soxxx if I were you I would run, not walk to the thinset and get started. I would build that part back and then go over the rest of the piece with your thinset also. You could mosaic it or do as Slow did with her latest and greatest fountain and stain it back brown. I will let her tell you what she used but I know she watered down concrete stain and sprayed it on I think.
I don't know what part of the country you are in but I have had Mexican clay pottery separate even in our relatively warm humid climate. I think it has something to do with what part of Mexico the clay came from.
Good luck, I believe you can salvage it.

Great advice. That's what I'd do. Re the fountain, I watered down the thinset colorant and brushed/sprayed it on. Not sure how much w/come off. Haven't sealed it yet, but plan to re-apply in the spring and seal it, hoping for more permanent bond. As I learned when I was coloring thinset on "Shea's Tower", it doesn't take color well once it's dry. Should be added when mixing it up to apply, or sprayed/brushed on while it's wet.

Thanks for the prompt advice. I want to go right out and tackle it, but the surprise record setting 21o has kept me inside and thumbimg through glass patterns and "such."
I am high energy like slow.