Trivia: The character Harley Quinn was actually created for this series. She prove so popular, she was later integrated into the comics, and has also appeared in live-action feature films like "Suicide Squad" and "Birds of Prey."
Trivia: An idea of his own, Kevin Conroy was the first actor in animation to use two different voices for Bruce Wayne and Batman.
Trivia: Voice recording sessions were all done ensemble, with the cast together in the same room. In order to generate the manic energy required to play the Joker, Mark Hamill was the only actor permitted to perform his lines standing up.
Trivia: Tim Curry was initially cast as the Joker, and recorded lines for a few episodes, but was later replaced by Mark Hamill. It has since become one of Hamill's signature roles.
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.