Removing mortar from back splash tiles

I have some heavy Portuguese back splash tiles that I want to reuse in a kitchen reno I'm doing. I have removed them from the wall but many have mortar and some drywall stuck to the back. How do I remove the mortar such that the tiles can be re-used?

A grinder will remove the old mortar from the tile backs. Either a diamond wheel or a couple masonry wheels will get them clean.