Egg Tempera Cracking/Crazing

Sorry to hear about this, it’s discouraging, I know.  I often hear of iconography traditions that recommend periodic “nourishing layers” – but the reasons for doing so aren’t clear.  It’s true that a nourishing layer initially re-saturates a surface & colors appear richer, but this effect doesn’t last.  In fact, deep saturation is contrary to the nature of pure egg tempera; it’s a high PVC paint that naturally has a matte to satin appearance.  (To make egg tempera’s natural finish more saturated requires varnishing – a complicated subject addressed in other posts). 
Durable paint requires a proper relationship between pigment and binder, known as CPVC (critical pigment volume concentrate).  CPVC has been discussed in this forum (enter CPVC into Search); there’s also an excellent article by Sarah Sands at  https://www.justpaint.org/pigment-volume-concentration-and-its-role-in-color/.   CPVC varies depending on the paint.  Once it’s achieved, there isn’t a benefit to adding more binder to paint; in fact, excess binder is generally a problem.  It can cause cracking, yellowing (in oil paint), and efflorescence onto the surface of excess lipids (fatty acid migration). 
For egg tempera, the ratio of pigment to binder (pure yolk) is approximately 1:1 (with very minor variations among a few “fatter” or “leaner” pigments).  Once you’ve tempered (mixed yolk and pigment) properly, there’s generally no need to periodically add additional layers of pure binder.
One exception is if you’ve “undertempered” (haven’t added enough egg yolk to your paint).  A very thin nourishing layer might be justified on top of an under bound paint layer.  However once an undertempered layer is “nourished”, the solution is to correctly temper subsequent layers of paints, not continue to add nourishing layers.  Even when a nourishing layer is called for, it’s best to keep it minimal.  I recommend a ratio of 1 yolk to 6 to 8 parts water or wine (I prefer water).  
Presuming you have proper tempering in the underlying layers + that extra rich nourishing layer = definitely too much binder on your surface.  How to remove it is a difficult question.  Egg tempera paint’s CPVC has a very high ratio of pigment to binder; so much pigment, in fact, that particles aren’t fully encased by binder and slightly protrude above the paint surface – visualize a sheet of rough sandpaper, greatly enlarged.  This makes the surface of egg tempera paintings rough and porous (on a microscopic level; this also accounts for tempera’s naturally matte finish).  Any coating (tempera paint, a nourishing layer, varnish) applied on this absorbent, irregular surface sinks into its nooks and crannies, and thus is difficult to remove.  As an experienced tempera painter I can offer a few suggestions how to approach the problem – but I am not a chemist or conservator, and am hoping one chimes in.
1. Whenever a painting needs restoration work, the most responsible answer is “take it to a conservator with expertise in that paint”.  I recognize this is impractical for most artists, so onto other options.
2. You can try to rewet and blot up the layer, as you’ve been doing.  Water works just as well as wine, in my experience (more about wine, below).  There are a couple of problems with this approach. First: To fully remove a nourishing layer sunk into an irregular tempera surface is very difficult to do without also removing surrounding pigment particles. To really get the nourishing layer all out, you’re probably going to remove some paint as well.  Two: Tempera has a great capacity to absorb and release its solvent (water) without damaging the paint film; however my experience is that repeatedly saturating a temper painting too quickly can cause fine craquelure to develop throughout the depth of the paint layers.  I don’t know why this is – my guess is that too much solvent, too quickly in and out of the paint film, swells and shrinks the paint film too rapidly; and/or draws out plasticizing elements in the paint; or doesn’t allow the binder to coalesce into a polymer; or…?  This is where conservators input is welcome!  
Suffice to say that if you’re going to soften and blot up the nourishing layer by wetting the surface, wait a bit between solvent (water or wine) application.  If I’ve been building up a lot of paint layers very quickly, the test I use to see if a surface is ready for more is this: If the painting has been allowed to dry for a minute or two but I still can feel residual coolness when touching the surface, that tells me a lot of moisture has accumulated in the underlying paint and it needs time to dry before applying more layers (either paint or, in your case, water/wine).  
One more consideration here – the longer the unwanted nourishing layer sits, the more the yolk polymerizes.  At some point (anywhere from 2 weeks onward) it will become increasingly insoluble in water and you won’t be able to remove the nourishing layer by dissolving it. 
3.  You could let the surface cure (polymerize) for a bit (1 week or more), then do a light sanding to remove the nourishing layer.  I’d use a sanding sponge (cushioning on the pack minimizes scratches) around 400 grit up to 1500 grit.  Higher grit sanding sponges are available from fine woodworking and autobody stores.   As with #1 above, you’ll inevitably remove some paint as you sand, and thus would have to repaint affected areas.  
That’s all I can think of for now.  Hopefully someone else can recommend less disruptive solutions. 
One final note.  There are two reasons that I know of to add alcohol to egg yolk medium.  One is to preserve the yolk.  Alcohol in general (wine specifically contains mostly water, ethanol, and various acids) can extend the life of egg yolk medium by a few days.  This was an important consideration before refrigeration.  However if you start with a fresh yolk, and refrigerate it when not in use, you should be able to get 2-3 days from it anyhow, without adding preservative. (On a few especially hot days each year I put my egg yolk medium in a dish of ice while I’m painting). After 2-3 days of painting I’ve generally used up the yolk anyhow; and, if not, eggs are an affordable, readily available binder. Alcohol actually can potentially mask the smell of a deteriorating yolk, and it’s important to know when an egg is going off. 
The other reason to add alcohol is to help hydrophobic pigments disperse. However most pigments disperse readily & don’t need the help of alcohol. For the few hydrophobic pigments in my palette, I prefer to add isopropyl alcohol to pigment paste (rather than medium); then, I’m adding alcohol only to colors that actually need it, not to those that don’t. 
In short, alcohol isn’t an essential part of egg yolk medium. The simpler the medium (one less material, one less step involved in making the medium), the more efficiently I work. Also, the more complexity in a system, the more prone it is to complications. For all these reason, I don’t bother with alcohol.  If you are accustomed to alcohol in your medium, or need your medium to last a bit longer, or have many hard to disperse colors, it’s perfectly fine – just not requisite. 
As Cennini notes, “Begin by adorning yourself with these vestments: love, reverence, obedience and perseverance” – most especially the latter, sigh, at times like this.  Good luck.  
Koo Schadler
 
 
 

Koo
Thanks so much for being our newest official moderator. You have been an unofficially one in terms of egg tempera for quite some time.
Also, thanks for covering these questions. I have been struggling with teaching studio classes online at the University and that has dominated my time over the past week and more.

​Your most welcome, it’s my pleasure.  Koo