Slip and underglaze let you put colour and pattern onto a pot long before it ever sees a glaze bucket. The great advantage is control: you are decorating a firm surface with a brush or a trailer rather than hoping a glaze will behave. But timing matters more than any tool. Most of these techniques work best when the pot is leather-hard — firm enough to pick up without leaving fingerprints, still cool to the cheek and slightly damp. If the clay has gone bone dry, freshly applied slip will simply flake away as it shrinks at a different rate. If it is still soft, your drawn lines will drag and slump.
Where a pot is too dry to take slip, you can usually still work with underglaze, which is essentially a pigment suspension in a binder and grips a bisqued surface happily. So the practical rule is: slip for leather-hard, underglaze for leather-hard or bisque.
Slip is simply clay thinned with water, sometimes with a colouring oxide or stain added. Because it is clay, it shrinks and matures with the pot, which means it can be layered thickly without the flaking you get from a foreign material. Mix your own and sieve it to remove lumps; the consistency you want is single cream. If it settles quickly, a pinch of sodium silicate or a drop of washing-up liquid will help it flow without making it watery.
Slip can be brushed, poured, dipped or sponged. Brushing shows the marks of the brush, which is part of its charm; dipping gives a flat, even field for the other techniques below.
Trailing uses a slip trailer or a bulb with a nozzle to draw raised lines and dots. Keep the nozzle moving — if you pause, you get a blob — and hold it just above the surface so the slip sits on top rather than cutting in. Practise on a tile first; the pressure on the bulb takes a little getting used to.
Inlay is the reverse: carve your design into the leather-hard pot with a needle tool, then flood the lines with a contrasting slip and scrape the excess back with a metal kidney once it has firmed up. The colour stays only in the grooves. It suits fine detail and looks wonderful under a transparent glaze.
Underglaze comes as a ready-mixed liquid or a dry powder you combine with water and a binder. You can treat it much like watercolour, but remember that it will be far more intense after firing than it looks wet. Apply two or three coats, letting each dry to a matte finish before the next, and build up shading gradually rather than trying to get full depth in one go.
Colours can be blended on the pot while still damp, and you can lighten a tone with white underglaze or a touch of the base. Do keep notes: some pigments shift dramatically under a glaze, and zinc-bearing glazes in particular can turn a reliable blue into a muddy grey. Test tiles are not optional here — they are your best friend.
If you want crisp geometric shapes, masking is the quickest route. Ordinary masking tape pressed firmly onto a leather-hard or bisqued surface resists slip and underglaze well; peel it away at a low angle while the decoration is still damp for the cleanest line. Wax resist is the alternative and is superb for organic shapes, allowing you to paint a resist pattern, let it dry, then wash colour over the whole pot — the waxed areas stay bare.
Sgraffito means scratching through a layer to reveal the clay beneath. Coat the pot with one or two layers of slip, let it dry to leather-hard, then draw with a loop tool, a needle, or simply a cocktail stick for finer work. Aim for a consistent depth of a millimetre or two — too shallow and the line disappears under the glaze, too deep and you weaken the wall.
Brush away the burrs with a soft brush before the pot dries, or they will fire into scratchy ridges. The magic of sgraffito is the two-tone result: a bold field of colour with the body showing through in fine, gestural lines. Combine it with trailing for lines that are raised in one place and incised in another.
Both slip and underglaze are normally bisqued around 1000°C before glazing, which sets the decoration and makes it safe to handle. Most underglazes then need a glaze over them, both for sheen and for food safety — an unglazed underglaze surface can be slightly porous. If you want the decoration to remain the star, choose a clear, non-zinc glaze and apply it thinly so it does not cloud the detail.
Keep a notebook of every test: body, slip recipe, underglaze brand and shade, glaze, firing temperature. In ceramics, the pot you like is rarely an accident — it is a recipe you can repeat. Start with a few tiles, work up to a small bowl, and you will soon find the combination of masking, trailing and sgraffito that feels like your own hand.
Bay Pottery shares practical, down-to-earth guidance on pottery and ceramics crafting for readers across the UK.
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