The Framers Forum

This came in for re-framing, the silicone had failed, or the backing had, or both.

You can see that not only has the glaze been eaten away but there is a serious dip in each corner. There is mountboard facing paper stuck to the silicone, but no tile material, that's just been eaten away by this 'orrible muck!

Thanks for posting that John.

It's clear to see that there has been some nasty chemical reaction.

Whether there may be, or not be legitimate uses for silicone in some levels of framing is not something I would wish to provoke a debate about, but it is well to recognise that most common RTV (room temperature vulcanising) silicone adhesives and sealants use the acetic acid curing process and this acetic acid takes a long time (in some cases years) to disperse from the silicone.

If this tile was made from fired clay there may have been less likelyhood of such an outcome, this reaction will most likely have occurred between the acetic acid and the calcium contained in what is probably a plaster tile. A reaction between an acid and most metals will result is the production of a chemically produced salt. A volume of salt forming at the interface of the calcium and the silicone would of course have a weakening affect upon the bond between the silicone and the calcium in the tile.

In this case the metal in the reaction is almost certainly calcium from a plaster tile and yes, calcium is a metal. There are some RTV silicone adhesives and sealants which do not produce acetic acid when curing, unfortunately these types are a specialist product and are considerably more expensive to purchase.

A more likely explanation might be that the original framer removed the glaze on the back of the tile to give the silicone a better key.

Silicone has been used in bathrooms for years, and if it affected tiles in this way, wouldn't it have been noticed?

Not to fix tiles - that's what tile adhesive is for and no one cares what happens to the back to notice, unless they fall off the wall like this fell in its frame. Silicone in bathrooms is mainly for sealing and it's not the same stuff as this acetic acid-curing stuff which will etch pretty much any surface except glass.

I'll check with the customer if the framer gained permission to remove the glaze (plus a load of clay as well to make those dents) but what is left is a gritty surface that rubs off easily on your finger, no 'key' at all; to make one you'd have to remove the mess that IMHO the silicone has made and is still part of.

Am I right in believing that you think that silicon contact with the front of the tile would not cause the same damage as you have observed on the rear?

Roboframer wrote:this acetic acid-curing stuff which will etch pretty much any surface except glass

Anyway - it failed, regardless.

It just goes to show that if you use an adhesive fixing only on tiles, then the glue is going to fail sooner or later. It might be 100 years later, but it will happen.

There is no way you can fabricate a mechanical fixing that won't be visible to some extent. My preferred method is to use a narrow cushion moulding to hold the tile and then fix this on top of a wide, flat section. Sometimes with another moulding around the outside. It gives the effect of the tile standing proud and covers only a minimal area of the tile.

This case might serve as a useful wake up call to any who do not know about the out-gassing issues and potential corrosive problems resulting from the use of acetic acid curing silicone compounds.

The manufacturers on-line data sheet can give some very important knowledge to those who need to learn these things.

I reckon chewing-gum would last the longest. Probably longer than the tile.

That silicone has the ability to corrode a tile in this fashion is a huge discovery, probably something that the building trade is completely unaware of.

Is there not some way that your findings could be presented to, say, The National Tile Contractors Association? Perhaps they have a forum.

They'd probably laugh me off their forum, the adhesives they use cure far quicker and provide a better bond to more suitable surfaces - if they used this stuff you'd end up with a bathful of tiles, if you weren't hospitalised by the fumes first.

If their adhesives were so fantastic though, they'd be using them to fix door hinges instead of screws.

Objects mounted in frames with adhesive - most any kind of adhesive - can fail in 3 ways. The adhesive can give up the ghost and both substrates can too, one or all 3 has happened here and it looks like the acid in the muck has done something to the back that I had read plenty about but had yet to see - that's all.

Roboframer wrote:this acetic acid-curing stuff which will etch pretty much any surface except glass.

For an acid to etch something there has to been a chemical reaction between the acid and the something. One of the notable properties of glass is that it is an extremely stable compound and as such requires a very powerful acid to overcome the powerful atomic level bonding between the elements which are combined to produce the glass.

To a lesser degree there are similar stable molecular bonds in fired clay tiles and bricks. If this was not so, tiles and brick would not be used as durable building materials. The added factor in this case is that some tiles are manufactured from plaster to avoid the need and therefore the expense of firing them at high temperatures in an oven.

Therefore not all tiles would exhibit this same problem. Framers need to know what things are made of before deciding to use silicone. Mechanical methods of fixing have a big advantage over adhesives where it is neccessary to avoid unforeseen problems and especially where full reversibility is a priority.

Silicone is not neccessarily the cheaper, or simple option. Mylar is cheap, stable, fully predictable and in many cases the quick and easy option.

There is no reason why Mylar should be primarily considered as a conservation fixing method, but it is a method which with practice can be a faster procedure with lower material costs.

In particular there is no need to wait for the silicone to set before the job can be finished, no mess to worry about if the silicone gets somewhere where it can be seen and no unwanted chemical reactions, or out-gassing.

It's not about using a conservation oriented method, this is the easy way to do these things and to be sure that there will be no future problems. Do it right and do once only!

BTW, Mylar is much cheaper than silicone.

I have a word file on silicone from TFG that goes back to 2000. I could bore the pants if I pasted the whole thing here - it would probably take 3 topic pages, but heres a quote from it I like, which is from a long time ago.

"I remember when silicone was pretty much the default adhesive for things like coins and arrowheads.

The first framers to suggest that maybe this wasn't the ideal process were considered somewhat radical - fringe lunatics, in some cases".

And now some of those first framing pioneers are respected educators - backed up by conservators - but hey - let's not mention that 'C' word - I mean WOOOOOOAAAAHHH - easy turbo!

Your customer could stick an object to a board with some gunk - so could any primary school kid .... but you're a framer, offer some framing skills! But anyway, and I say again - IT FAILED.

I don't know a lot about the properties of silicone, having only ever used it as a sealant in DIY jobs at home.

However, when the original poster states that "glaze been eaten away" "there is a serious dip in each corner" it kind of suggests that he knows even less about the subject than I, specially when here is clear evedence in his own photos, particularly the third, that this effect was caused by some sort of rotary abrasive device.

I wouldn't want to get into an argument about silicone's suitability for conservation framing without doing some research on the subject, but I would want to keep an open mind on the subject until all the evedence is in. If ever I get a job, conservation or otherwise, where I feel that silicone might be of use, I'll be sure to look into it.

OK, let's say the surface was keyed - a rotary device would have been overkill; a bit of sandpaper would have done, but still, this surely was not the keyed surface that was left because it is loose - left glazed or keyed, the silicone has done something to the surface.

See the line going away from the bottom left corner? That was a stray bit of silicone, probably a drip from the tube, it wasn't touching the mounting board, the tile is convex, and that is the mark left after I removed it.

I would also have thought, if the surface had been keyed, that the silicone would have squashed over that area in places and if so there would be glaze left under those areas, or maybe not all of the keyed areas would be covered, but that's not the case, the lumps fit perfectly in the rough areas.

................. and it failed.

Sorry Roboframer, the convoluted points in this thread, such as using silicon instead of door hinges, or something, has confused my simple mind to the point where I'm not sure whether or not you are sticking (so to speak) to the notion that your photos demonstrate that silicone can eat away the glazing of a tile.

Can you clear that one up please?

Looking at the pictures again, the peeled away face paper would suggest to me that the tile has been separated from the mountboard by force. Could the framed package have been dropped?

Before I came to the Fine Art Trade Guild, and even before I was a glass blower, I trained as a ceramicist.

Without seeing the actual tile it's hard to be certain, but I think what you're looking at here are the spaces where the maker has wiped off the glaze so that the tile doesn't stick to the kiln. They will have arranged a series of props (little hard ceramic towers) then balanced the tiles on the props before firing. I suspect that the reason they're indented is because the tiles have sagged slightly in the firing process. Personally I can't see the point of glazing the back of tiles - it just makes them harder to affix, but there you go!