Copley's expert rendering of _____ displayed a level of talent uncommon in 18th century America. Hair, Facial expressions, foliage, or cloth? I think it was foliage.
If you are speaking on John Singleton Copley, this statement from a web site might provide a clue:
<<Copley demonstrated a genius, in both his American and British periods, for rendering surface textures and capturing emotional immediacy.>>
I'd go with facial epressions. Being able to paint cloth (a surface) does not seem like that big a deal