Slip LayerWhen the newly pressed clay medallions have sufficiently dried, the sharp rims that could easily chip or crumble are removed with a moist sponge. The clay medallions are then fired in a pottery kiln at 1060 degrees centigrade to become bisque or biscuit. After firing and cooling off, the bisque medallions are covered with a white engobe, i.e. a thin, white baking slip layer that is used to mask the inherent touch of yellowness of the pottery clay. The composition of the engobe is a production secret. Sometimes the medallions are referred to as porcelain. However, technically speaking the Verwoerd medallions are pottery, as is virtually all Delft.
In addition to using brushes for painting glaze on top of the slip layer, they can also be used to actually splash, flick or spatter the glaze