MITRA Forum Question Details

​I have sent this to a couple of our tempera experts.

​The vast majority of egg tempera paintings are on wood panels. However, there are a few examples of egg tempera on flexible supports, such as​ stretched canvas. One notable example is Sandro Botticell’s, Birth of Venus, in the Uffizi, Florence, Italy. This painting is now in good condition after undergoing restoration in 1987. ​I have examined this painting close up in 2007 and while there are many cracks, there are less than I would have expected for a tempera painting on canvas. Nevertheless, the condition of the painting likely has more to do with its provenance than any special technique Boticelli may have used in its execution. Another painting executed in the same technique by Botticelli, Primavera, also in the Uffizi, was painted on a poplar panel.

I doubt that a tempera painting with a glue ground on canvas would survive rolling up and transportation. Painting on an acrylic ground may solve this issue, but present other issues for the egg tempera painting itself, such as lipid efflorescence.

While this may appear to be a green light to painting on canvas, I would strongly suggest testing your technique and materials before committing to it.

The moisture content of the paste or adhesive could be an issue if it affects the paint. It may be advisable to prepare the back of the canvas with a light coat of an acrylic product (allow to dry before painting), to reduce the amount of moisture which penetrates the fabric, and to support good adhesion with the acrylic-based mounting adhesive. (The acrylic coating might not be advisable if using a clay based adhesive.)​​

Acrylic dispersion grounds, unless specifically formulated for such a purpose, are generally not absorbent enough to serve as a successful ground for egg tempera, at least not traditional uses of egg tempera. In my experience, few replacements to traditional chalk or gypsum bound in animal glue grounds perform like the traditional product. I have found that even grounds made using traditional materials but produced on a larger scale (spraying, etc) tend to be a bit less absorbent. Now whether this small difference matters to an individual artist would completely depend on how they paint and their expectations.

Additionally, I received an email from our resident ET expert Koo and she has said that while she is quite busy at the moment, she will weigh in on this as soon as her schedule allows.

Hi Brian,

A less-than ideally made painting can last a long time under ideal circumstances, and a well-crafted painting can fall apart under bad conditions – so it's hard to say definitively, “don't work with egg tempera on canvas".  Yet there are many reasons not to.

1.  Chief among them is egg tempera and traditional gesso's high PVC.  Anyone who wants to understand egg tempera in greater depth should understand high PVC: it's one of the medium's defining characteristics.  Attached is a handout on the subject.  Koo Schadler PVC.pdf

2.  Already a less flexible medium than oil or acrylic to begin with (due to its high pigment load), egg tempera becomes more brittle over time as mobile lipids, which give plasticity to the paint film, move around (into the ground if it's porous; or, up to the paint surface).  I don't know how soon this happens, nor have I seen a definite statement as to when egg tempera paint polymerizes.  The time line for oil (6-12 months) is often given for ET, but I think it's more likely 3 months, more or less (given the thinness of the layers, porosity of the paint film, and personal experience).  Even if we could determine polymerization, I don't know when the lipids start their traveling about.  There is much unknown; for example, Dr. Stoner, the Wyeth family conservator, notes that lipid efflorescence in A. Wyeth's ETs seems to be exacerbated by humidity; so, even if a painting isn't very old, could humidity increase the likelihood of efflorescence? Amidst all this uncertainty, a conservative view is to presume an already less flexible paint becomes brittle at some point, and to give it maximum stability from the start. 

3.  As for painting on an synthetic polymer ground, I agree with Brian B. – there's nothing like a traditional chalk/gypsum + animal glue ground for egg tempera.  Both traditional gesso's porosity and its hygroscopic binder create maximum absorbency, and absorbency gives the best working properties for ET (because it allows for controlling water content, which is key to controlling the paint).  More importantly (in terms of permanence), maximum absorbency encourages good adhesion.  My experience is that egg tempera requires, in addition to dispersive adhesion, mechanical adhesion – a physical intermeshing, like sewing or Velcro, of paint to ground.  I believe mechanical adhesion is critical for long term adherence of ET; I say this not only from decades of direct experience handling the paint, but also because, when people share images with me of delaminating ET paint, nearly always it's on a synthetic polymer ground or non wood-based support. You get a wee bit of mechanical adhesion with a regular acrylic gesso, but not nearly as much as with a traditional gesso (or even a very high PVC, synthetic polymer gesso).  

There's little doubt in my mind that traditional gesso under ordinary conditions will not last well on canvas, especially rolled up. The few synthetic polymer, high PVC grounds that perform moderately well for egg tempera have such a high pigment load (which is what allows them to work for tempera) that they'd suffer the same fate (cracking and delamination) as traditional gesso.  A regular acrylic polymer ground can be rolled – but this very fact indicates lower PVC and decreased absorbency, which means it won't give as good working properties to the paint nor, more importantly, as good adhesion between paint and ground. 

All of this begs the question…how did Botticelli work on canvas?!  I've asked this question many times (including on this forum; see below).  I haven't the benefit, as George has, of a close up examination (aside from craning my head towards the painting, many years ago, before there were beepers saying 'stay away'; the Ufizzi security guard still scolded me).  From what I could see the paint looks applied thinly (a bit like washes). I also wonder if he may have added oil to both ground and paint to improve flexibility.  Botticelli worked in oil and egg/oil emulsions at times; i.e. his Primavera is now believed to be an egg oil emulsion (see Baldini, Umberto, Primavera, The Restoration of Botticelli's Masterpiece).   

This suggests an option: Have you considered working in tempera grassa?  If you haven't yet tried the medium, it's very easy to make (simple recipe attached:Koo Schadler Tempera Grassa.pdf). This would impart more flexibility and greater adhesion of the paint to a regular acrylic gesso.  While still not ideal (I'm so used to working on rigid supports that, I confess, I have a bit of a bias against canvas) it's a more viable option than pure ET on canvas.

Koo

PS.  Here is, slightly abbreviated, the reply Kristin DeGhetaldi posted a few years ago in response to my question asking how Botticelli worked with ET on canvas:

“It is often impossible…to make definitive conclusions regarding binding media based on visual assessment alone.  One really needs to have the condition report of the painting in hand and ideally a very thorough analytical report. And even then it can be impossible to know what Botticelli and his workshop truly used. Botticelli's works in particular have likely been restored multiple times over…It is no secret that Botticelli and his workshop pushed tempera to its absolute limit as a medium… So the Birth of Venus could very well be an example of true tempera grassa [egg oil emulsion].  Or the ground may have been carefully prepared in a unique manner. The fact that it is on canvas has and likely always will continue to intrigue…If this painting has been analyzed recently the results have yet to be published. Yet even if cross-sections are obtained or samples are collected for destructive medium analysis…I doubt that all of our questions would be adequately answered. It would be wonderful to conduct a technical study of the painting with some top-of-the-line analytical techniques but often funding…is the main hold-up.  In Italy this is often the problem combined with a whirlwind of political hoops that one has to jump through in order to carry out a technical study of this magnitude on a painting that may as well be the country's unofficial 'banner'." https://www.artcons.udel.edu/mitra/forums, June 29, 2017.

​Thanks so much, Koo

One more point I'd like to make regarding synthetic polymer gessoes.  Even if you really raise the ratio of solids to make a very high PVC gesso (as the acrylic or vinyl-based gessoes designed for egg tempera do, such as Natural Pigment's Tempera Ground, ArtBoards Gesso, Claybord, etc.), you are still dealing with a binder that, once cured, no longer absorbs but rather repels water (synthetic polymers are 'plastics', after all).  It seems to me that one of the important differences between a synthetic polymer gesso (even one with very high PVC) and traditional gesso is that the latter's binder remains water absorbing; this imparts an additional absorbency that a plastic-based gesso can never achieve. This isn't to say that very high PVC, synthetic-polymer gessoes should never be used for egg tempera; they seem to behave moderate to well for dry brush techniques (less so for water intensive methods, such as Petit Lac, lots of layers, etc.).  And they are admirably convenient (not an insignificant factor, given how few people know how or have time to make gesso from scratch).  Still, a plastic-based gesso can't reach the same level of absorbency as traditional gesso, and this affects their working properties for ET (I know, from much personal and student experience, the paint is more prone to lifting on a synthetic polymer gesso – primarily, I think, because the paint's water content has less places to go/stays more present, and water is the diluent for ET!). 

I don't know how much a synthetic polymer gesso decreasing adhesion, but seemingly it does somewhat from delamination issues I've seen.  But does a plastic-based gesso increase fatty acid migration? Egg temperas on traditional gesso effloresces too, after all.  The question is: animal glue takes up water, but can it also take up lipids?  Does an animal glue binder provide storage not only for ET's water content, but also egg oil?  Anyone know the answer to this?

Koo

​I agree - I've seen problems occur literally with a few weeks or up to decades later.  ​To be fair to tempera, lots of paints can delaminate after x number of years for many different reasons, some having nothing to do with the paint/ground (even a well made painting can fall apart with enough humidity or other trauma); and it can be hard to pinpoint the cause without a thorough understanding of every aspect of the materials, methods and atmoshperic conditions (any one or which, or combination, can cause the problem).  

​As for egg yolk sticking to a car...yes, pure yolk is very different from yolk + pigments. Pigments aren't in and of themselves, adhesive, of course (i.e. throw dirt at a car and it won't stick!).  And much of tempera's vulnerabilities (porosity, absorbency, rigidity) comes for its high pigment content.  So a "binder" on its own doesn't have the same potential issues as "paint" (binder + pigment).  Egg yolk on a car also get's baked onto the surface (sunlight is the best way to cure egg yolk); one can't bake a painting in the same way.  So it's hard to make too direct of a comparison. 

Koo