Reference
Defects
Twenty defects across the whole life of a pot — forming, drying, bisque, glazing, glaze firing, cooling — with what they look like, the usual causes, and what to change. The same reference Claybench uses to read a photo of a bad pot.
Causes are listed most-likely first. A defect can have several at once; change one thing per test.
Drying
S-shaped crack in the base of a thrown form, visible from the underside or inside after trimming.
Cracks radiating from or along the rim of a thrown vessel, usually appearing during the leather-hard to bone-dry transition.
Crack at the junction between a handle, spout, knob, or other attachment and the main body.
Form loses its intended shape -- plates go oval, bowls go off-round, rims go wavy.
Glaze firing
Small holes in the glaze surface, typically the size of a pinprick, sometimes with a raised rim.
Glaze pulls away from the surface of the piece, leaving bare clay islands or beading into pools.
Raised blisters or bubbles in the clay body, visible on fired work, often with glaze bulging over them.
Glaze flows off the piece onto the kiln shelf or forms thick drips on the bottom edge.
Glaze beads up or pulls away from surfaces that were not fully cleaned of bisque dust.
Large glossy bubbles or craters in the glaze surface, often concentrated in one area.
Glaze surface is rough, dry, matte when intended to be glossy, or easily scratched.
Glaze has run, lost its intended surface quality, or absorbed clay body color deeply.
Form loses its vertical structure during firing, leaning or collapsing.
Form is fused to the kiln shelf by glaze that ran off the foot.
Small depressions or craters in the glaze, often with dark or foreign material visible.
Cooling
Fine network of cracks in the glaze surface, visible immediately or developing days to months after firing.
Glaze flakes off the piece, often along rims and edges, leaving sharp bare spots.
Ringing crack or complete break running around or across a piece, occurring during cooling and often audible as a 'ping'.