Given the task definition and input, reply with output. Generate a correct and concise answer for the question based on the words in the context.

Context : The Portrait of Frédéric Chopin and George Sand was an 1838 unfinished oil-on-canvas painting by French artist Eugène Delacroix. 
Question : What is the surface of Portrait of Frédéric Chopin and George Sand made of?

canvas