Slips and Surface Decoration
Slip is liquid clay used to join, cast and decorate; surface decoration is where design marks are made and preserved through the glaze fire. The exam tests which technique suits which stage of dryness.
Slip and engobe
- Joining slip - same body, creamy, with a little sodium silicate.
- Casting slip - deflocculated, SG 1.75–1.85.
- Engobe - slip with flux and colourant, applied to leather-hard or bisque ware as a coloured ground under the glaze.
- Terra sigillata - ultra-fine deflocculated slip, applied thin and burnished for a sheen.
Techniques
- Sgraffito - scratch through a slip layer to reveal the body. Apply slip at leather hard, carve when firm but not dry.
- Mishima / inlay - incise, fill with contrasting slip, scrape back at leather hard.
- Slip trailing - squeeze a line of slip from a bulb or bottle onto the surface.
- Majolica - tin-white glaze, then oxide or stain painted onto the raw glaze before firing.
- Wax resist, stamping, marbling, agate ware, burnishing, decals, on-glaze enamel.
Worked example
A bowl is to be sgraffito-decorated in two colours. Throw it, dry to leather hard, apply white slip about 1 mm thick in crossing brush strokes, let it firm to touch-dry, then carve with a loop tool. Carving too early drags the slip; too late it chips and the line lifts.
Traps
- Applying slip to bone-dry clay - it flakes off in the kiln.
- Slip and body with different shrinkage - crawling and peeling.
- Carving into a damp body and sealing the cut with slurry.
- Forgetting that iron and copper stains change colour under different glazes.