Continuity mistake: When Selina dons her Catwoman costume, she pulls on her mask which was detached from her costume like a face mask, but later on when she returns to her apartment after the museum robbery, she removes her mask which is now attached to her costume like a hood.
Continuity mistake: When Selina talks to Scarface in the limousine, her eyebrows change from black to blond back to black again between shots.
Continuity mistake: When Catwoman first encounters Batman during the flashback, her whip is coiled around her shoulder. When the camera cuts back to Catwoman after the shot of Batman, the whip disappears altogether.
Other mistake: When Catwoman scratches Batman's face in the flashback, her fingers and claws stretch comically.
Continuity mistake: During the flashback, Catwoman's whip disappears and reappears on her belt in a couple of scenes.
Continuity mistake: Catwoman/Selina gains and loses her pupils throughout the episode.
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.