What were mummy's portrait masks made of?

the Encyclopedia Britanica-1991, Mask,... funerary masks and death masks were used in aincient Egypt and were associated with the return of the spirit to the body. Such masks were generalized portrates and, in the case of nobility, were made of precious metals. Gold death masks also appear in Aisa and the Incan civilization....

Most mummies' masks were made of cartonnage -- a paper product.

http://www.virtual-egyptian-museum.org/Collection/Material/Collection.Material-FR.html