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.
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).