Many things beyond VisualArtwork have materials that can be described. We had http://schema.org/material for VisualArtwork but it was renamed to give us http://schema.org/artMedium.

Proposal here is to bring back 'material' as a superproperty of artMedium but applicable to more types of thing (suggest Product + CreativeWork). Definition would be "A material that something is made from. "typical values would be: leather, wool, cotton, paper." (Text/URL, to allow for controlled vocabulary).

(The property would be repeated for multiple values, rather than parsing out comma-separated lists.)

/cc @lazaruscorporation re VisualArtwork and @mfhepp re Product