Robin's Reckoning (1) - S1-E51
Continuity mistake: During the flashback, Batman's suit has a full black bat emblem on it without the yellow circle, but in the scene where he flips one of the gamblers over his shoulder, the black bat changes to the one with the yellow circle.
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.