Before looking for the cause of a defect, you need to give it the right name. Here are the most common ones, with the leads potters explore first. Every case is different: for a diagnosis, start a topic with the technical details and photos.
Crazing (crackle)
What it looks like: a network of fine cracks in the glaze, sometimes only visible days or weeks after firing.
What's happening: as it cools, the glaze shrinks more than the clay body. It's put under tension and cracks.
Leads: most often, the fit between the clay and the glaze is to blame. Changing the glaze or the clay, or adjusting the glaze recipe, fixes the problem for good. A thinner coat of glaze can reduce it, and on earthenware, firing the bisque a little higher often helps.
Good to know: on a piece meant for the table, crazing is best avoided. The cracks trap dirt and weaken the piece. On a decorative piece, on the other hand, crackle can be a sought-after effect.
Shivering
What it looks like: flakes of glaze popping off, especially on rims and edges. They can be sharp.
What's happening: it's the opposite of crazing. The glaze is squeezed too hard by the clay body.
Leads: revisit the fit between the clay and the glaze. A piece that shivers must not be used at the table.
Crawling
What it looks like: the glaze has pulled together into islands or beads, leaving the clay bare.
Common causes: glaze applied over dusty or greasy bisque, a coat too thick that cracked as it dried, a glaze that shrinks a lot as it dries.
Leads: wipe the bisque with a damp sponge to remove dust, avoid coats that are too thick, and let the glaze dry thoroughly before loading the kiln.
Pinholes and blisters
What it looks like: small holes (pinholes) or blisters on the surface of the glaze.
Common causes: gases escaping from the clay or from the layers under the glaze during firing, glaze applied too thick, an end of firing too short to give the glaze time to heal over.
Leads: a well-fired bisque, a thinner coat of glaze and a hold at the end of the firing often help the glaze heal over.
Runs
What it looks like: the glaze has run, sometimes far enough to glue the piece to the kiln shelf.
Common causes: a very fluid glaze applied too thick, or a firing too hot for that glaze.
Leads: a thinner coat on the lower part of the piece, an unglazed band above the foot, the right firing temperature. While testing, set the piece on something that protects the shelf.
What next?
Our diagnostic tool and the Fix crazing or shivering tool give you leads. For the community's opinion, start a topic and fill in the technical details.
