Pottery at Home
Pottery That Survives the Oven
Most handmade stoneware can be baked in. The pieces that fail almost always fail for one reason, and it is a reason worth understanding before the dish goes in.
A dense, well-fired stoneware dish is a genuinely good baking vessel. It holds heat, it releases it slowly, and it browns the underside of a gratin or a pie in a way that thin metal does not. It is also the piece of kitchenware most often broken by a habit its owner did not know was a habit.
Thermal shock, described once
Ceramic material expands slightly when it is heated. If one part of a dish heats much faster than another, the hot part tries to expand while the cold part holds it back, and the resulting stress has to go somewhere. In a material that cannot flex, it goes into a crack.
That is thermal shock, and it is not a sign that a pot was badly made. It is a limit of the material. What varies between pieces is how much sudden change they tolerate before the limit is reached.
What makes a body more tolerant
Three things help. A coarser, more open body with grog in it interrupts the path a crack would travel and accommodates a little movement. A thinner and more even wall reduces the temperature difference between the inside and outside surfaces. And a well-fitted glaze, one whose expansion rate closely matches the body's, avoids adding its own tension to the equation.
Pieces intended for oven use are usually made from a specific body chosen for those properties, and they tend to be visibly different: slightly sandy where the clay is bare, thicker at the base than a serving bowl, and often with a wide flat foot rather than a narrow ring.
The question is never how hot a dish can get. It is how fast it is being asked to change.
A stoneware dish will sit happily in an oven at high heat for hours. The same dish can fail in seconds when the change is abrupt.
The habits that cause failures
Cold dish into a hot oven. Taking a dish straight from the refrigerator and putting it into an oven that is already at full temperature is the most common cause. The base and the outside heat immediately while the contents stay cold.
Hot dish onto a cold or wet surface. A stone counter, a steel draining board or a damp cloth will pull heat out of one small area very quickly. A wooden board or a dry folded towel will not.
Cold liquid into a hot dish. Adding stock from the refrigerator to a pan that has just come out of the oven applies the same shock in reverse.
Direct flame or an electric element. Almost no handmade stoneware is made for the stovetop. Direct contact heat concentrates on a small area and is a different problem from the even radiant heat of an oven.
The broiler or grill element. Intense heat on one surface only, while the rest of the dish stays comparatively cool, is as severe as anything a dish is likely to meet.
Habits that work instead
Put the dish into the oven while the oven is still coming up to temperature, so that the dish and the air heat together. Let refrigerated food stand in its dish for half an hour before it goes in. When the dish comes out, set it on wood, on a cork mat, or on a dry folded towel. Let it cool on the counter before it goes anywhere near water.
None of this is demanding, and after a few weeks it stops being a decision.
Reading a dish before using it
Turn it over. A wide, flat, unglazed base suggests a piece intended for the oven. A narrow foot ring and a very smooth, dense body suggest a serving piece.
Look at the glaze under a light at an angle. A fine network of hairline cracks is crazing, which means the glaze and the body are expanding at different rates. Crazing is common, it is often deliberate in certain historical styles, and it is not dangerous in itself. On an oven dish, though, it is worth knowing about, because a crazed glaze lets moisture into the body and moisture in a body that is then heated quickly is an additional stress.
Tap the rim gently with a fingernail. A dense, sound stoneware piece rings. A dull thud can indicate an underfired body or an existing crack that has not yet opened.
If a dish does crack
A clean crack through a stoneware dish is the end of its life as ovenware. Repairs hold well enough for a piece to be displayed, but a repaired dish should not be heated again, and a dish with a crack that has been cleaned and glued is not suitable for food. That is a straightforward call and it is better made immediately than after a second failure in a hot oven.