Heart of Ice - S1-E3
Continuity mistake: In the wide shot of Batman hanging upside down in Mr. Freeze's lair, his face is blank and the bat outline from his chest emblem is missing. (00:14:10)
Continuity mistake: The Joker reads “Charles Michael Collins” from Charlie’s card, but later, when he shows it to the camera, there is no middle name on it.
Character mistake: The news program Stromwell is watching at the beginning of the episode displays a picture of Commissioner Gordon with the caption "Comisioner." (00:02:00)
Factual error: The note the Joker has that says Charlie has changed his name to Don Wallace lists his address as 12 Marigold Lane, Springdale, Ohio, a city near Cincinnati; however his area code is listed as 614, which is the Columbus, Ohio area code.
Answer: Because he stole a chemical from his lab.
But Batman knew that with it, Clayface could permanently retain his shape and Clayface desperately wanted to be Matt Hagen again. In a sense, Batman stopped Clayface from becoming human.