Liquid clay is called 'slip' or 'slurry'. The pottery plants of the time needed slurry for two reasons: To feed the filter press and to make irregular shaped ware using moulds. When clay is mixed with water it forms a creamy consistency that is quite pleasant to the touch and has some remarkable properties as we shall see.
2. Ceramic items that could not be jiggered were
formed using a method called 'slip casting'.
In this process the clay slip is poured into plaster moulds that define the outside
shape of a piece and left to sit for a number of minutes. During this time
the absorbent plaster draws water out of the slip and a stiff watered-reduced layer
forms against the mould surface. When the desired thickness is achieved the
remaining liquid slip is poured back out of the mould. After further hardening
the mould is split and the clay object is removed and set out to dry and later
cleaned. The dried
and often fragile pieces are called 'greenware'.
The operator had to know how to control the specific gravity, viscosity, and gelling properties of the slip to assure that it worked properly in moulds like those shown here. Properly mixed slip is a very heavy and sticky liquid; it flows well and turns to a gel if left sitting for a few hours. The slip mixer was under considerable pressure given the large quantities needed in the plant and the serious consequences of mixing it improperly.
Since casting slip is so heavy, the equipment used to mix it had to be very powerful and durable. The equipment in this room was made of concrete to avoid rusting problems associated with metal containers. Four large blunger tanks are visible, however there are also tanks under the floor.
The slip was often contaminated by particulate impurities from the raw materials and small iron and copper filings from the equipment. All slip had to be run through the magnetic separator and rotating screen to remove these impurities. If not dealt with these would produce small black or green spots on the fired ware.
The clay mix used in these shops is a white earthenware clay that is painted with low melting glazes and fired at around 1800F or 1000C. However the slips typically used in this plant were porcelains and fired to much higher temperatures than what is done in hobby ceramics today.
