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Moderator Answer
(koo schadler)
Egg tempera artists are rarely prolific since it's a slow medium; so there's constant pressure to produce and get work out into the world – I'm sympathetic.
To correctly temper and varnish egg tempera, it's important to understand PVC, pigment volume concentrate. It's been discussed previously on this forum (enter into search engine), I'll try to keep it simple. Basically every paint has an ideal ratio of pigment to binder (it varies among binders and, to varying degrees, among pigments). For egg tempera, the ratio is approximately 1 part pigment : 1 part binder (with minor differences among a few colors, a subtlety we needn't address for now). Most people add a percentage of water to yolk to make egg yolk medium; many people work with pigment pastes that have a percentage of water. So, to temper properly, you have to mentally subtract out whatever water is added to medium and/or pigment pastes to achieve the correct ratio of 1 part pigment : 1 part egg yolk. This sounds mysterious and daunting to newcomers, but experienced tempera painters know the feel/behavior of properly tempered paint and, with practice, good tempering becomes second nature.
Once paint is properly tempered, you can use as is (consistency of light to heavy cream, depending on your egg yolk medium) or add water (thin to consistency of milk or watercolor). Whatever water is added evaporates away, leaving behind the correct ratio of pigment to binder. As long as paint is tempered correctly at the start, the consistency can be altered by adding any percentage of water. This is why you see tempera artists working with thick paint, thin paint, and everything in between.
An under tempered paint has too little yolk - it tends to dry chalky, not adhere well, create dust, smear. An overtempered paint has too much yolk - it tends to feel greasy and slippery, layers are hard to build up, may eventually crack. While you needn't get the ratio absolutely perfect, you want to be in the ballpark (more or less a 1:1 ratio) to create well behaved paint and good adhesion.
As the painting coheres over time, an over tempered paint naturally expresses to the surface excess lipids; this is "fatty acid migration" (FAM). Other factors can play a part in FAM (i.e. artwork which moves between dry and humid environment seems prone), but fundamentally I think FAM results from too much binder in the paint. This is why I joked that the lack of FAM in my work must indicate that I temper perfectly each time (when, in fact, it's probably because I varnish, which is thought to suppress FAM).
From what I hear, some Wyeth paintings show FAM - which isn't necessarily a surprise. Seeing his work in person I've noted varying degrees of shine on the surface: slightly over tempered in parts, a whisper under tempered in others - not enough to jeopardize the paint but enough to create slightly uneven finishes and, perhaps, occasionally FAM. At least, that is my understanding (please correct me, Brian or Joyce, if I am oversimplifying).
Some icon traditions encourage the application of regular "nourishing layers"; i.e. thinned washes of egg yolk medium. Nourishing layers are initially gratifying (because they re-saturate and intensify values and colors, like a varnish). But there isn't a need for them (unless previous paint layers are very under tempered) and they throw off the correct ratio of pigment to binder, injecting unnecessary fats into the paint layers (which can eventually create problems like cracking or FAM). So you don't want to work with an "egg rich" paint (as you've described it to me); you want a "properly tempered" paint.
Okay, now onto varnishing. One of the characteristics of egg tempera is that it's correct ratio of pigment to binder (or critical PVC) yields a paint with a very high percentage of pigment (a high solid content). In fact, there is so much pigment relative to binder that pigment particles protrude above the surface of the paint film to create an irregular, porous, open surface. I'd like to insert an illustration here, but not sure how to do it...Brian, help? (or the questioner, who I know, can email me).
This irregularity creates the matte appearance of egg tempera. Varnishing fills in the porosity and smooths the surface, which increases saturation and shine.
When a varnish is applied directly atop an irregular, porous egg tempera painting, the varnish sinks into that porosity. This is why varnishes on egg tempera can appear uneven and may take a long time to dry (coatings that dry via oxygen aren't well exposed). A coating directly on egg tempera also becomes nearly inextricably part of the painting (it doesn't sit on top of the paint film, but sinks into/meshes with it).
One solution to these challenge is to first apply an "isolating" layer to egg tempera – a thin coating that dries quickly through evaporation of its solvent, and thus doesn't have a lot of time to sink into tempera's porosity. Then, on top of this "isolator" a painter could (if desired) apply another coating, for more protection or to control shine.
For example, I first "isolate" my temperas with either shellac (a controversial choice discussed in other posts) or B-72; then I finish with "wax medium" (wax, solvent and synthetic resin; there are many commercially produced wax mediums such as Natural Pigments, Gamblin, Grumbacher, Renaissance Wax). The isolating layer protects the painting; it also allows me to apply wax medium before the tempera has fully cured (the mechanical action of rubbing on wax can smear an uncured tempera). Wax medium creates a softer, more organic finish that I prefer to the isolator finish (and adds a bit more protection).
While the natural finish of egg tempera is matte, isolating/varnishing allows for any degree of saturation and shine. Many tempera artist object to altering the natural tempera finish but, as Dr. Stoner notes, it's really up to the artist (nevermind that icons have been varnished for centuries; and many [pretty much all?] the Renaissance temperas we see in museums have, at this point, been varnished).
I am skeptical about mixing varnish into paint – each dries at different rates through different means; I think the varnish could interfere with the polymerization of egg yolk into a stable paint film. But I am not a conservator - Joyce or Brian better address this question. It is possible to add a drying oil or resin (synthetic or natural) to egg tempera, to create a tempera grassa paint – but that's a whole other topic and I've already gone on too long.
Koo Schadler