Slip is clay suspended in water, and it does three unrelated jobs: it glues pieces of clay together, it decorates a surface with colour, and it is poured into moulds to cast pots. The same word covers all three and the consistency needed for each is quite different.
Slip as glue
Joining two pieces of clay needs scoring and slip, and the slip should be made from the same body being joined so it shrinks at the same rate. Thick, like double cream, is right. Score both faces, wet them with slip, press until it squeezes out, and clean the seam. A join made without slip will part as the piece dries, every time.
Slip as decoration
Coloured slip painted, trailed or poured onto leather-hard clay gives the whole slipware tradition. Here the slip carries a stain or oxide and is thinner, and it must be applied while the pot is still leather hard so the two shrink together. Slip on bone-dry clay flakes off, which is the single most common slip-decorating failure.
Casting slip is a different product
Slip for pouring into plaster moulds contains a deflocculant, which lets it flow at a much lower water content than plain slip. That matters because a casting slip with too much water shrinks too much and cracks in the mould. Casting slip is bought ready-made or mixed to a recipe; ordinary studio slip is not a substitute.
Making it from reclaim
The cheapest slip is your own dried scraps slaked in water and sieved. Dry the scraps completely first, add them to water rather than water to them, let them slake without stirring, then mix and sieve. It costs nothing, it matches your body exactly, and it keeps the reclaim wet, which is the safe way to store it.
Questions people ask about clay slip
What is clay slip?
Clay suspended in water. It is used as glue for joins, as coloured decoration on leather-hard clay, and, in a deflocculated form, for casting in moulds.
Can I use any slip to join clay?
Use slip made from the same body, so the join shrinks at the same rate as the pieces it holds.
Why did my decorating slip flake off?
It was almost certainly applied to clay that was already too dry. Slip decoration goes on at leather hard so both shrink together.