Nontraditional use of egg tempura

There are several companies (Sennelier, Daler Rowney, Zecchi) that sell commercially produced paints labeled “egg tempera”.  These are not, in fact, pure egg tempera paint (if pure egg tempera paint – just yolk and pigment – were put in a tube, it would quickly putrefy!)  Tubed “egg tempera”  is actually an emulsion of egg protein (albumen) and a drying oil.  Such paints would be more accurately labeled as “egg oil emulsion” or “tempera grassa”.  Being water-soluble indicates there is a greater percentage of yolk binder than drying oil in the emulsion. (If there were a greater percentage of oil than yolk, it would be solvent soluble).  There is only one way to have pure egg tempera paint, and that is make it yourself.
Tubed, tempera grassa paint is a perfectly fine medium; however it does not have the same properties as homemade egg tempera.  The oil content makes the paint more painterly and a bit more flexible.  It can be applied more thickly than pure tempera, although not as impasto as oil.  If applied too thickly, it will develop cracks. 
Pure egg tempera reached its peak of renown in the 1400s – a time of more tightly rendered, linear, representational imagery.  So those qualities, to an extent, are not inevitable to egg tempera; they’re, in part, the result of the visual preferences of the era in which egg tempera dominated.  It’s true that egg tempera has a great capacity for fine, precise linework – but it’s also perfectly possible to work in a looser, more painterly style with the medium. I love to see artists work this way as it helps egg tempera to break out if its 15th century shell and expand the medium’s range.  
One of the unique characteristics of egg tempera is its capacity for layering; it’s possible to accumulate literally scores of distinct layers in a single day (whereas if you applied 100 layers of watercolor or gouache you’d end up with mud, since the underlying layers remain soluble; and if you painted 100 distinct layers of oil it would take you as least as many days).  The only other medium that I can think of that has a similar capacity for layering is acrylic (which, as “plastic” has a different feel from tempera, which feels more “organic”, so to speak).  Many, many layers (of local color, glazes, scumbles, whatever) can impart a lot of atmosphere and depth to an image.  So by applying just a few thick layers versus many, many thin ones, you’re not taking advantage of a special attribute of the medium – but that’s not a disadvantage if you’re getting the results you want in just a few layers.

Pure egg tempera has a high PVC – lots of pigment relative to binder. There’s so much pigment that pigment particles protrude slightly above the level of the binder, which creates a porous, more open and absorbent surface.  This makes the surface of egg tempera more vulnerable to atmospheric moisture & things landing on and getting embedded into the surface.  Tempera Grassa’s surface isn’t as vulnerable as pure tempera but a bit more so than oil.  To protect the surface, you can either frame under glass or varnish.  Varnishing is a complicated subject in egg tempera; there are pros and cons, and it takes consideration and experience to do well.  This has been discussed in other threads (search the website for varnishing egg tempera). 
Finally, to clarify – the correct term is egg tempera.  It comes from the idea that pigments are being “tempered” with a binder of egg yolk. Tempura refers to Japanese fried food.  A common mistake but good to clarify. I think that addresses all your questions. 
Koo Schadler

Dear all,
I am working in homemade tempera grassa, oil:yolk=1:1 with various water content added.
I would like to start painting with thicker impasto and then move on to thin glazes and scumbles in subsequent layers. I would want brushstrokes to show through to the end.

Are there any suggestions?​
My ground is half-chalk, but I could move over to traditional gesso if needed.
Kind regards,
Damir P.

I concur with Brian about not going too impasto.  I’ve played a bit with how thickly I can apply both egg tempera and tempera grassa, and depending on various factors (water content in paint, pigment, thickness, percentage of yolk:oil in tempera grassa) cracks can develop – sometimes visiible to the naked eye; but I’ve also looked under a magnifying lens and raking light, and perceived hairline cracks in thickly applied paint.  So initially, as you’re developing a feeling for how far you can push it, you may want to use a lens and strong light to check for fine cracks in your impasto paint. 
My understanding is that the main reason thick passage of tempera crack is that water comprises a large volume of wet tempera paint; hence much of a thick blob of tempera paint disappears quickly (via evaporation of water) and, consequently, a thick egg tempera paint film doesn’t have time to coalesce into a stable network.  The result is cracking.   Is this about right, Brian?
One way I’ve been able to apply slightly thicker areas of tempera without getting cracking is to let a small pile of properly tempered paint sit for a bit. I occasionally mist it, but only very lightly – I don’t fully rehydrate it.  Gradually the paint loses its water content and eventually, over the course of a few hours, reaches a consistency that is akin to softened butter (or, for that matter, oil paint).  I can apply this thicker quality tempera paint with some degree of impasto.  Because it’s lost water content gradually (over the course of a painting day) it seems to tolerate being somewhat thick and doesn’t crack.  Granted, it is not the 1/4” dab of paint that can be achieved with oils.  Still, it is genuinely thick for egg tempera (perhaps 1/16” or so) and appears very dense and opaque relative to the more characteristically thin layers that surround it.  You may want to play with this idea with tempera grassa as well – though I’d be interested to hear what Brian thinks of this practice, and if there are any potential pitfalls. 
Koo Schadler