How Do You Bake Without a Thermostat? Store the Heat in the Walls

A masonry oven works as a heat battery rather than as a heated box. A fire burns inside the chamber for a couple of hours, and the brick, stone or clay absorbs that heat into its mass, with the interior surfaces reaching a high temperature and the whole structure warming through. The fire is then raked out and the chamber swept, and the door is closed while the temperature evens out. Baking proceeds on the heat held in the walls and floor, which radiates back into the chamber and conducts directly into the base of whatever sits on the floor. The temperature falls steadily over several hours, which is a constraint and also the basis of how such an oven was used.

A single firing supported a sequence of different tasks:

• •The hottest phase suited flatbreads and pizza, baked in a couple of minutes
• •Then bread loaves, which need a fierce initial heat and tolerate a falling one
• •Then pies, pastries and roasts as the temperature came down
• •Then slow dishes, beans and stews left for hours
• •Then custards, meringues and fruit dried overnight in the residual warmth
• •Then the last warmth used for proving dough or drying wood and herbs

Bread baked this way differs from bread baked in a domestic oven in ways that are explainable rather than mystical. The floor is hot, dense and in direct contact with the dough, which drives a rapid expansion in the first minutes and produces an open crumb and a well-risen loaf. The heat arrives largely as radiation from the surrounding mass rather than as moving hot air, which dries the surface less aggressively and allows a thicker crust to develop without the interior overbaking. Steam released by the loaves themselves is trapped in the sealed chamber, keeping the surface flexible during the expansion, which is what a domestic baker imitates by adding water to a hot tray. The falling temperature suits a long bake.

Working without any instrument required tests that were reliable enough to bake by, and bakers developed several. Throwing a handful of flour onto the floor and watching how it browned gave a usable reading, with instant blackening meaning too hot and slow colouring meaning too cool. Holding an arm inside and counting how long it could be endured was the common rough method. The colour of the interior surfaces indicated the state of the firing, since soot deposited during burning clears from the dome at a characteristic temperature and leaves it pale, which told the baker the mass was charged. A scrap of paper or straw was used the same way. Experienced bakers judged by the feel of the radiant heat on the face, which sounds imprecise and was reproducible within a narrow band.

The social arrangement around these ovens is as interesting as the technology, because building and firing one was rarely a household matter. Fuel was expensive and a firing consumed a great deal of it, so heating an oven for one family's bread was wasteful, and the usual arrangement was a shared or commercial oven serving a village or a street, with households bringing their prepared dough to be baked. In much of medieval Europe the lord held the legal right to operate the oven and required tenants to use it and pay for it, which was a resented and enforceable monopoly. Commercial bakeries elsewhere baked customers' dough for a fee, and the practice of bringing a dish to the baker to be cooked persisted into living memory in many places.

A fire heats the mass of the structure, is raked out, and the baking happens on stored heat that falls steadily over hours. That falling curve was worked through in sequence, from flatbreads at the hottest phase down to overnight drying. Direct contact with a hot dense floor and trapped steam explain why the bread differs, and the cost of fuel meant these ovens were usually shared or seigneurial rather than domestic.