Pottery studio failures: underfiring, crazing, and how handling breaks pots

Most home studio failures are not accidents. They are four mechanisms, and each one announces itself before the pot leaves the shelf.

What to take away

Why underfiring hides until the pot is in use

Clay does not become ceramic at a temperature. It becomes ceramic at a threshold. Preservation guidance on ceramic collections puts the line at about 900 C, where particles begin to bond through sintering.

Below that line the body stays porous and takes on liquid. Above it the ware turns harder, denser and less permeable, and also more brittle. Both changes are permanent.

A university materials course measures the same shift. Unfired green clay beams broke at roughly 37 newtons; fired beams took about 54. That course also records that water of hydration leaves the clay around 350 C, well before the body has bonded at all.

An underfired pot looks finished. Then it weeps, stains, and fails in the freezer or the dishwasher. No glaze applied afterward rescues it, because the problem is in the body.

The four mechanisms and what each one asks of you

Read the table as a diagnostic order, not a list. Underfiring comes first because a porous body makes the other three worse: salts travel further into it, crazing reaches the clay, and a chip exposes more surface.

Crazing is a structural problem, not a surface one

Preservation guidance describes crazing as fine cracks in the glaze layer. The glaze contracts faster than the clay body beneath it, so the glaze has to give somewhere, and it gives in a web of cracks.

On functional ware, every one of those cracks is a route into a porous body. Staining is the visible result. What sits in the cracks between uses is the part you cannot see.

That is why crazing gets fixed by changing the glaze or the body, not by refiring or by applying the glaze differently. The mismatch lives in the materials, and a second trip through the kiln does not remove it.

Handling breaks more pots than firing does

Conservation practice for ceramics is short enough to copy at home. Lift an object underneath its center of gravity, never by a handle, rim or spout.

Remove lids and pedestals and carry them separately. Never hand a pot from one person to another; set it down on a stable surface instead.

For slippery or fragile pieces, nitrile or vinyl gloves beat cotton, which snags on rough feet.

Those four rules cover almost every avoidable breakage in a home studio. They pair with the firing discipline and the making habits that prevent the rest.

Storage is where handling rules quietly fail. A shelf of greenware stacked two deep in a cold room will crack before it ever reaches the kiln, and the crack appears as a firing fault you then chase for a month.

Reading a failure before you change anything

Work backward from the mark. A white bloom that wipes off and returns points to soluble salts, which means the ware sat in contact with soil or salt. A web of fine lines in the glaze points to fit. A pot that feels heavy and cold after a soak points to underfiring.

Weigh a fired test tile, soak it a day, weigh it again. A large gain in water means the body never vitrified, whatever the controller display said.

Keep a few failed pots and label them with the cone, the body and the glaze. A shelf of known failures is the fastest reference you will own when a new fault appears.

Common questions

How do I know if my ware is underfired?

Weigh a fired test tile, soak it for a day and weigh it again. A large gain in water means it did not vitrify, whatever the controller said.

Can I use a crazed mug?

For your own use, with care. The cracks hold moisture and residues, and on a porous body they can reach the clay, which is why crazed ware is not sold as functional.

Is any of this different for hand-built work?

Not in mechanism. Uneven wall thickness makes hand-built pieces more prone to drying cracks, but firing and glaze fit behave the same way.

Should I keep failed pots?

Keep a few and label them. A shelf of known failures is the fastest reference you will have when a new fault appears.