Continuity mistake: During his introduction Scarface's scleras go from light yellow to white then back to light yellow.
Continuity mistake: When Scarface's gang goes to steal the platinum from the ship and burn a hole in its hull, there is already a stack of platinum bars in their boat before they enter the ship.
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.