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I've been working with a traditional egg-tempura technique that I was taught, but I have had some problems with the paintings later on.

The recipe is essentially one egg-yolk (without the yolk's membrane) mixed with one spoonful of vinegar and one spoonful of water. Powdered pigments are then stirred into a small portion of this mixture at a time and applied with a small brush (usually between a 0 and a 2 to avoid bubbles and control the flow of the paint) to the panel.

About a year later some tiny dark spots appeared on the painting, that may be the result of egg mixture rotting. The recipient of the painting says that it could be from being stored in an outdoor unit exposed to heat over a summer, but I am worried that it could be my recipe or technique.

This image shows my process in case that is useful.

Madonna & Child in traditional egg-tempera

Additionally the only main difference between my method and the traditional technique that I was taught is that I worked on a gessoed wood panel instead of a wood panel with a thin layer of plaster. (I'm including this information in case the relative porousness of the support could have affected the paint over time). (Note: The spots only appeared in the painted area, not in the gold leaf).

  • I think a lot of things could cause this, but if the tempera itself is bad, this should soon show up in all areas (esp. those with the same pigments). What do these spots look like? Is it a fungus, or a decoloration? And is the gesso a chalk skin glue mixture, or an acrylic paint? – Joachim Jan 10 at 21:30
  • On a sidenote: is the green of your shadows actually coming through the skin layer? It doesn't appear to be the case, judging by the photographs, seemingly defeating its purpose. – Joachim Jan 10 at 21:34
  • Joachim, the spots are small and dark, about the size of a dot that would be made by the tip of a felt pen (like a Sharpie). When the owner of the painting sends me a photo, I will add it to the post; I've seen it in person, but do not have photo documentation. I do not know if it is a fungus, rot, or a mysterious discoloration. I do not believe that it is a mark on the surface. I believe it is a change in the color of the paint itself. The gesso was acrylic based, applied in several layers, and sanded very smooth before painting on top. – A. Staffelbach Jan 10 at 21:51
  • Joachim, addressing your side question, it's hard to say for certain if the verdaccio (green under painting) is doing much in the finished image. It is supposed to add vibrancy to the color of the skin. Egg-tempera has just a little transparency and is meant to be build up in layers, so it could be subtly influencing the finished look. I did it because it is what is traditionally done. – A. Staffelbach Jan 10 at 21:59

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