Varnishing Egg Tempera

These are all very good questions that will take a bit to ruminate over but for the time being I suggest looking at a couple resources regarding the polymerization of ET:Bakkenist, Tonnie, René Hoppenbrouwers, and Hélène Dubois, eds. Early Italian Paintings, Techniques and Analysis: Proceedings of the Symposium held at the Stichting Restauratie Atelier Limburg, Maastricht, 9-10 October 1996. Maastricht: Limburg Conservation Institute, 1997.

Beltinger, Karoline, and Jileen Nadolny, eds. Painting in Tempera, c. 1900. London: Archetype Publishing, Ltd., 2016.

Massing, A. (2003). A short history of tempera painting. In Making Medieval Art (P. Lindley, ed.), pp. 30–41, Paul Watkins.
We will get back to you soon to address the other questions regarding varnishing.

First and foremost, you have very definite and well thought out reasons for choosing to varnish your tempera paintings. But we do want to state for other artists reading this thread that there is no technical need to varnish tempera paintings…this is completely an aesthetic choice.After reviewing the current scientific literature on the chemical properties of tempera, it appears that there is not a general consensus on WHEN an egg tempera film might be considered completely and fully “cured.” There are likely several reasons for this.
Even though sophisticated analytical techniques have been applied to naturally and artificially aged tempera paints, there are too many factors to consider (e.g. pigments present, fatty content of the egg, environment, etc.) when attempting to characterize the precise mechanism(s) involved with film formation. Jaap Boon et al.’s article in the “Early Italian Paintings Techniques and Analysis” publication is perhaps the most detailed summary to date although publications by Patrick Dietemann at the Doerner Institut have also reviewed this topic albeit his studies focus mostly on tempera grassa (egg and oil) film formation.
To get back to the question of when to varnish. Honestly the methods that you have proposed in terms of testing the hardness of the film seem adequate to us and 3-6 months sounds fine as well. The general advice for varnishing oil paintings is between 6-12 months and given that both oils and tempera paints share similar fatty components (fatty acids), it would make sense that tempera paints would be ready for varnishing in half that amount of time, probably even sooner.
As you state, however, any varnish applied to the surface will become incorporated to a certain degree into the egg tempera film, the degree to which will depend on the PVC. It could be argued then that it really does not matter when one varnishes an egg tempera painting….on the other hand if you review the publications that I have already listed you will note that there are still complex cross-linking reactions and chemical changes that continue to occur long after the paint film is touch dry, which is why 3-6 months is probably a safer bet. That being said it would be fine to apply an isolation layer so long as this is recorded on the back of the painting (you want to avoid some future restorer misunderstanding this layer and attempting to scrub it off!) and it is a material that has been shown to age well in terms of retaining saturation and not being prone to discoloration. Opinions regarding the final appearance of a varnished painting can (and should be) very personal, however, combinations that sound appropriate to us would be PVA (in acetone or ethanol) followed by Regalrez or Laropal. If the Regalrez or Laropal needs to be removed, it can be done without biting into the PVA layer beneath.

We want to make sure that everyone understands that when we write PVA we mean PVA solution (a 50/50 mixture of PVA AYAA and PVA AYAC [or 100% Molwilith 20] in 95% ethanol/5% water. PVA can also be dissolved in acetone and mixtures of acetone and ethanol. We do not mean PVA dispersions or emulsions like those that some use as a size. Baade, Brian

Koo, your point is well taken. Also, your expertise in all things egg tempera makes me reticent to even weigh in on this but here goes anyway. In many ways varnishing is always an aesthetic decision for all easel painting mediums. Yes, a varnish provides some mechanical protection and a barrier against grime in all mediums. Historically, this is an interesting concept as the presence of natural resin varnishes, their propensity for yellowing, and need for eventual removal (especially in the past when there were few solvent choices as well as clumsy and misinformed treatments by underskilled restorers) has caused far more damage to oil paintings than was ever prevented by a thin surface coating. Thankfully, the creation of more stable varnishes and the evolution of modern conservation practice has greatly improved this situation. I do not want to overstate this but I do think that there are subtle differences between the physical function of varnish on oil, and perhaps even acrylic paintings, as opposed to contemporary egg tempera works. As we all know, even discounting issues of sinking-in, the various pigments in oil paintings dry to different levels of sheen and saturation. There is also a pronounce difference between the overall saturation of an oil paint film when it first set and when it has dried a few weeks. Most artists apply varnish to their oil paintings not to protect their paintings but to reestablish saturation and the original unity that they created. So, I think that, for most, the application of a varnish on oil paintings is done to make it look like the artist intended AND how it looked when they were painting. This is somewhat different than the situation for egg tempera paintings. As for acrylic dispersion paintings, for these I personally consider varnishing primarily an aesthetic choice. The acrylic medium allows for the creation of paintings which range from rather matte to very glossy. Varnishing may or may not be a component of the intended gloss. The lack or presence of gloss or saturation is apparent almost as soon as the water has evaporated from the paint film. However, varnishing an acrylic painting does help to prevent the incorporation of soil into the possibly porous surface and may serve to mitigate migration of surfactants/etc. Certainly, one could make a similar case for the reason to varnish egg tempera paintings. These paintings are very porous and therefore likely to incorporate or imbibe grime into their surface if displayed in a dirty environment. I do think that the primary difference in the function of varnish on tempera works is that the color effects and surface quality of egg tempera painting remain the same or similar after the work has dried. There is generally not the need to reestablish unity of saturation. Egg tempera paintings tend to have a relatively matte surface that does not GREATLY vary in glass and saturation (at least not to the same degrees). Individual painting practice will obvious influence this. In fact, unless the varnish is very minimal, varnishing an egg tempera painting may even change the color effects and composition contrast of the work as compared to when the work was just finished. Of course, a knowledgeable painter can certainly plan for these changes or even use them as part of their intended final effects. Again, I do not want to belabor the point, while varnish does provide a protective function, it is always an aesthetic decision. As to the books we mentioned, there are very niche publications. Unfortunately, I am not sure that they would be generally available. You would likely have to go to a library associated with one of the Art Conservation programs (U of D, Winterthur, Buffalo, NYU, UCLA) or perhaps you could try an inter-library loan. Beyond that you would likely have to purchase them.

I will copy what I wrote above to give a more simplified answer: Opinions regarding the final appearance of a varnished painting can (and should be) very personal, however, combinations that sound appropriate to us would be PVA (in acetone or ethanol) followed by Regalrez or Laropal. If the Regalrez or Laropal needs to be removed, it can be done without biting into the PVA layer beneath.

Hi Koo I do agree with everything you write. To be clear, there is really no technical reason why someone should not varnish an egg tempera painting as long as they know the result, that the saturation will change and the varnish will become a part of the painting. As you know, practically if not all old master temperas have been varnished at some point and most still have a varnish. As to current best practice, I do not think there is a consensus right know. What Kristin wrote above is probably as close as we are at the moment and even then, I am sure that some would be of another opinion.