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Old 06-05-07, 12:19 PM
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Default Reviving the surface

As most of you probably know, after a significant amount of time has passed, ET dries to a rather different surface. I would like to know if anyone has any suggestions for how to revive the surface for further painting. One I want to work on again has begun to resist the application of further layers. What can I do? I have tried a bit of alcohol, not to ease my woes, but to try and make the surface less water repellant. It seems to work a bit but is it OK to do?

jeff
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Old 16-05-07, 11:07 AM
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I take this as a very bad sign that no one is answering this. I have decided now to experiment. First experiment has involved pulling out one of my old and not so good painting and swabbing parts of it with acetone in order to remove any oils from the surface. I report now that this has made the surface very reponsive to renewed painting. I suspect that the drying process separates these oils to the surface where they will later form an increasingly hard layer as they too dry completely. I think that a coat of ET medium after swabbing the parts that need repainting will completely renew the binding and effective paintability of the surface.

Please tell me where I am wrong.

jeff
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Old 16-05-07, 12:07 PM
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Jeff,
How much time has passed between painting sessions? Months, years, or weeks?
I've been able to return to a painting after several months, successfully, by laying down a couple of light coats of dilute yolk over the old paint.
Dennis
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Old 16-05-07, 01:30 PM
Alessandra Kelley Alessandra Kelley is offline
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This is one of those issues I wish the chemistry people would look at. Is anybody aware of a museum curatorial program that deals with this? Or any conservators or the like?

I would be hesitant to use a solvent to remove oils from a paint film, but then I would be hesitant to muck about with the film anyway. Good luck, Jeff, and please do let us know how it turns out.
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Old 16-05-07, 08:54 PM
David McKay David McKay is offline
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One of our members (Hisstah) I believe, is a conservator. Although I have not seen her post for some time, she may be a good person to ask about this. David
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Old 17-05-07, 03:46 AM
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The following exchange between conservator Ross Merrill and Koo Schadler appears in the Winter 2007 newsletter of the Society of Tempera Painters:

KS:
This is what I’ve understood regarding the drying time of an egg tempera painting: after a certain point the egg oil and albumen within one’s egg yolk medium “polymerize” (the proteins link up and form a water insoluble paint layer); the process of polymerization takes anywhere from 6-12 months (depending on numerous variables, such as number of layers, drying conditions, etc); once the paint layers have polymerized (or fully cured), one ideally should not work in egg tempera on top of the painting anymore, as the surface is no longer as absorbent, and subsequent layers of tempera paint may not stick well enough to the polymerized surface to be considered archival. Is this right?

Recently I went to an egg tempera show where a tempera painter was taking work from early in his career -- pieces that were 20-40 years old -- and reworking and finishing them to sell. Obviously, it is physically possible to do so; but would you advise for or against it? What is the rule of thumb concerning how long one can work on an egg tempera painting? (An icon painter recently told me that Russian icons are reworked in tempera over and over again, century after century...)

RM:
Tempera paints “dry” by evaporation of the water followed by oxidation of the oil in the egg. The rapid evaporation of the water allows layer to be built up quickly with virtually no drying time between layers, although such practice has the potential to create premature cracking in the film. Oil films, both egg and linseed oil, adhere to the substrate through mechanical bonding, although in both cases there is chemical bonding as well.

The pigments in the paint layer have a great effect on the quality of the bond and the integrity of the paint film. Black pigments, all of them, are slow driers in oil paint and I would assume in tempera as well. Manganese containing pigments such as umbers are fast drying due the presence of the manganese, a natural drier. Again, talking about oil paint to which egg tempera is closely related, earth colors, especially ochres, recently have been found to absorb airborne moisture in aged oil painting, rendering them somewhat soluble in water. Lean ochre films are, of course, much more soluble in water. Zinc in any mixture results in a very brittle paint film. Zinc is often incorporated into titanium white and lead white paints, and is used in Claussens oil grounds. (Their ads say two layers of acrylic gesso followed by two layers of zinc in oil. The most flexible of materials [acrylic gesso] below the most brittle of materials [zinc in oil] seems to be the recipe for future disaster.)

The oxidation of the paint film can literally take decades and there is strong evidence that the “drying” of oil paint continues indefinitely, although at a gradually slowing rate. Regarding oil paint films, both egg and oil, assuming the surface is clean and free of grime and oil residues, a new layer can bond to the older layer without a problem. Such practice can lead to cracking of the upper film if there is a poor bond to the lower layer. In short, I cannot see a reason not to paint over a dried paint film. There are lots of examples of the practice from Wyeth and Peter Hurd. In some early Italian paintings an oil glaze was applied over the egg tempera underpainting with highlights in egg tempera. These painting have stood up well, although I cannot cite an example off the top of my head.

KS:
I’m interested that you compare the oil in an egg to the oils (be they linseed or poppy or walnut, etc) used in oil painting. Are the oils in egg yolk as strong a binder as the other painting oils listed?
Also, when I’ve gone back to work on a cured egg tempera painting, I’ve found the surface to behave a bit like tempera applied on clayboard, or acrylic gesso the surface feels slippery, and the paint tends to bead up and misbehave a bit. To avoid such problems I either:
(a) Very lightly (and carefully!) sand the surface, to open up the paint layers a bit, or (b) Apply a nourishing layer (a thinned layer of egg yolk medium) over the entire surface. What do you think of these options?

RM:
I just chatted about this issue with one of our organic chemists and of course, he has a much too long “chemistry” response. The short answer is as follows: Egg yolk forms a very tough paint film. Leave it on a plate for a week and see what I mean. However it is less flexible than oil paint and forms a very brittle paint film, hence the need to paint on a wood panel or rigid support. One of the conservation scientists feels that the egg tempera forms a much tougher paint film than oil paint (more resistant to cleaning solvents, etc) and is the main reason that tempera paintings have survived so well. As the egg tempera oxidizes, it forms a very closed impenetrable surface. Since the new egg tempera paint has water in it, it is
difficult for the new paint to wet up the surface. An old recommendation was to rub a sliced onion or potato over the surface to break the surface tension (it also leaves a layer of starch). Using a surfactant or wetting agent will solve your problem although there may be some minor complications. Very dilute acetic acid (vinegar) has been used as a wetting agent by Italian conservators and artists for a very long time but if too strong, it can react with the gypsum in the gesso ground. Acetic acid (as used in photographic processes) can be used but must be carefully diluted. A detergent or soap can be used in a dilute solution but must be completely washed off, which can minimize its effectiveness. The safest wetting agent is ox gall, which is often used by watercolorists. Brush on and wipe off. I don’t have a recommended dilution to use but if there is some left on the surface, it will combine with the water in the egg tempera paint without causing a problem so long as there is not a high concentration. Sanding, of course, is the most drastic solution and of course, may damage the paint layer. The same wetting problem happens with oil paintings and the solution is to rub an oily painting medium over the surface and wipe it off as in the 19th century “oiling out” practice. I hope this is of interest and helpful.

KS:
Yes, very much so. By the way, I realize it may sound crazy, but I often sand my egg tempera paintings (while they are still in progress -- not when they are nearing completion). I use a very fine grit (1200-1500), thin, flexible sanding sponge. If done with attentiveness and care, I find it doesn’t scratch, and it seems to “help” the surface along -- the painting becomes more receptive to subsequent layers of paint. (However if any other tempera painters want to try this I advise that they do so with caution -- I don’t want to be held accountable for marring someone’s masterpiece.)
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Old 17-05-07, 10:58 AM
Alessandra Kelley Alessandra Kelley is offline
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Helpful! Thanks, Dennis.

I would only add (in case of random browsers who haven't heard this before) that sanding a painting -- any type of painting -- can be a hazard for breathing toxic dust and should be done while wearing a NIOSH (or the equivalent)-approved respirator.
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Old 17-05-07, 11:41 AM
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Just what I wanted! My painting is only about 3 months since I last worked it, but those have been quite hot months and the surface is beading significantly. I did try a layer of medium but that didn't work. I am reluctant to use the acetone of course though it has worked very well on my tester and without any abvious damage at all. In fact the surface looks unchanged but for the fact that it accepts the paint readily. I have got some more enquiries out to conservators to see what that also turns up.

Thanks all.

jeff
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Old 17-05-07, 02:18 PM
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Perhaps not really terribly helpful with the actual question, but as background info...

Quote:
I’m interested that you compare the oil in an egg to the oils (be they linseed or poppy or walnut, etc) used in oil painting.
There's a breakdown of triglyceride compositions from egg yolk here:

http://www.pubmedcentral.nih.gov/pag...06&pageindex=3

showing a moderate degree of polyunsaturation (about 14% linoleic acid moities within the triglycerides), which does support the interpretation that one of the mechanisms involved in "drying/hardening" might be some degree of oxidative polymerisation of the oils (though note that there's essentially no linolenic acid) - egg yolk's triglycerides are a much poorer "drying oil" than any of those used for oilpainting.

Speculating, a little, I'd guess that the major contribution is denaturation and uncoiling of the proteins within the egg-yolk, and that the resulting insoluble random-coil meshwork is then tangled together with whatever degree of meshwork gradually forms from triglyceride oxidative polymerisation. In the longer term, perhaps additional covalent crosslinkage occurs between proteinaceous and fatty meshworks?

Dave
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Old 19-05-07, 03:11 AM
David McKay David McKay is offline
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I am wondering if some of the problem may simply be surface dirt such as oil, dust etc. I would be tempted to carefully (lightly) clean the surface with mineral spirits. I realize that this may take a small amount of pigment and/or egg oil from the paint, but since it is going to be reworked in any case, it may not be a bad idea??? David
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