Question asked 2017-01-17 08:43:08 ...
Most recent comment 2017-01-17 08:53:00
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
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
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from the coatings world are likely outdated as shellac long ago ceased to be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
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.
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.
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.
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.
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