score:39
It's a pounce pot, being used to dry the wet ink without having to blot it.
As noted here, the pounce itself could be made from any of powdered gum sandarac; crushed pumice (origin of pounce I believe), cuttlefish bone, or eggshell; or allum mixed with resin. This was used both to size the writing surface as well as to dry the ink after writing, and the choice of pounce might be dictated by the requirements.
Vellum as well as (unsized rag) paper were the most common writing surfaces, though as noted school desks and privy walls were also not unheard of. Each of these surfaces posed its own challenges to proper calligraphy, from the greasiness of vellum to the roughness of wood or unsized paper, which an appropriate choice and use of pounce could alleviate.