Slips & Firing Techniques:

Oxidizing & Reduction

The vast majority of ancient pottery is coated with slip, fine watery clay that produces a glossy surface when fired. The use of slip is in fact more for practical reasons than it is for aesthetic.  Despite the decorative results, its primary function is to seal the vessels’ surfaces and make them less permeable to their contents. Slips should not be confused with glaze, which involves a chemical process that results in a vitreous or glass-like appearance after firing.  Such exteriors are rare in ancient pottery.

The colour of pottery is also affected by the method in which it is fired; as the iron oxide content in clay will react to the amount of oxygen inside the kiln. For example, by firing in an oxidizing or smoke free atmosphere the iron content will produce a red surface, while a reducing, or smoky atmosphere will produce a black finish.  The reduction technique required a damp fuel source and a sealed kiln to contain the smoke, which would turn both the slipped and bare areas black or gray. Romans however, generally preferred shiny red pottery and therefore relied mostly on the oxidizing technique.