Glueing old tiles back on an (unused) FP surround
I recently asked about repairing 100-year old tiles on a fireplace surround. Worthy gave me good advice about removing them, resetting and regrouting. So I got all the loose tiles off and patched the place where the wall itself was crumbling. Only then, however, did it dawn on me that there was no mastic to scrape off the wall before resetting with new thinset or mastic.
These tiles were set directly into the mortar troweled on the brick firebox. So...if I were to reset with new thinset or mastic even the lightest coat would make them protrude more than the tiles which are still firmly adhered. And the mortar is so strong, that I can't imagine how I could scrape enough of it off to set the tiles flush.
Is it possibly then to simply glue the tiles back on? Obviously this wouldn't do if it were a kitchen or bath where water would be an issue. But they are on the surround of an old, never used gas fireplace...not only not used, it never COULD be used as there is no venting.
If this idea has any merit, what would be the best type of glue to use?