Vol. II · Heat Plate 7 of 11oven, thermostat, preheating, temperature swing, oven walls, baking stone, door openingHT

The Oven.

The dial sets a target, not a temperature. The air swings around it; the walls, which hold most of the heat, catch up slowly.

An oven heats with an element switched on and off by a thermostat. Below the set temperature minus a margin the element comes on; above the set temperature plus the margin it goes off. So the air never sits at the dial's number: it climbs past it, falls below it and climbs again, every few minutes. Home ovens commonly swing ten degrees either way, and many run hot or cold overall by as much.

The air is the small part of the oven's heat. The walls, racks and any baking stone hold far more, and they lag well behind the air. When the "preheated" light comes on, the air has reached the set temperature but the walls have not — and the walls are what keep the oven steady when a cold tray goes in or the door is opened. The workshop runs an oven for an hour.

Below: the oven's air (solid) and walls (dashed) from switch-on, with the dial's setting, the thermostat's band, and the effect of a baking stone and of opening the door.

What you will learn

Explain how an oven thermostat cycles.
Measure an oven's swing and mean temperature.
Explain why preheating should continue after the light comes on.
See what a baking stone and an open door do.

Workshop

A 2.5 kW oven under an on–off thermostat with the band you set (stove bench; air and walls as two lumped bodies, a teaching model). The door, if opened, stays open for 30 seconds.

The light is about the air

Set to 200 °C, the oven's air first reaches the dial after 16.6 minutes, then swings between 190 and 210 °C. The walls, which hold most of the heat, are only at 152 °C at that moment and reach 198 °C only later — they are the slow body that the air leans on. With a 5 kg baking stone in the oven the air takes 22.6 minutes to first reach the dial and the stone keeps absorbing heat for long after: bread bakers preheat for 45 minutes to an hour for that reason.

Opening the door for thirty seconds dumps most of the hot air, and the oven drops to 163 °C before the element and the walls bring it back. The walls are what make the recovery quick; an oven whose walls are still cold recovers slowly. Two practical rules follow: preheat longer than the light says, especially for bread and pastry; and check your oven with an oven thermometer on the middle rack, because the dial may be off by ten or twenty degrees.

At the stove

Put an oven thermometer on the middle rack, set the oven to 180 °C, and read the thermometer every two minutes for half an hour after the preheat light comes on. You will see the swing and the oven's true average. Note the difference from the dial on a strip of tape by the knob.

Most domestic ovens hold temperature with a simple on–off thermostat; some newer ones use electronic control that pulses the element to narrow the swing. Commercial deck ovens for bread have thick stone floors precisely to store heat.

Terms

Thermostat: A switch that turns the element on below and off above a set band.
Swing: The range the air temperature cycles through around the setting.
Preheating: Heating the oven, including its walls and racks, before the food goes in.
Thermal mass: Material that stores heat — walls, stones, heavy pans — and steadies the temperature.

An oven's air swings around the dial, and its walls lag behind. Preheat longer than the light says, and check the real temperature.

Sources

A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.