Reclaimed Tiles 200 years old - Tile Advice

[Wishiwasatoptiler]

Hi
Picked up this job, 200 year old reclaimed tiles from a cattle barn in france. There 300x400x 25mm approx, bowed.
There to go on to wet underfloor heating.

Clay is really all the info i have with no glaze.
There extremely dry if that makes sense and i have concerns they may suck the moisture from the mix and debond, any ideas how i can reduce that risk.
Any recommendations addys, sealers

and grouts, I leaning towards ditra mat as I can't see these tiles liking the underfloor heating.

Thanks in advance
D