Egyptian Zoomorphic Duck Juglet, Incised and
White Filled UC 13479 - copyright of the
Petrie Museum of Egyptian Archaeology, UCL

Some pots are made wholly in the shape of animals, known as zoomorphic, or humans, known as anthropomorphic. Some types of decoration may have a functional aspect, for example, clay strips that appear decorative may be applied to strengthen a vessel at a particularly weak point. In practice several decorative techniques are often combined on a single pot.

Cookworthy Dish with Stiffener - reproduced
by Permission of Plymouth City Council
Museums and Archives

7.6 Drying

At a microscopic level, clays have a sheet-like form, therefore a high surface area to volume ratio. When mixed with water, some of it is chemically attracted to the surfaces of the clay particles forming stable water films, so it takes more water to wet them and more energy to dry them out. Before a pot can be fired, it must be thoroughly dried to prevent the free and surface water, also known as ”water of plasticity’, in the clay turning to steam in the kiln and shattering the pot. Drying starts at the surface and moisture migrates there, but reaches a point when it balances and naturally stops. Temperature, atmospheric humidity and movement of air past the pot are factors affecting the rate of drying. A porous body dries in minutes sufficiently to handle. Once most of the unbound water has been removed, the clay particles come into contact with each other and generate friction, providing rigidity, and the partially dry pot is described as “leather-hard”. It still contains the chemically bound water and a small amount of water in the pores, but it cannot easily be reshaped. Leather-hard items are reasonably robust and can be polished and decorated carefully without damaging them, and this is the stage attachments such as handles can be applied. Further drying causes the remaining unbound water in the pores to evaporate and diffuse to the surface and is replaced by air. However, it becomes increasingly difficult for this unbound water to migrate to the surface, particularly in very plastic clays having small particle size. When this process is complete the pot is described as “dry” as opposed to leather-hard. Sponging will remove blemishes from a leather-hard item, but a dry one requires “fettling” (scraping with a tool). Although it seems a simple process, drying pottery was a difficult task for earlier potters, as the Mediterranean weather is often too dry, causing cracking, and the Northern European weather too wet and cold to dry at all. 

Removing the water from the pot causes it to shrink so it has to be slow and even, otherwise cracking or distortion will occur as the drier parts, having lost more water, are subject to greater shrinkage. Typically this takes 2-3 days, however, there were laws in ancient Athens that decreed building bricks had to be dried in the sun for 5 years to ensure dimensional stability! Managing the shrinkage can be a problem, as, depending on the type of clay, a pottery object can shrink on drying by around 5 to 15%, with bodies made out of fine clays shrinking the most and needing the longest to dry. As an example, with a complex model, such as the Worcester Arab Stallion that needs 38 different moulds to produce, 4 to 6 weeks has to be allowed for drying prior to firing. It has to be propped to prevent the legs from bending and the model from collapsing before it dries out and during firing. Because of shrinkage, the props must be made of exactly the same clay as the model so that the props shrink together at the same rate and do not themselves cause the model to distort.