slip-cup

from The Century Dictionary.

• noun In ceramics, a baked earthenware utensil in which the potters' slip, or diluted clay, is placed; a quill-box. It is provided with one or more quills at one side, through which the slip is poured in decorating pottery. A slip-cup differs from a pipette in being open at the top, while the latter is closed, having only an air-hole or vent by means of which the potter regulates the flow of slip by the pressure of his thumb. The latter term is used by European potters.