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I left my stone to soak with my other stones over the holiday and the coticule has delaminated from the slate backing.
What was used to bond the stones together? Would thinset work? Should I just use it as is?
With "thinset" you mean a kind of tile setting mortar, right?
Would work fine. I presume that your Coticule was a vintage one? They used a mixture of hide glue and beeswax. Hide glue has only a poor long term water resistance.
Ardennes uses a a waterproof tiling cement, that's why I assume your de-laminated Coticule is a vintage one.
If so, you need to get rid of all the old glue residue, by applying heat (hairdryer) and scraping it off. Make sure the stone slices lay fully supported on a solid surface, while you're working on them.
Once clean, any gap filling glue is great. Only in cases where a tightly fitting natural stone separated, I recommend CA glue. Because it's quick and easy, but unfortunately, it has no gap filling properties.
Thinset is a tile setting mortar. I bought my coticule from a US vendor about a year ago so it is not vintage. You can see the stone parts. If you look at the corner of the glue on the stone there is a small air bubble that lets you see the thickness. The glue does not want to chip off as easily as thinset would. I will work up my nerve and clean it.
I stand surprised that Ardennes' glue failed prematurely. Although a Coticule is not meant to be soaked in water, I know for fact that the glue they use is a modern high-performance tiling cement.
In the pictures, it looks as if the glue separated cleanly from the slate. Perhaps it was neglected to clean the slate tile, before gluing the Coticule part onto it? The glue surface does look "dusty", in a way.
Any glue with some filling properties qualifies to re-glue a Coticule to its slate base. Hot glue (the glue gun type) works. So does typical panel adhesive. Of course 2-component epoxy would be strong and durable beyond anything.
Bart, that is what I observed also. I think the dusty look is from the stone dust that was in the water. The bottom of the coticule is not level and the adhesive has to fill that. The slate is grooved for the adhesive I do not think I showed that side. I think the adhesive does not like the slate as well as the coticule. In any case the separation was caused by misuse. But this should be a hint to others that might have been careless without seeing my mistake.