Slip is a mixture of clay and water, forming a thick liquid. It can be used as a kind of glue to stick a handle onto a cup when both are “leather hard” (hard enough to handle but still soft enough to cut with a knife). Slip can also be poured into plaster moulds to form intricate shapes. The plaster absorbs water from the slip, causing it to dry on the outside first. If the rest of the slip is poured away, hollow vessels and ornaments can be made.
In pottery, pieces of clay sometimes need to be joined to each other. A handle on a cup for example. The way this is done is by scoring (scratching marks in) the edges that need to be joined, thereby creating a key. Then liquefied clay is pasted onto the two edges before pressing them together. This creates a bond.
The liquefied clay is called slip. Sometimes the words slip and slurry are used interchangeably. However, it is also argued that technically slip is thinner than slurry. And that slurry is actually the particular kind of thick, gloopy slip that is used for bonding pieces of clay.
Generally, slurry that is used for joining pieces of pottery consists only of particles of the clay being used in the pot itself suspended in water. This is important because different kinds of clay shrink and vitrify at different rates. If you join pieces of clay together with slurry that has a different shrink or drying rate to the pot itself, the join may not be very strong.
There is no particular hard and fast ratio for clay to water for joining clay. How liquid the slip needs to be depends on how wet your clay pieces are. If the clay is still relatively wet the slip does not need to be too thick. However, when the clay has begun to dry out, then the slip needs to be thicker in order for the pieces to key into one another.
Slip for joining clay is sometimes called ‘joining slip’. It can be bought or made. However, it is a good idea for the slip to be have the same clay body as the pot itself. Therefore, it is often recommended that the potter makes the slip themselves.
Slip is also used to decorate items of pottery. The term engobe is used to describe a clay slip coating that is applied to the body itself. This can either improve the texture of the item or add color. Engobe is opaque and can be white or colored. Colored engobe usually contains stains and metal oxides.
Engobe is different from slip in that it has lower clay content than slip. It also contains more silica and flux than regular slip. This means it shrinks less than slip when it dries.
Some people claim that engobe is half way between being a slip and a glaze. Fired engobe surfaces have a bit of a sheen that fired slip surfaces do not have. Others, state that engobe and glaze are quite different. They point out that if an engobe creates too much of a glazed effect, it loses its opacity, which is its most important property.
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