I glaze oil paint very thinly atop my tempera paintings, but before doing so I isolate the tempera with shellac. (If I don't, I find the absorbent true gesso ground and tempera painting suck up too much oil and the paint drags). I use only bleached varieties of shellac, mixed myself, in very thin formulations (a 1:12 shellac/denatured alcohol ratio). From what I've read about shellac, whatever percentage of yellow appears in it when initially mixed does not increase over time and my mix starts out virtually clear (and my 13 year old test strips have remained clear). I've read that it does become a bit brittle, but its going on top of a solid panel support. I have checked with conservators in the past and have been given their approval for this process, but nonetheless I run into painters who are surprised I use shellac in this way - they say shellac is too brittle, yellowing, and prone to cracking. So my questions are...
1. Does anyone have any thoughts on this process - its pros and cons?
2. Is it true that shellac does not increase in yellowing over time (i.e. is it photo stable? Is that the correct term?)
3. What percentage of shellac is needed to successfully isolate the tempera/true gesso ground? I find my 1:12 ratio works okay, but wouldn't mind applying a slightly heavier solution (maybe 1:10 or 1:8) to more fully isolate the tempera. How much shellac could I use and not experience too much brittleness, yellowing, etc...?
4. If the shellac is quite dilute (as in my ratios above) could I paint tempera grassa on top of the shellac? What is the ratio of shellac in india ink? (Since pure tempera can go over india ink, I'm presuming the slightly more powerfully binding tempera grassa would adhere to a full coating of shellac if the shellac were about the same ratio more or less as india ink).