Flaking Glaze or Underglaze
Pottery With Flaked Glaze
Pottery or ceramics with chipped glaze or underglaze (looks similar to chipped nail varnish). , flaky pieces of
Shards of glaze/underglaze coming off pottery ceramics
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.
What is Shivering?
Shivering is when flakes of glaze peel, crack, flake or chip off edges of ceramic ware. Sometimes the pieces of glaze or underglaze are small and razor-sharp. It happens because the thermal expansion of the clay body is incompatible with the glaze or underglaze (e.g. the bisque and glaze shrink or expand 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
What else prevents glaze or underglaze from adhering?
Bisque can be contaminated by the decorator with oils, creams, grease from food etc. Ensure hands and working surfaces are clean. It is best to wash hands with soap and water - even baby wipes contain oil-based products and can prevent adhesion.
Do not force dry glazes and underglazes with heat guns or hairdryers.
Do not apply excessive coats of glaze or underglaze. Check the viscosity of your glaze and make sure colour is not "globbed" on.
Make sure underglaze colours or clear glazes are fully dry before firing (any moisture from wet wares can turn to steam and cause a poor fit between colour and bisque).
Under-fired bisque, over-fired bisque and contaminated bisque can cause problems - always buy bisque from a reputable bisque supplier.
Check firing temperatures are correct for the clay, glaze and underglaze you are firing. All products need to be compatible and have the same recommended firing temperatures. Do not guess and always check with your supplier.
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.
"Hard spots" or "hot spots" on bisquewares look like a yellowish discolouring on the bisque (and will show up through transparent, translucent or light glazes and underglazes. It occurs when slip (liquid clay) is continually poured in the same spot when filling moulds.
Can you repair a shivered piece?
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).