Recently, artist Jackye Mills emailed me about a problem she was having with her first mosaic project, and it really caused me a lot of angst because the artwork was a strong design that was otherwise well executed. I hated the thought that a first-time mosaicist could do such a good job on something so ambitious only to lose the project due to a technical issue.
The problem was that the tiles were popping out of the thinset mortar that she used to attach them.
Since Jackye lives in a desert climate and said that she had done the work out in the sun, it was obvious to me that the problem was due to the thinset drying out before it could cure. REMEMBER: Cement products such as grout and thinset mortar are made from portland cement, which hardens by bonding water at the molecular level, NOT by drying out. If you let grout or thinset dry out as they are trying to harden, they simply won’t harden very well, and then they will be crumbly and not adhere well to surfaces. I don’t ever tile with mortar in direct sunlight if I can help it and will rig tarps if needed to minimize the effects of sun and wind. I also run humidifiers if I am working in a dry climate, including indoor climates with strong AC or heat running.
BUT, drying out during hardening isn’t the only reason why thinset might fail to bond securely to glass mosaic tile.
An Alternate Cause of Failure
While grout and thinset failures are almost always due to drying out while hardening, another potential explanation for poor adhesion is oil or dust on the tile. Sheets of stained glass are cut to size by being scored with a Pistol-Grip Cutter, and these cutters use Cutting Oil, which can contaminate the surface of the glass. For this reason, we recommend that sheets of stained glass be rinsed with detergent before being cut into pieces with a Mosaic Glass Cutter.
From the detail photograph of the missing tiles, we can see that the missing tiles had embossed patterns on their bottom sides. This indicates that they were molded tiles and not pieces of stained glass. The conclusion is that the failure was in the thinset itself (due to drying) and not an oil film on pieces of stained glass.
One solution to the problems in this particular project would be to rub your hand firmly over the surface encouraging any potentially loose tiles to go ahead and pop off, and then you could individually remount these with thinset mortar.
However, if many of the tiles started coming loose, then my solution would be to cover the mosaic with Mosaic Mounting Tape and then scrape it off the backer with a putty knife or paint scraper. Then the mosaic could be remounted.
If only some of the tiles came loose by rubbing, I would reattach them individually as first recommended, and then I would “grout” the mosaic with thinset mortar, which comes in gray and white and can be dyed by mixing with concrete dye. The reason I would use thinset for my “grout” is that if any tiles came loose during grouting, I could reattach them. Also, the adhesive properties of the thinset would help ensure that any tiles close to failure were reinforced.
I would work gently and not try to clean the mosaic off too much while the thinset was still hardening.