This hash been hashed out quite a few times already:
Jim Miller, June 19 2005:
Curing time
Sister says -- and some others agree -- that silicone is OK for framing if you wait until it has completely cured before closing up the frame. How do we know that? And how do we know when curing is complete? I've heard framers answer eight hours, overnight, a week, and up to a month. Some say it's cured when the acrid odor goes away. Who's right? Is the answer always the same, regardless of brand? Is curing time the same for a small, thin layer on a coin, as it would be for the large globs needed to hold a heavily-textured, weighty mask?
Chemical composition
Silicone adhesives aren't all the same. I guess some basic ingredients are common among them, but who knows what other chemical elements are in any particular brand? More to the point, who knows what chemical reactions could result within the closed-up environment of a frame -- even after complete curing? The adhesive chemists I've asked say that's a tricky question, and I've never heard a clear answer.
Strength of surfaces
This question applies to all adhesive bonds, not only silicone. What Emibub said above is worth repeating: "...adhesive failure rate is pretty high. It is only as strong as the surface you are gluing it to, like the surface paper on a piece of matboard. Glue will peel away from that over time." That is, the fibers of the paperboard will tear away under stress. Likewise wood, textiles, and other fiberous surfaces may shred and cause bond failure. The logical fix for that dilemma is to glue a larger surface area. But what about an unnoticed spot of skin oil or grease? What about a coating of varnish or paint that chips or peels off?
Removability
Sister said the silicone peeled right off of her tableware. Yes, some silicones will peel off of some very smooth, non-hygroscopic, coated, oily, or dirty surfaces. Is it safe to assume that whatever silicone is in hand will peel off of whatever we glue with it? Certainly not. How do we know whether any given brand is removable from any given surface? I can personally testify that it sticks quite well to porcelain bathtubs, ceramic tile, and anodized aluminum.
We always have choices about our mounting methods and materials, and our decisions may be based on any number of considerations.
When the fastest, cheapest way is best, perhaps silicone is the right choice. But if the mount is expected to last a long time, or if the issues above matter for the project at hand, then we can and should make a better choice. And on my worktable, that's nearly every time.
Rebecca, July 27 2004:
This is the link to the article, which is about tests to identify natural vs silicone rubber caulking:
http://palimpsest.stanford.edu/waac/.../wn16-208.html
This is what the author had to say re using silicone caulk in framing and exhibition contexts:
"The off-gassing thing with fresh silicone caulk is pretty well known. Silcone rubber will turn a lead coupon white in short order as the acetic acid forms lead acetate crystals on the surface. In my opinion, the biggest problem with using silicone caulk as an adhesive how difficult it is to get off the object. Even if the object is reasonably hard it can still stain.
I have seen silicone staining on stone arrowheads and tools that were mounting with it. Even glazed ceramics can have small micro-fissures in the glazing that allow the oil from the silicone to penetrate. It is nearly impossible to get out. The only thing you will ever be 100% certain you can get it off of is glass. On metals , the residual acid will etch the surface guaranteed.
There is also another aspect of using silicone caulk that you might want to alert the framers to. This not yet proven but there is some evidence that silicone caulk will give off some sort of volatile oily component over time. It has been noted on the inside of exhibit cases where the glass was siliconed in. The glass gets this hazy look to it. It is very difficult to clean off. I have heard from exhibit guys that all they can do is smear it over to the side as best they can so that the central view field is not affected. I don't know if it is a certain kind of silicone caulk or if they all do it.
Scott Carrlee, Conservator
Alaska State Museum"