Coloured slip is clay slip carrying a stain or an oxide, and it is the basis of the whole slipware tradition. The colours available are broader than most people expect, and the timing of application matters more than the colour chosen: slip goes on leather-hard clay, not dry clay.
How the colours are made
A white or pale base slip is coloured with commercial body stains for clean, predictable colour, or with raw oxides for the older, more variable palette. Iron gives reds and browns, cobalt gives blue, manganese gives brown-black, and stains give the yellows, pinks and bright greens that oxides struggle with. Black slips usually combine iron and manganese or use a black stain.
Timing is the whole technique
Slip is applied to leather-hard clay so that pot and slip shrink together as they dry. On bone-dry clay the slip shrinks more than the body it is on and flakes away, sometimes not until after the bisque firing, which is why the failure often appears long after the mistake. This single rule prevents most slip disasters.
Trailing, dipping and sgraffito
Slip trailed from a bottle gives the raised lines of traditional slipware; dipping gives a solid ground to draw through; brushing gives soft, uneven cover. Applying a contrasting slip and then scratching a drawing through it to the body beneath is sgraffito, and it is the technique that gets most use out of a bucket of coloured slip.
Making your own
Take a white base slip from your own body or a bought casting slip, weigh in stain at a few percent to start, and test. Stains are potent, so a small jar colours a large quantity, and the cost per bucket is low. Mix in a ventilated space with a respirator, because stains and oxides are fine powders and several are materials to handle carefully.
Questions people ask about red slip
When do I apply coloured slip?
On leather-hard clay, so slip and pot shrink together. Slip on bone-dry clay flakes off, sometimes after the bisque firing.
How do I make black slip?
With a black body stain, or with a combination of iron and manganese in a base slip. Stains are more predictable.
Does slip need a glaze over it?
For functional ware, yes. Slip is not vitrified on its own and a clear glaze both seals it and brightens the colour.