Glaze Shivering, Pinging Off, Cracked
Glazing Problems On Pottery
Problems with your Firing?
When glaze starts to lift away from the bisque, (more commonly this occurs on edges, rims and handles), this is known as shivering or pinging. It is like chipped pieces of glass and can be very sharp. It occurs when the bisque and glaze shrink at different rates.
Most things expand when hot and shrink/contract when cool. A clay body and glaze fuse together during firing. The aim is to get the right combination of expansion and contraction for a good glaze fit.
- If the clay body contracts more than the glaze, during cooling = SHIVERING
- If the glaze contracts more than the clay body during cooling then it will crack = CRAZING
This problem occurs:
- decorators contaminating bisque with oils, creams grease or food (preventing adhesion)
- forced drying e.g. hairdryers.
- due to excessive glaze application/globbed on colour (check the viscosity of your glaze)
- underglaze colours or clear glazes are not dry enough before firing (when heated, moisture from the colour can turn to steam and cause a poor fit between colour and bisque).
- under-fired or over-fired bisque and contaminated bisque (always buy bisque from a reputable bisque supplier).
- because of compression and expansion differences
- due to firing temperatures
- to avoid excessive compression - consider reducing the soaking period
- to avoid excessive expansion - consider lightly sponging rims and handles of piece before bisque firing. Use care not to over sponge, which will promote peeling.
- Silica in clay bodies
It is unlikely that you will be able to repair a shivered piece. However, if the area is small (and as experiment), trying mixing your underglaze with clear glaze and applying it to the bare patch and lightly over the edges of the area that has shivered. Then re-fire your item on using a programme with a slow ramp (items that fire too quickly on a second firing are prone to cracking/splitting - especially large or thick shapes).