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I have a fellow painter asking me about a piece she is creating in silverpoint and egg tempera. As she rightly points out, the sulphur in egg yolk causes silver to oxidize (in fact, a bit of yolk is sometimes added to metalpoint grounds to speed up oxidation). She may leave parts of the silverpoint visible in the final painting and doesn't want the silver to tarnish. She is thinking of applying a layer of rabbit skin glue over the underdrawing, to seal it off, before painting in egg tempera on top. I have a few questions about this:
1. Because rabbit skin glue is so hygroscopic, would a very thin layer of platina shellac be a better option for sealing off the silverpoint uderdrawing before moving onto tempera?
2. I know there are many examples of egg tempera adhering well atop india ink underdrawings, so I understand ET can adhere to shellac - but if an entire panel is coated with a layer of shellac (as suggested above), is adherance of the tempera a bit more problematic? If so, would a careful sanding of the shellac layer be sufficient to improve adhesion?
3. How porous are rabbit skin glue and shellac? Would they entirely protect the silverpoint from oxidation (from either egg yolk or atmospheric sulphurs), or are they not sufficiently sealing? Is a final varnish necessary to make sure that the parts of the silverpoint that remain visible don't tarnish?
4. When silverpoint underdrawings were used in the Renaissance, were they pretty much invariably covered with ink layers before egg tempera was applied? Or are there Renaissance examples of egg tempera painted directly atop silverpoint? I'm wondering, if egg tempera is applied directly onto silverpoint (no ink layers in between), do the tempera paint layers sufficieintly seal off the silver from oxidation, or does enough oxygen travel through the tempera to reach the underlying layers and cause the silver to oxidize if in direct contact with the tempera paint layers? If so, would the resulting oxidation on the silverpoint compromise to any extent the adherance of the tempera to the silverpoint?
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In answer to your questions 1 through 3: There are no coatings known today that form an impervious barrier to water vapor and gas, so it is rather doubtful that any coating can prevent silverpoint from tarnishing. A coating may delay tarnishing, but will not entirley prevent it.
In conservation of silver cultural objects a number of different coatings have been used to prevent corossion. One in particular, Agate Lacquer #27, has been used to mitigate corossion of silver objects. However, even this coating cannot completely prevent it and must be reapplied at certain intervals.
Very interesting - thanks, George.
Well, in answer to my own question, in the excellent book, "Art in the Making, Underdrawings in Renaissance Paintings", I found the following,
...[various metalpoints] also react with the atmosphere to produce darker lines over time, and this phenomenon even appears to take place in silver and tin metal-point underdrawings in paintings, in spite of their protection beneath layers of paint." So, as you say, George (no suprise that you are correct) there is seemingly no protection of the silver from oxidation.
Thanks George. I just found some time to sit down and address this question and it appears that, like The Elves and the Shoemaker, it was already dealt with ;)
Even if one applied a perfect coating, which would prevent
oxidation, it would need to be so substantial that the adherence of the tempera
would be completely compromised.