A form of surface finishing applied to pottery, a mixture of clay and water applied before firing to improve the pot's smoothness and decrease porosity (it makes it more watertight by clogging the pores of the earthenware). Slip often contains the pigment which imparts, after firing, the ground color of the vessel. The slip, being clay-based, is subject to the same color variation through different firing conditions (oxidizing or reducing) as the clay itself. Hematite slips, intended to be red, occasionally fire to a shiny black finish in a reducing atmosphere. Slips may or may not be polished after drying, and all sorts of decorative techniques may be used to alter this coating. Used as a decoration, slip is applied fairly thickly to form white or tinted patterns; this technique is called slip-painted.