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Old 04-08-17, 06:48 PM
Koo Schadler's Avatar
Koo Schadler Koo Schadler is offline
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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?

Thanks,

Koo Schadler
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  #2  
Old 04-08-17, 07:46 PM
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MBergt MBergt is offline
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I've never experienced any cracking in the "paint" of an egg tempera painting since I starting painting in ET in 1976. I have had some paintings where I painted on a hand made wooden panel and the wood itself moved causing cracks. Even in museums, I'd say most of the cracking issues are caused from the panel, gesso or support in some way. Rarely do I see an issue in the paint. That seems more common to oil paintings which take so much longer to dry and thereby cause the surface tension that leads to cracking.
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Old 14-08-17, 04:36 AM
arbrador arbrador is offline
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Sorry for the delay in my response.

Koo- you mention craquelure which I believe has it's own limited definition and is different from other types of cracking. Wikipedia has some fascinating information about craquelure which I'll copy below for convenience.

If your ET comrade is experiencing fine cracks then it might be "craquelure" which is defined as fine pattern of cracks at least according to Wikipedia which of course may be totally wrong!

What you say about the cracks eventually flaking off sounds like it might be an issue of insufficient egg binder. In my painting, titanium white is often the culprit. Even though I know and you have driven home in your workshops, that "tit white" requires more binder, I still find it to be an unpleasant pigment. Could you ask this person if the painting contains a lot of tit white?

Anyway, maybe ET fans out there can make some sense of craquelure as it relates to ET painting.

Thanks, Lora

Here goes Wikipedia: this is partial Search Wikipedia for the complete entry.

Craquelure (French: craquelé, Italian: crettatura) is the fine pattern of dense "cracking" formed on the surface of materials, either as part of the process of ageing or of their original formation or production. The term is most often used to refer to tempera or oil paintings, where it is a sign of age that is also sometimes induced in forgeries, and ceramics, where it is often deliberate, and usually called "crackle". It can also develop in old ivory carvings, and painted miniatures on an ivory backing are prone to craquelure.

Craquelure in paint[edit]
Normally, craquelure is formed by the aging of paints. It can be used to determine the age of paintings and to detect art forgery, because craquelure is a hard-to-forge signature of authenticity.[1][2]

Authentic[edit]
Authentic paint craquelure occurs because paint dries and becomes less flexible as it ages and shrinks.[3] In the case of paintings on canvas, the canvas slackens as it ages as it cannot endure the long-term stress of stretching.[4][5] Paint at the center of a painting is the least cracked, whereas paint at the edges is the most cracked, or stressed.[4] The precise pattern of craquelure depends on where, when, and under what conditions the picture was painted,[6][7] and subsequently kept. Cracks caused by stretching or slackening the canvas are quite different from cracks due to other factors, such as drying and ageing of the paint. The paint cracks when the stress upon it is greater than the breaking stress point of the paint layer and the paint will crack approximately at right-angles to the direction of the stress, relieving that stress.[8] The stress at the corners is more than double that of the center.[8]

There are distinct so-called French, Italian, Flemish, English and Dutch "styles" of craquelure, relating to differences in the typical techniques used (and the typical period when each country produced most of its art); however, these names simply refer to typical patterns, and an Italian painting might show a "French" style of craquelure. The distinctness of these styles has been largely confirmed by studies[which?]. The English style arises from the use of bitumen in paint that was pioneered by Sir Joshua Reynolds, and can be especially disastrous, leading to large blisters in the surface. Radiating circular patterns of cracks are a result of impact.[9] One study used images to get subjects to classify paintings according to the following "rules", with some success:[10]

Italian - usually perpendicular to the grain of the panel
Flemish - usually ordered network
Dutch - usually connected network
French - usually curved cracks
Italian - can have small to large islands
The pattern of changes in humidity and temperature that a finished painting experiences affect the way craquelure develops, with sharp changes especially likely to create craquelure. Modern acrylic paint is especially likely to develop craquelure if it experiences low temperatures.[11] The precise pattern depends on chemical characteristics of pigments used—from the finest light colors to the less perceptible dark, the painting style of the painter, and whether wood or canvas was used as a background. It also furnishes a record of the environmental conditions the painting has experienced during its lifetime, such as temperature and humidity,[5] and can also reveal details about the painting's history of handling, transportation, and restoration.[6][7]

Induced[edit]
Induced craquelure can be created by a variety of techniques, and in paintings is often used by forgers of Old Master paintings, which would normally show some. Art forger Eric Hebborn developed a technique and Tony Tetro discovered a way to use formaldehyde and a special baking process.[12] Craquelure is almost impossible to accurately reproduce artificially in a particular pattern, although there are some methods such as baking or finishing of a painting by which this is attempted. These methods, however, generally achieve cracks that are uniform in appearance, while genuine craquelure has cracks with irregular patterns.[13]
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Old 14-08-17, 12:40 PM
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Koo Schadler Koo Schadler is offline
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Hi Lora,

I've seen this painter's work in person, and can't say if the problem occurs exclusively in titanium white because her paintings have so many varied layers, some with white, but some just glazes of color. She and I tend to think it has to do with too much water in the paint and over-saturating the surface - she lays on so much wet paint that the surface feels consistently cool and moist, even when the paint is dry to the touch, which suggest there is still lots of water in the ground and paint layers. I too, at times, thin my paint with a lot of water, but then control how I apply it (so it doesn't go down as a puddle); or, if I do put on paint puddle-like, I quickly sop up excess water. Having too much water accumulate within a painting seems detrimental, perhaps for reasons #3 and #4 explained in my earlier post, but I don't know for sure.

It's not a problem of insufficient yolk - I've seen her temper her paints, she does fine, and this is further supported by the gorgeous, semi-gloss finish she is able to get when she polishes her painting.

I wouldn't say you want too much yolk combined with titanium, more like equal parts, because having excessive yolk can cause cracking. But I agree with you that titanium white can be problematic - I've heard, a few times over the years, from other painters who've encountered slight cracking in titanium white (even when properly tempered), particularly when built up (even thinly) in the lights and highlights of a painting. I'm not sure why this would be. Hence this post, trying to ferret out similar experiences from other painters and get more information on the circumstances surrounding cracking.

As for calling it craquelure, I'm probably not technically correct doing so, but maybe it is the correct term. Again, I'm not sure. I can almost guarantee that the majority of (maybe all) references to craquelure refer to oil paintings - so whatever is said about "craquelure" isn't necessarily applicable to egg tempera. Don't yet know what to call what I've seen....

I have a few conservators I can ask about cracking and will post what I learn. (The reason I haven't asked them yet is that I can be a question pest and try to limit myself so that I don't alienate valuable sources of information!). Conservators are great because of their scientific know-how; however it's also important to hear from painters, because hands-on experience can differ from what science says. So hopefully we might hear from more ET painters on this subject.

Thanks for chiming in.

Koo
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  #5  
Old 22-08-17, 04:49 AM
arbrador arbrador is offline
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Hi Koo,

In my last painting I used the watercolor "pour" technique. This goes way beyond the controlled icon technique of "petit lac" because pouring the paint on and letting it run off the panel causes some serious pooling of paint. I mention this because I noticed in some areas a fine pattern of cracks have appeared.

This is not academic but the term "craquelure" has always meant. to my mind, a pattern of tiny cracks is is somewhat attractive to the eye or at least does not interfere with the image. I would distinguish this from using the word "cracking" which connotes a defect. So although a little worrisome in terms of longevity I decided to admire my "craquelure" because it was fine and did not interfere with the image.

The painting is stored away otherwise I would try to post a picture. So in my case, it may not be too much water but maybe too paint that sits in a puddle too long although I may have diluted some of my paints too much.

I'm not saying that we should just learn to love our craquelure because those of us who paint very delicate paintings certainly don't want any eye sores but I'm trying to say that some craquelure may be acceptable to some ET artists and may not cause further damage or deterioration.

I found the following on the National Gallery website and they certainly include ET as victims of craquelure but as you've said, conservators often don't know much about tempera because they mostly don't practice it. See what you think. I'll look more carefully next time I'm in the medieval or early Renaissance section of a museum:

"The craquelure on a painting is the network, or pattern, of cracks that develops across the surface as the paint layers age and shrink.

Easel paintings in most types of paint, particularly egg tempera and oil, develop cracks which join up into a complex, extensive network."


Thanks!
Lora
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