Overglazes and Underglazes
Glazes can be laid on top of each other to create even more effects. This is called overglazing. Some "overglazes" are simply other glazes that are applied on top of another unfired glaze that will mature at the same temperature. True overglazes may also be applied after the base glaze has already been fired. These overglazes will require the ware to go through a third firing, at a lower temperature than the base glaze was fired to.
Underglazes are not glazes themselves, but are colorants applied to the bisqueware before the glaze is applied. When fired, these colors show up through the transparent glaze covering it.
Slips and Engobes
Slips and engobes are essentially the same thing. The difference in term is basically a difference in regional language preference. “Slip” is more common in Europe, and “engobe” is more common in North America. Both words refer to a liquid slurry consisting of clay or clay mixed with coloring agents. Slips and engobes are used to decorate wet greenware, adding color, texture, or two dimensional design.
Clay is a master chameleon. With skill, clay can successfully visually mimic all sorts of substances, from metal to old shoes.
Clay is impressionable. Textures can readily be added to wet pots through impressing a variety of tools and objects into the surface.
Clay is also carve-able. Marks and designs can be incised into leather-hard greenware. By doing so at the leather-hard stage of drying, the cuts retain their crispness. Leather-hard greenware also allows for more ease when incising more intricate patterns.