Hello MITRA,
I have an egg tempera student with a background in icon painting who makes her medium by combining 1 part yolk with 2 parts wine. For my class, she changed the ratio to 1 part yolk to 1 part wine (since I work with a thicker medium).
I’m not a fan of adding alcohol to medium: I prefer just egg yolk and water (cracking a fresh egg as needed; and adding isopropyl alcohol only to the few colors that actually need help dispersing). The simpler a paint system is the less that can go wrong, and the easier problems are to diganose. But I realize many painters are used to adding wine or vodka to medium, and of course they are free to work as they please.
This morning her medium, which she keeps refrigerated when not in use, had “curdled”. I explained that this is (I believe) protein denaturation: alcohol is causing the molecular bonds in the protein portion of yolk to break down, the same as cooking an egg.
So, two questions:
1. Why doesn’t this happen everytime alcohol is added to egg yolk? Does egg oil sort of “encapsulate” the proteins and protect them from the denaturing effect of alcohol? I understand there is less protein in yolk than in egg white, but still there is some, and clearly (from her example) it’s capable of denaturing.
2. Any guesses what casued her medium to denature?
Hope some of you chemists out there can shed light on this.
PS. The above question was asked by me, Koo Schadler. I’m an egg tempera moderator on this site, and for some reason the program, which usually signs my posts for me didn’t recognize me. I clarify this becasue I don’t want you, Brian, to send this question to me – since I clearly can’t answer it! Hoping you, George, Sarah or some other technically astute person can.
I am guessing that the egg was older than thought.
It is true that ethanol denatures egg proteins. However, there are a few things to consider before assuming that this was the culprit here. First, the ethanol solution needs to be rather concentrated for it to disrupt protein side chains. Additionally, most egg protein is contained in the white, although the yolk does some amount. About 17% of egg yolk is protein. For most artists a small dash of ethanol should have little to no affect on the binder. On the other hand, why add ethanol at all? It is far smarter to just use fresh eggs. They are cheap and this assures that your binder is fresh.
I don’t bother with ethanol either. Trying to shift some of the outdated practices in iconography (wine/vodka based mediums, olifa, working with mercury/lead based pigments in powder form), but it’s a very strong and much beloved tradition (for many good reasons, of course). Thanks for you input, Brian.
Just a followup question: could the wine have turned and become sufficiently acidic to curdle the yolk? Wine that has gone to vinegar would introduce acetic acid in addition to the tartaric, malic and lactic acids already present.
Rereading my comment, I see that I was not as clear as I should have been. I should clarify my comment about the relative proportions of protein in eggs (white vs yolk) Yolks have about 16-17% protein and whites have around 11%. What I mean by whites having the most protein is that they have basically only water and protein, so the final film would be practically only protein. Yolks have 16-17% protein but 27% oils, meaning that the dried film would be close to 1/3rd protein and 2/3rds lipids.
Additionally, Koo, I understand that you do not use wine/brandy, vodka, Everclear, etc. in your medium, I just wanted to make my position clear.
Michael, I am glad that you mentioned this. I did not think about it as I generally worry more about bases in conjunction with proteinaceous media and acids more with pigments like ultramarine. However, acids could attack the calcium carbonate or sulfate in the ground.
I do wonder about acidification now that you mention it. Perhaps if one is keeping a very old bottle of some cheap Chianti (substitute cab Sauv, P. noir, etc.) around for this purpose, it may be practically vinegar over time. I would like to do a little experiment with this idea when I return to the lab mid-February.
Yes, that’s an interesting idea. I know you’ll keep us posted of your experiments, Brian – thanks. Koo
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