Skip to Main Content
Sign In
Toggle Navigation

Open the Navigation Management window, which can be used to view the full current branch of the menu tree, and edit it.

  • Instagram
  • Facebook

MITRA Forum Question Details

Image Picker for Section 0


  • ShellacApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2017-01-17 08:43:08 ... Most recent comment 2017-01-17 08:53:00
    Egg Tempera Other Rigid Supports
    1. Does shellac discolor with age?  I’ve read both that it’s photostable (whatever degree of yellow is present when it’s initially made, depending on the cut, does not change over time) and also that it gets more yellow with age.   I have a 18 year-old test strip that shows no color change so far, but perhaps that’s not long enough to say.  Has there been testing on the yellowing of shellac?

    2.  My understanding is that shellac gets brittle with age.  If I’m using it as an isolating layer on tempera (which also gets brittle with age) on a panel, does the solidity of the support address this concern?  Or is brittleness always a concern, regardless?

    3.  I believe shellac becomes increasing resistant to solvents as it ages – is this irrelevant if its purpose is to isolate?  In the case of isolating, could this be a sort of benefit, like a paint film curing and becoming insoluble, so to speak?

    Thanks as always for your help.  Koo Schadler

Answers and Comments

  • EditDeleteModerator Answer


    I understand that this issue is important to you. First, all references that I have read in the conservation literature state that it yellows with age. This is borne out by my own experience. I have a paintout of water-white, bleached shellac that has become quite yellow after only 4 years of ageing. How do we reconcile our experiences? I applied my shellac quite thickly as I was using it on a panel to demonstrate the natural ultraviolet induced visible fluorecence of natural and synthetic resins. You can view a pdf of this chart in visible and UV exposure on this page by downloading the “UV Chart”. Your sample was probably no thicker than you would need for you particular isolation requirements. In that instance, the yellowing is probably not pronounced enough for you to notice. Scientific tests for this really require very sensitive colorimeters to determine if any change has occurred. For your purposes, visual confirmation is really all you care about.

    On the other hand, the literature that I can quickly reference is primarily focused on paintings and easily accessible references from the coatings world are likely outdated as shellac long ago ceased to be important in industry. I would bet that the most recent studies/tests on shellac probably come out of the furniture or object conservation world. I will send out an email to a few others to see if they have anything more concrete to add.

    Shellac does get brittle with age and as you mention, so does the egg tempera painting. In adverse conditions heavy applications of shellac can separate into little alligator-skin-like islands or even become powdery. You have probably seen this on very old doors or furniture. Again, this is probably not going to be an issue considering the thin applications you use.

    Overtime, shellac certainly does become less soluble in the solvents that originally easily dissolved the resin. This occurs even more quickly with bleached shellac. This may or may not be a problem as you mention. It could be an issue if the isolating layer ever needed to be removed. In your case, some of the solvents that are quite damaging to relatively young oil paints have little effect on pure egg tempera paints.

    Those are my thoughts on the subject. Others may have a different viewpoint. As I wrote, I will send out a query to a few conservators in other specialties to gather their thoughts. In the end, as with all materials choices, you need to weigh the benefits vs the deficits and decide what is important to your own particular needs and artwork.

    Baade, Brian
    2017-01-17 16:44:30
  • ApproveRejectUn-ApproveUser CommentHi Brian, I use a very thin solution of 1 part bleached shellac to 8 parts alcohol and it's virtually transparent. Where I applied a more dense coating (1 shellac:4 alcohol) on my test panel, it was yellow from the start but doesn't appear to have visibly changed, but it may be that it hasn't been exposed to sufficient UV rays (it sometimes lives in a folder, for teaching purposes). I appreciate seeing your chart with its visibly yellow shellac square. I like shellac and have gotten adept at applying it, but am considering switching to B72, which conservators most often recommend to me as an isolator. I tried a solution of 1 part B72 resin to 5 parts acetone and it seemed to work well (albeit with a caveat that I'll save for another question, another time...). Dr. Stoner discussed using a 4% solution of B72 in xylene to coat some Wyeth temperas (to suppress efflorescence). I like the idea of the thinnest isolation coat possible. What would you recommend for a ratio of B72 to solvent for isolating, and is there a benefit to xylene over acetone? I need to get more practiced before applying B72 to finished paintings, which as you know take a long time in tempera - I'd hate to make a mistake or discover unanticipated complications. So more experimenting is needed, and in the meanwhile I'll stick with shellac. I understand it's an outdated practice to use it on paintings and generally frowned upon - hence my questions, to make sure I'm not doing something too terribly wrong. Thanks, Koo
    2017-01-17 19:43:15
  • EditDeleteModerator AnswerI received the following general comments about the use of shellac on fine and decorative artworks from Martin Kotler, conservator of frames at the American Art Museum, Smithsonian Institute. 1. Shellac is stable but does change (yellow) over time, but that is also a question, how many layers were applied and to what surface? Is 18 years a sufficient time lapse, here again one layer two layers or more and the environment that the test or product will exist in. Treatment of an 18th cent. Lily Frame made in England and its surface revealed so many problems, a graying of the shellac but my final assessment was the frame was made with “Green” or not dry Pine, wet climate verses dry and what happens over time. 2. In gilding frames, a diluted or thinned shellac is used as a sealer or barrier over the many various karats of gold, silver and metal leafs, patinas (oil pigment, caseins colors or chemical, acids) are then applied to the gilded surface adding additional tones to enhance or create a historic period or mimic collected grime to the frames surface, and then sealed with either a clear lacquer or wax. 3. Commercial shellac (Clear or Amber) verses Crystals suspended in alcohol, Spray shellac or brushed and what is the desired effect over time. 4. Is the shellac pure or was Gamboge and or Dragons Blood added. 5. Shellac becomes brittle over time and can “gray” besides yellow, but what was the intent of the artist and what is it applied to. Note; I had to remove a shellac layer applied as a (finish) over a Rosewood veneer on a frame, it was applied very thick and it had become so discolored that one could not tell that the surface was Rosewood under the shellac layer, the frame was maybe 100 years old. (Environment)? 6. Shellac over Wood, Egg Tempera or Casein or even Crayon, I have experience all and found various toning changes, but here again, what affect is the artist trying to produce? Here I believe shellac seals the surface but has a permanent effect on the artist materials. Note; I tried a few years ago but failed during a conservation evaluation with the curator, not to acquire a work of art into the collection, because of shellac over crayon which is a future failure, my argument that we cannot remove safely the (amber shellac) without harming the crayon below. I guess the answer is shellac over some materials make sense, e.g. (Gilding) where the richness of the gilding is enhanced over time by the shellac sealer as to over another material that will only alter over time and finally what is the artist desired effect? I am not aware of any noted tests on shellac long term.
    Baade, Brian
    2017-01-18 11:11:45
  • ApproveRejectUn-ApproveUser CommentIt's very helpful that you can put us artists in touch with so many conservators. The consensus seems negative regarding shellac and paintings, albeit there seems to be an acknowledged and meaningful difference between thick, multiple layers of toned shellac as a top coat versus a very thin and dilute layer of bleached shellac as an isolator that's not intended to be removable (the latter perhaps contributing no more yellowing than the oil in oil paint?). Still, I get that shellac is problematic and will make an effort to transition to synthetics. I need more practice with them, but like most tempera artist I produce a very limited inventory of work and it's hard to sacrifice pieces to experiments, but I'll keep at it. If you know of any published studies or personal experiences among your conservator and/or painter friends regarding the behavior of B72 or Molwillith over freshly painted egg tempera, I'd be interested. I know they are well-studied and respected materials but I've occasionally run into surprises when two previously unfamiliar materials are layered atop on another. Thanks for all your help. Koo
    2017-01-18 11:57:42
  • EditDeleteModerator AnswerHi Koo First, I would defer to Joyce on issues relating to varnishing tempera paintings with B72 and relative proportions. She is probably more experienced with this than anyone. Keep in mind that she is generally varnishing an older tempera and one that was probably already varnished in the past. I will shoot her a link to this thread to see if she has any additional comments. I would suggest that if you are going to use B&@ that I would dilute it in xylene and apply it is a well ventilated space, preferably with a respirator. Xylene is certainly more toxic but it would be a far better solvent for brush application. Acetone dries so quickly that it is very difficult to apply an even layer without overlaps, (like those seen when one pauses when applying a watercolor or tempera wash, except the overlap is gloss rather than a deeper color). Xylene has a much slower evaporation rate allowing for a more even coating. I would personally think that PVA solutions diluted in ethanol to the proper consistency would make an ideal isolation medium and would work very similarly to what you are used to. As you mentioned, you would need do a bunch of mock-ups and tests before to used it on your artwork.
    Baade, Brian
    2017-01-18 14:40:07
  • ApproveRejectUn-ApproveUser CommentHello to Koo and to Brian, I use B-72 in xylene because I work on both tempera paintings and oil paintings. Acetone is too strong a solvent to use near oil paint as it could pick up the oil paint if the coating were brushed on and especially if the oil paint is young or even elderly but mixed with varnish. Thus, my B-72 is in stock solutions mixed with xylene. [I just came out of the spray booth having brushed about 5% Paraloid B-72 in xylene onto a large painting from which I removed a fairly comprehensive efflorescence by brushing in 2011 (but did not coat with B-72 at that time). It has returned about 6 years later, now with even more efflorescence so I have now coated it and hope it won't need to return in another 6 years! Acetone also evaporates more quickly than xylene, which I would find disconcerting, but it is much less toxic than xylene so that's a good thing. Conservators like to be able to remove coatings if something goes wrong; regular shellac does usually come up with alcohols (which may or may not be safe near the paint film) but we've found several 19th-century American paintings with bleached shellac coatings [verified by our analytical lab] and those coatings won't come up with ANYTHING we tested; they had to be left in place. So I would certainly prefer treating paintings coated with B-72 rather than any kind of shellac 10 to 80 years from now [more than 10 I would probably have to bequeath to my former students.] Tempera is much safer near acetone than oil or oil-resin paints, in any case. Joyce Hill Stoner
    2017-01-18 15:03:13
  • EditDeleteModerator AnswerJoyce mentioned something that I should have made clear. Acetone will not dissolve true egg tempera in any manner but it should never be used as a solvent for anything that will be applied to an oil painting. Acetone is far too polar and will quickly and indiscriminately dissolve oil paint.
    Baade, Brian
    2017-01-18 15:40:20
  • ApproveRejectUn-ApproveUser CommentThanks for chiming in Joyce, I appreciate that you have lots of experience in this area. I actually prefer the rapid dry time of shellac because it sets so quickly that is sits more evenly atop the surface of egg tempera and is less likely to sink in; but I also know, as you say Brian, that overlapping strokes can be a problem (although less so with a very thin coating, and as an isolator). It's taken me a while to get adept at shellac, not get those strokes, which is one reason I am reluctant to switch. But I understand the benefits of synthetics and will practice more with B72 (in both solvents, to see which works best for me) and Molwillith. Joyce, have you found that a coating over tempera is a pretty good guarantee, as far as you know, to stop efflorescence? Does the type of coating matter? (FYI, I've never seen efflorescence thus far in my paintings, and almost always coated them with some sort of varnish). Koo
    2017-01-20 11:17:46
  • ApproveRejectUn-ApproveUser CommentDear Koo, if you like rapid evaporation, B-72 in acetone would be good. 4-5% B-72 in acetone or xylene becomes pretty invisible but nicely retards efflorescence. I hadn't realized you WANTED a visible coating. You could experiment with thicker B-72, say 15%. I worry about the bleached shellac because if something goes wrong with it, it would be much more difficult to remove--B-72 not so much. I had a phone interview with Paul Cadmus some time ago, and he said his temperas NEVER effloresced. We don't quite know why some do and some don't. With Andrew Wyeth's temperas, the ones that effloresced the most were ones he took back and forth between Maine and Chadds Ford WHILE he was painting them. Michael Schilling of the Getty hypothesized that temperas that undergo fluctuations in RH during their creation might be more likely to effloresce. That fits with what AW's works exhibited. Since your paintings (I believe) tend to be smaller, you may not be carrying them around to different environments and they may not be as likely to effloresce. Rothko's Houston Chapel black shapes carried out in egg tempera (to make them more velvety) effloresce a lot-- and being large and subject to changes in RH, that makes sense in that case.
    2017-01-20 18:08:27
  • ApproveRejectUn-ApproveUser Commentoops, should have put my name with the previous comment-- from Joyce Hill Stoner 20 January 2017
    2017-01-20 18:08:56
  • ApproveRejectUn-ApproveUser CommentThanks for sharing your extensive experience, Joyce. Very interesting about efflorescence - I didn't know about the RH influence. My paintings are in relatively stable RH while I'm working on them and, as you say, small. I'll do a survey among my ET colleagues and see who has experienced efflorescence and if there are correlations to size and RH. In fact I don't want a visible coating; I meant merely that I like how a quick drying coating doesn't sink in, seems to coat more the surface more evenly, but ideally I'd prefer it to be completely clear. So I will experiment with B72 in the 4-5% ratio you suggest. Thanks a million for yours and Brian's help. Koo
    2017-01-20 19:49:20

Page Settings and MetaData:
(Not Shown on the Page)
Page Settings
MetaData for Search Engine Optimization
MITRA Forum Question Details
This page cannot be accessed until you accept the Terms of Use, which can be found here.
Please note that this Terms of Use system uses cookies. If you have cookies disabled you will not be able to accept the agreement. If you delete our cookies you will need to re-accept the Terms of Use.
  • The Department of Art Conservation
  • 303 Old College
  • University of Delaware
  • Newark, DE 19716, USA
  • Phone: 302-831-3489