly exposed to oxygen and light polymerizes more quickly than one that is under glass, within a frame. If a paint film takes longer to polymerize, does this matter? Or, how much does it matter? I don't know. Paintings are rarely made or live under ideal conditions…my tendency is to think that, while it would be ideal if an ET paint film were fully exposed to oxygen for several months before being varnished and/or framed under glass, doing neither of these isn't greatly consequential.
Wish I could be more specifically helpful, Zarina – but I'm not a chemist/conservator and, even if I were, there still might not be clear answers, given how much is unknown about ET. Of course, if any conservators reading this know more, please let us know.
Koo
