loribee2October 31, 2010

Waaaah. I've been casting cement garden mushrooms. I mosaic the top and paint the stem. My most recent one failed miserably. I covered the mushroom cap with small tumbled stones using thinset as my adhesive. All was well until I went to grout the thing a couple weeks later. One by one, the stones started falling off. At first I was going to just glue them on after the fact, but it seemed once one fell, others followed. I ended up throwing the whole thing in the trash!

Any speculation on what went wrong? Not enough thinset? Should I have burnished the adhesive side of the stones somehow? Maybe I was too vigorous in my "scrubbing" the grout between the joints? This is only my second 3D mosaic. The first mushroom cap went fine but I used marble and granite tiles instead of these shiny river rocks.

I'd like to think it was my material and not my technique because I'm working on another one right now using vitreous tiles and hope I don't end up with the same problem! Definitely, I'm going to be more gentle with my grouting as a precaution.

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Only two things I can think of are: Does your thinset have the admixture (polymer)? If not, it isn't sticky enough. The other is: perhaps you didn't use enough thinset. I have experienced the rocks coming loose if I don't squish them into the thinset and have enough to sorta surround them.

    Bookmark   November 1, 2010 at 9:42AM
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I'm not sure about the admix. I bought a pre-mixed tub of it at Lowes. But I think you're right about not using enough. Probably, when dealing with these polished roundish rocks, I should have really squished it in. Given that most of the rocks really did adhere, this half dozen or so that popped out were probably the ones that didn't have enough thinset around them.

The sad side of learning a new craft! :-(

    Bookmark   November 1, 2010 at 10:28AM
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oh that is so sad. i once made some finials and the same thing happened. i hadn't even had them out in the yard yet. i finally determined that it was most likely that the thinset had gone bad. Here in TX it's very humid and my thinset, being stored in the garage, had formed clumps. I simply broken them apart and then mixed and used the thinset. But I think it was because it had already had moisture in it that it went bad. That's not a scientific theory by any means, but I did buy new thinset. I store it inside so it doesn't get the humidity and I haven't had any problems since. just a thought...

    Bookmark   November 1, 2010 at 11:11AM
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I went looking for my reference book on pebble mosaics and can't find it. I just remember reading in it that the artist installed pebbles end first into the thinset, not lying flat. She said that if you just pressed a polished stone into the thinset, it would pop back out. I hope I am making sense here. So you may never know what caused the failure, but I would guess either the stones are too polished and not adhering well enough or possibly the thinset is old and failing, or a combination of both, but I don't think it's your technique. When you have a durable mosaic, it shouldn't be necessary to be gentle with the grout. In other words, we shouldn't be counting on the grout to hold our tesserae in position; the adhesive should do that. Wish I could find that book! I will keep looking.

    Bookmark   November 1, 2010 at 1:43PM
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I would definitely like to think it was the pebbles and not my thinset, as the thinset isn't very old. It is good to know the stuff goes bad, though. Like I said, it comes pre-mixed in a tub from Lowes and I'd assumed I could keep using it until it dried itself out.

I've been using the same tub for my latest project, and so far, the vitreous glass is holding pretty tight. I'll keep my fingers crossed it's just that polished pebbles are more temperamental than regular tiles. And yes, Silvamae, I did totally get what you meant by end first. I'd actually quite intentionally looked for "flat sides" to stick to my mushroom cap, so I most likely exasperated my own problem.

Question: Have any of you used Guerilla Glue as an adhesive for outdoor mosaic? I ask because it tends to slightly foam up around the object you're glueing and I'm wondering if, for things like this where I've got plenty of depth to work with, it might be a better choice.

    Bookmark   November 1, 2010 at 4:24PM
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I hate to sound negative, but I've used the pre-mixed adhesive, and quit ages ago. Don't trust it, and it definitely isn't for outside projects. I get Mapei Ultra Flex II from Lowes, and wouldn't think of using anything else. I also store it in the garage, and some old thinset that had clumped, I just sifted it and used it w/success. I sooooo recommend y'all try it. Yes, used Gorilla Glue, and never again. I used it on a little garden spade, and it puddled and foamed and ruined my mosaic. Still display the spade outside. The little area where the foam shows isn't too noticeable. I still believe if you squish enough thinset around the pebbles, they'll stay put. I, of course, am partial to the Ultra Flex, and have had so few of them pop out that I wouldn't hesitate using them again. I used many of them around the area of the fish plaque in the Fountain Surround. None have come loose - it was in the little path I started by my swing patio, and I wasn't too careful w/laying them down. Some day I hope to finish that path.

    Bookmark   November 1, 2010 at 6:14PM
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LOL Slow, you can never sound negative! If you ever fear that, I can point you to a couple threads where people actually call each other names!

I appreciate your sharing your experience. I have seen you mention the Mapei before. I will definitely make that my next purchase!

    Bookmark   November 2, 2010 at 1:03AM
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Do you think that part of the problem might the extreme smoothness of the polished pebbles?

    Bookmark   November 2, 2010 at 10:26AM
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I don;t want to vut in, but to replay waht Slow said, throw the darned premixed stuff in buckets in the alley or maybe use to make leaves but mix your own Ultra Flex II by Mapaei and your problems will fade with the sunset. I have two large projects that I am redoing because I loved the premixed....ugh, until all started falling off....

    Bookmark   November 14, 2010 at 2:25PM
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I have had a recent problem with pebbles falling off when everything sticks. I believe it was that the pebbles are shiny and may have something on them to make them that way (varnish or whatever). Most of my pebbles fell off but the tiles and glass and other doo-daws didn't. i.e. problem with shiny pebbles.?? Ya think

    Bookmark   December 2, 2010 at 11:29AM
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I hope so as so far I haven't had a problem with any other materials I've been using!

    Bookmark   December 6, 2010 at 9:18AM
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