Then later when they hit camp or eat a meal. Have them roll for constitution. Failure leads to spitting out a broken tooth and a future visit to the blacksmith to get it pulled.
In all fairness, picking up the handful of smashed tile and shoveling out into one's mouth are a compound action that is unlikely to be "free", but the rule of cool wins in this instance, for sure. Had the Intimidation roll failed however, I would've given the opponent advantage on their next attack against the geek as the tile chunks turned their mouth into hot chum.
Now I'm starting to wonder... Are zippers a thing? Hell, I know piercings are. Thankfully you need to see the object you cast Heat Metal on. Otherwise I would pity the man who got a Prince Albert pierced.