Missing Tiles Detail View Mosaic Art

Why Did My Tiles Come Loose From Thinset?

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.

Mosaic Art by Jackye Mills
Mosaic Art by Jackye Mills makes great use of color contrast, iconic design, and borders. Note the “color wash” transition in the border from red to light yellow. Note how well the cyan blue trim in the border contrasts with the warm color wash and ties in visually with the three blue dragonflies.


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.

Mosaic Table by Jackye Mills
Mosaic Table by Jackye Mills in progress.




6 responses to “Why Did My Tiles Come Loose From Thinset?”

  1. Michele Follger Avatar
    Michele Follger

    Love your blog!

  2. Lorna Avatar

    I am embarking on a project and obviously came up with the idea materials and what not before actually figuring out HOW TO!
    My childrens primary school are celebrating 200 year of existence, and the 6th class are making a wooden bench and are going to embellish the ‘leg’ stumps with mosaic – 1818 on one and 2018 on the other, and with these they are going to make 4 stools (tree stumps) mosaiced using ‘their’ imagination.
    How do you get mosaic tiles to stick to what is primarily wet timber? – any advice! – Someone suggested putting tar on the stump tops before using the tile adhesive. . . I’d love some help!??
    Thanks Lorna

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Lorna,

      We have warnings about using wood as a mosaic backer in many of our blog articles. Wood swells and warps with slight changes in humidity. Long before the wood starts to rot, most of the tiles will have popped off. The solution is that you need to find something else to mosaic to use for the stools, or if you are committed to using the wood, then you need to consider something other than mosaic. I wish I could be of more help!

  3. Gail Christofferson Avatar

    I use weldbond as often as possible, but currently have a project that I need to use thin set. I have my pieces on mesh.How do I avoid the thin set oozing up between my Mosaic pieces? Do you suggest that the thin set be the same color as the grout to be used?

    1. Joe Moorman Avatar
      Joe Moorman

      The easiest way to minimize thinset pressing up into the grout gaps is to put down a controlled amount using a 3/16-inch notched trowel. If thinset does press up, it is best to make sure that grout matches that color (or you scrape it out). That is why I sometimes use thinset as my mortar.
      I hope this helps.

  4. Annette Casto Avatar
    Annette Casto

    While curing my mosaic piece after grouting or thinset I keep misting with a fine spray of water to keep damp for several days. This helps cure properly and have never had a problem with losing pieces.

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