The physical medium of which the object is composed. For photographic materials, Medium is the photographic process used to create the image. For text, graphic, and three-dimensional objects, Medium indicates the material of which the object is composed.
Three-dimensional object. Stone; Wood; Clay; Cloth; Leather; Glass; Bone; Paper: Metal; Fur; Plastic; Mixed media; Other
Photographic (NOTE: in Release 1.0. all terms have been consolidated into a single list, regardless of the value in the 'format' field.).
If format = Photoprint (direct positive):
Daguerrotype (copper/silver-plated); Ambrotype (glass); Tintype/Ferrotype/Melainotype (iron, japanned back); Unknown [source: rulib]