A decorated pitcher with a base, an oval body, and a wide spout. Some of these types of pitchers are also marked with tall, slender bodies. The main purpose of an ewer was to carry water for drinking or to wash an individual. A good modern example of this type of pitcher is Winart Pottery from Oklahoma.
Ewers are basically pitchers that have a handle and a flaring spout. They are often decorated. In olden days they were filled with water and used in bathrooms to flush a toilet before it could be done with a flick of a handle. The name has nothing to do with the shape or size of the vessel.
A ewer, regardless of what type of material it is made from, is defined as a large, wide mouth, water pitcher. Your question seems to suggest, having a water pitcher at hand when working with clay.