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Old 04-08-17, 06:48 PM
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Koo Schadler Koo Schadler is offline
Tempera Painter
Join Date: Dec 2003
Location: Alstead, NH & Zirahuen, Mexico
Posts: 317
Default Cracking in Egg Tempera Painting

Hello Fellow Tempera Painters,

I have a cohort who's experiencing cracking in the uppermost layers of her paintings. It starts out as very faint, fine lines that gradually increase with successive layers; the lines grow and evolve into fine craquelure, and eventually flaking paint.

I've been thinking about this, trying to understand what's going on. From what I know, here are some reasons why cracking may appear in tempera paint:

1. Excess binder.
As discussed in recent posts, adding too much binder to either gesso or tempera paint can cause cracking due to, as George O'Hanlon says, "the stress caused by the protein molecules shrinking as water evaporates".

2. Too thick a layer of paint.
Since tempera initially dries through the relatively rapid evaporation of its water content, if too dense a layer of impasto paint is applied, it can crack as it shrinks (akin to what a dried-out lake bed looks like).

3. Adding too much water to tempered paint.
Once the proper ratio of egg yolk to pigment is achieved (the paint is properly "tempered") it is possible to thin the paint significantly with water. However if TOO much water is added, at some point the various components of the paint become so separated and attenuated that it can create a weak paint film. Brian Baade, a painting instructor/conservator at the University of Delaware, described this problem; and although he was speaking of oil paints his explanation is relative to ET: "As to over use of diluents contributing to a weakened oil film, think of it like this, while in theory a diluent is not changing the percentage relationship between pigment and binder, there is a point where pigment particles are so separated and the binder so attenuated that the resulting paint film is friable and underbound".

(The solution to this problem, by the way, is to add a wee bit more yolk to very, very watered down paint.)

4. Over saturating underlying paint layers with water.
There has been research on the effects of solvents on paint films, including various solvents (acetone, alcohol, water) applied to egg tempera. Apparently solvents, most especially spirit based solvents, but also water, can induce swelling of paint films; and if a curing paint film is compelled to repeatedly expand and shrink, this stress can weaken the bonds being formed in the polymerization process and create cracks (at least this is how I understand it; hopefully we may get a paint chemist to chime in on this one).

5. Stresses in the ground and/or support.
Cracks in the gesso (either from too much binder, or the panel being dropped, or temperature extremes, etc) and/or panel (changes in humidity) can telegraph up through the paint layers.

I'm interested in an informal survey to develop a better understanding of cracking in egg tempera; how often does it occur, what causes it, how to address it. So, my questions to fellow tempera painters are...

1. Has anyone out there experienced cracking in their paint layer?
2. If so, would any of the above reasons pertain?
3. Any other thoughts on why cracking might occur?


Koo Schadler
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