I am happy to say that all 38 tiles were fired without any breakage. They are going to be installed later this week. The contractor has come up with a way to install them so that they can be removed as one piece if the owners ever decide to move. I think the plan is to mortar them to a separate board which will then be attached to the wall, rather than simply mortaring each tile individually to the wall.
And a special thanks to "All Fired Up" on Bethesda Avenue for being so accommodating and fastidious firing these for me.