Using Silicon Adhesives for Mounting

Feb 11, 2004
Chesterfield Missouri
I'd appreciate your help in understanding the use of silicon adhesives and related conservation issues.

I know that silicon gives off methanol and ammonia gasses as it cures. I also know that it is safe to put fresh and saltwater fish in aquariums that have been sealed with silicon glue after curing (24 hours.)

My question is when it not acceptable to use a silicon glue to adhere items when mounting and why?

That's an excellant question and I have my own ideas about when/when not to use silicone adhesives for mounting objects in shadowboxes. I have no foundation to back up my ideas other than common sense so I will refrain from listing them here but I think that generally the use of silicone is limited to things like attaching foamcore spacers to float mounts (it works quite well when attaching things to suede board), attaching things that have no significant value to the owner, or using silicone for mounting things of a temporary nature/short lifespan. As you stated, there is harmful outgassing as silicone dries and you need to give it time to dry completely. As to any long term outgassing, I don't have a clue.

I would advise using some common sense when mounting with silicone and, when in doubt, use another mounting technique.

I would not use silicone with heirlooms (for obvious reasons) or objects of a valuable nature, any kind of fabric (the silicone will penetrate the fabric and become impossible to remove without damaging the fabric), hard objects with much indented detail (the silicone is difficult to clean out of deep seated details on medals and raised lettering/details on objects), and anything that you know would cause damage to the object either by chemical reaction or by future attempted removal of the adhesive.

The only time I was actually advised to use silicone adhesive was by the AIC back years ago on mounting collector plates into a shadowbox. They said that the silicone would stay flexible, not harm the glaze on the plates, and be more easily removed if required than any adhesive that would harden and have to be chiseled off or scraped off with an edged tool. They said that you could damage the glaze trying to remove a hard set adhesive where silicone could be more easily removed using solvents/tools that would not scratch the glaze. That was back in the early 90's and those recommendations may have changed as so many of the other recommendations of that era have changed and been updated.

Hello! I am new to the grumble and am looking forward to alot of new ideas and information.
the subject of silicone I can add some valid input.
There have been times when I have siliconed mirrors in a picture frame just to give an additional strength, especially when the rabit is too shallow to brad for fear of breaking the mirror.
What one has to do BEFORE placing the silicone is to spray a coat of clear finish to the back and sides of the mirror.
The silicone in time will eat the silvering on the back of the mirror.
This advice was given to me by the owner of Indiana Bevel in Carmel, IN.
Hope this helps.
I think your best bet would be to contact the company who makes your silicone caulking. We on The Grumble have all used silicone for many different projects, because it works. I don't think any of us know the long term effects the out gassing and material itself would have on various objects that have been glued with it.

Myself, I have switched over to Tacky Glue for gluing up objects, the stuff just seems a whole lot better, and no gas smells. Silicone will always have a place in frame shops though. There are some items that may have to be removed at a later date. Silicone can be easily cut through with a thin wire or knife. I also like it for gluing up side panels on shadowboxes for that same reason. Tacky Glue can be removed with water, trouble is it would destroy the backing that the object has been glued to.

I think the only person who could give us the straight scoop would be the manufacturers or possibly Rebeca.

I would also like to note that I have been using silicone since the 1960s. I don't think I have ever had a return from a customer for using it.

Since I don't have reason to use it (other than in my bathroom) I don't know an awful lot about the subject. I think there are different kinds of silicone caulking, probably each with their own set of good and bad points. Certainly some emit acetic acid as they cure. Also certainly they should not be used on porous materials (paper, unglazed ceramics.....) Aging might also cause some problems.

I've e-mailed a conservator who I believe has written an article on silicone caulking - with any luck he will be able to shed some light on the subject.

This is the link to the article, which is about tests to identify natural vs silicone rubber caulking:

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"

Thanks Scott for your prompt and informative reply!

I was instructed to leave the piece for 24 hours before closing up to allow it to off gas and only use silicone II product , which is getting harder to fine
The more i research i’m realizing that if you want to use it at all use the 100% silicone as it doesn’t have any chemicals added to combat bacteria growth
Sixteen years on most framers that have read anything worth reading on framing, avoid any sort of adhesive for mounting purposes and it’s not just to do with preservation.
There are many methods of fastening things in place that are far more reliable
I'm with Robo on this.

I favor mechanical attachments, which often involve adhesives in minimum-stress positions, because direct-adhesive attachments can fail three different ways:
1. Bond on the item's surface fails due to contamination or disintegration
2. Bond on the the substrate's surface fails for same reasons
3. Adhesive itself fails

If you must use an adhesive, especially under stress (including gravitational stress), then avoid silicone. Aside from its somewhat unpredictable chemical reactivity inside the closed environment of a frame, even after extended curing time, silicone is notorious for refusing to be removed from some surfaces, and refusing to stick to some others.