putting a strip of 4mm glass mosaic between 6mm tiles

[John Wallace]

I'm tiling my bathroom, which is in a corner of the house with dry lining on the two outside walls. The plan is to tile all the walls floor to ceiling with white tiles (6mm thick), with a strip of glass mosaic (4mm thick) a little below eye level. As a result of some uneven plaster, and a 1 mm change in depth where I put aquapanel behind the location of the bath and shower, the row of tiles below where the mosaic strip is to go is hanging on up to 4 mm depth of tile cement. When I tried putting the glass mosaic strip in today, it was going to need up to 6mm depth of cement to mount flush with the main tiles. I found that too difficult, so abandoned the attempt, and plastered about 2 - 3 mm depth of tile cement (BAL white star premix) on the wall where the mosaic is to go, and am currently waiting for it to go off (it is still very soft after about 4 hours!). This seemed the only practical thing to do. However, if this is a really really bad idea, would someone please let me know before the cement hardens, so that I can scrape it off (provided, of course, that you can provide a better idea).

 

[Ian]

You would have been better off using a rapid setting cement based adhesive for this but, you should be ok with the whitestar, just double check what the recommended depth is on the tub, can't remember if it's 3 or 6mm for whitestar.


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[Dontiler]

I think its 3mm max, tiles max size shouldn't exceed 300 x 300 too, I would change your adhesive to powdered cement base one, and it drying slow too completely sets in about 20 hours
Good luck

 

[Brian the Tile]

think i would scrape it out and make sure you dont let any set on the face of the tile and then get some white rapid set powder adhesive mix pretty thick and bob should be your uncle,good luck:smilewinkgrin:

 

[andyb1]

white rapid does the job!

 

[John Wallace]

This is proving to be a very tricky job. I reviewed the specs for whitestar (max 3mm) and rapidset white cement based adhesive (2 - 8mm), but on reflection, the problem is not the depth. Cement based adhesive sets by hydration of tricalcium silicate, which happens by virtue of being wet, so it doesn't matter what area of tile covers it up. Whatever the adhesive is in whitestar sets by exposure to air - not idea what the chemistry is, but guess it might be drying out and/or oxidation, both of which would start at the edge of the tile and work inwards (though drying out could be into the wall, and the substrate of the tile). I suspect the 3mm limit on depth for whitestar is because it would take forever to set if any deeper. Glass mosaic is made of 20mm squares, so there is no point at which the cement is more than 10mm from the edge of a square. On that basis, I decided not to remove the 3mm layer I have already put up, but to pursue the idea to its logical conclusion (put another 3mm on top and stick the mosaic on it). It didn't work. On inspection, my tile cement spreader has grooves in it that are 6mm deep, 6mm wide, and 12mm pitch (i.e. 6mm gap between them).There were two problems when I applied the mosaic strip (1) with no pressure it sank deeper than the tiles below it, and (2) the tile adhesive was as inclined to come up round the edges of the 20mm squares rather than spread underneath them. After 3 attempts (with a thorough wash of the strip of mosaic between them), I scrapped the strip of mosaic, and spread the tile cement out to make another layer about 3mm deep on top of what I have put on already, so now have a layer of tile cement about 6 - 7mm deep, that I have sanded do a smooth level finish. I have just sized up a sheet of mosaic, on this, and it looks as if it will fit nicely now, only its no good putting 6mm high ridges of tile cement at 12mm pitch behind them. Am about to cut an improvised spreader tool from 2mm ply, using something like 3mm grooves at 6mm pitch.

 

[Rich]

Wow. What a job. If the mosaics are thinner than the field tiles, I use a mosaic packer. I think they are caller 'Mosaic Pro mosaic spacers'. They are great, they sit behind the mosaics to bring them flush with the rest of the tiles. Bit late for that now. Sorry

 

[John Wallace]

Thanks, Rich, that sounds like exactly what I need. I have finished the 1140mm strip that goes from the window to the corner, and am quite pleased with the result, but even having worked out a way to do it, was not looking forward to laying up 5 - 6mm of tile cement (probably in 2 layers) all the way round the room. I'll look for the spacers.

 

[AliGage]

It's not advisable to use white star on glass, mosaic tiles. You need a cement based adhesive.

 

[John Wallace]

AliGage, thanks for the advice. Can you explain why white star is bad on glass mosaic tiles? Unfortunately, I have already done a strip with it, which I am quite pleased with, and at the back of a shower, where it is not unlikely to get wet. It is not impossible to remove it (at worst about 3 hours work if I scrapped the existing tiling above bath level and replaced the sheet of aquapanel entirely), but I don't want to do so without a compelling reason.

 

[AliGage]

I never did very well in chemistry at school so i hope i'm right with this one:

Glass is non-pourous. Therefore moisture resistant. Tubbed adhesive sets, cures, drys by disperssion of water. If what you are fixing is non-pourous then effectively it is resistant to the setting process.

Bagged adhesive works on a chemical reaction to harden, it's cementious.

That is about as much as i really know about the science behind it all, but i think basically, because of the way tubbed adhesive works to cure (using the moisture/water in the addy) and glass being non-pourous they will not bond well, if at all.

 

[John Wallace]

Thanks for the quick response, AliGage. As the glass tiles in the mosaic strip are only 20mm square, I think it is drying out OK, but I'll try an experiment with some scraps of mosaic on a scrap of aquapanel to confirm that. As you say, bagged adhesive uses hydration of the cement powder in it to form interlocking crystals that set it hard, so sets even if it hasn't dried. Chemistry isn't my best subject, though I did take it to A level, and covered the chemistry of cement during an engineering degree. The problem in this case is that I don't have any idea what is in White Star, and how it sets. Most adhesives go solid by some sort of polymerisation, where a short chain molecule assembles itself into longer chains. The polymerisation can be triggered by a catalyst, or by oxidation. Drying out of itself makes some things go solid (effectively the glue is in solution in water), but is usually reversible, so is unlikely to be used in a tile adhesive. I think water may be acting as an inhibitor in White Star, stopping the polymerisation, but that when polymerisation has made it set, it is not reversed by getting it wet again. Its very impressive how well it remains usable until the tub is almost empty - several times, I opened a near-empty tub, expecting to find it dry and solid, but it was still wet, smooth, and workable (yet the stuff would set on my fingers in minutes).

 

[AliGage]

Going a bit above my head there John. lol. Let me know your findings if you do any test though.

I don't use tubbed adhesive, but i have done in the past, quite a long time ago though. I'm pretty sure though if you wet/dampen something like white star it will "unset" in a manor of speaking. I.E soften up again. Don't know if that's 100% right as i've not used tubbed for a long while, but if it is and it helps you understand how it works i'm interested in knowing a bit of the science behind it myself.

 

[tilerjason]

4mm i think

 

[JHeff]

I had a problem like this last week, needed to pack out about 6mm for the mosaic strip to sit flush with the rest of the tiles. I got a sheet of 4mm Wedi, cut it into long strips 50mm wide (the width of the mosaic strip), then stuck them to the wall, then the mosaic on top. Finished perfectly flush.

 

[jwatson]

how u doing jason?? not seen you for a while

4mm i think

 

[robbbin]

What i do is cut a peace of plastic to a L shape, to a depth of the mosiac and the width of the border use rapid set fill the area and scrape off, when set fix border with rapid normally would tile up to border on all walls then when set tile up to ceiling.

 

[John Wallace]

Just an update, after finding I could only get the tile spacers mentioned by Rich online, which would take about 3 or 4 working days, (about 5 or 6 actual days because there was a weekend coming up), I did the mosaic along the second wall using a bed of tile cement. To make life easier, I cut a small piece of wood to shape so that I could level the tile cement at exactly the right depth (4mm below the surface) all the way along the wall (I screwed a 9mm strip of wood along the wall above the position of the mosaic so that the tool I made could be levelled on the tile surface below, and the strip of wood above). This worked well, except that I had to wait 2 days for the White Star to go hard before putting the mosaic up. I like JHeff's advice about using Wedi. If I can find that in the shops, might use it for the next wall. To avoid the delay in setting, I tried using finishing plaster for the same purpose in a few short lengths round the window alcove, but then was a bit dismayed by the advice on the tile cement to "let new plaster dry out for 4 weeks before tiling over" (!!!). Won't get on to the next wall for a few days, because now trying to solve problems putting a large heated mirror along the wall behind the basin.