Why a vessel cracks

An earthenware vessel almost always cracks from a sudden change of temperature rather than a fault in the clay. When one part of the wall expands or contracts faster than another, the stress between them passes what the material can hold and the wall gives — usually with a single loud crack and a line running down from the rim.

What thermal shock is

Clay conducts heat poorly. That is the very quality that keeps food hot and cooks it evenly in an earthenware vessel, but the same quality means the outer and inner wall never heat at the same moment. While the difference between them grows slowly, the material carries it. When it grows sharply — a cold vessel set on a fierce ring, a hot vessel touched with cold water — the outer layer expands or contracts faster than the inner one, and the difference resolves itself by cracking.

The calcite blended with the clay in Zlakusa is precisely the answer to that problem: it reduces shrinkage during drying and firing and gives the vessel resistance to thermal shock, which is what lets it go on a direct flame at all. But resistance is not invulnerability. A Zlakusa vessel takes what ordinary ceramics would not, and still asks for heat that arrives gradually.

Six situations that break a vessel

• A cold vessel into an already hot oven or onto a fierce ring — the commonest cause of all
• A vessel straight from the fridge onto heat, without an hour at room temperature
• Cold liquid poured into a hot vessel, or hot into a cold one
• A hot vessel set down on stone, metal, a wet cloth or in the sink
• A flame or ring narrower than the base, so one spot heats far faster than the rest
• An empty vessel left on fierce heat, outside the controlled seasoning bake

All six are the same mistake in different clothes: somewhere on the wall a difference in temperature arises faster than the material can even it out. A vessel rarely breaks at the moment of the mistake — often it survives that meal and gives way at the next, which is why cause and effect are hard to connect.

A crack — when the vessel can still be used

Tap the side with a finger. A clear, ringing note means the wall is whole; a dull, muffled answer means a crack, even one invisible to the eye. On glazed ware a fine line that holds water and still rings is often harmless, but on an unglazed vessel every visible line runs through the material itself, because there is no coating for it to sit in.

The practical rule: a vessel that seeps water or answers dully does not go back on the fire. It can serve as a bowl for dry things, as a planter or for serving at the table, but cooking in it means the crack will keep opening under heat — and a vessel full of scalding food coming apart in your hands is no small matter.

The old remedy of boiling milk in a cracked vessel does temporarily close a fine line, as the proteins thicken inside it. But that is sealing, not repair: the strength of the wall does not come back, and thermal shock will open the same line again.

When it is not your fault

A vessel that cracks during a properly done first seasoning — from a cold oven, gradually — most likely carried a fault from the making: a trapped air bubble, a grain of foreign matter in the clay, or an uneven wall. It happens to the best of makers, and such a piece is usually acknowledged at the workshop. Get in touch with whoever made it; in Zlakusa that is still a person you can telephone.

Cause in the great majority

thermal shock, not a fault in the clay

Commonest mistake

a cold vessel into a hot oven or onto a fierce ring

From the fridge

an hour at room temperature before any heat

The tap test

a ringing note — wall whole; a dull note — a crack

A vessel that seeps

does not go back on the fire

Cracked during correct seasoning

likely a fault in the making — ask the maker