Heat is something that we don't think of as an actual ingredient in breadmaking. It's not a material substance like flour, water, or salt. You can't pick it up. But it can be measured, experienced, and its qualities assesed; in my opinion it is just as important to understand as each of the other ingredients, if not moreso.
If you look at the ingredients of bread, and why bread making is so addictive, I think it's because we, as human beings, are constantly awed by transformation. Each of the basic ingredients - flour, water and salt - become transformed by their interaction with each other when making dough. But the real transformational agent is heat.
When we load the hot oven and look in after ten minutes or so, we are constantly amazed on a deep level at the 'kick', the visual effect of heat's transforamtive power over dough. I've been watching this process for a quarter of a century now, and I haven't become immune to it at all. A little blase, perhaps, but definitely not immune.
Qualities of Heat
Heat isn't allowed qualities, like flour and water - it is measured by degrees, and it is defined by how it travels. Heat is either convective, conductive, or radiant. But each of these transporation mediums create certain qualities which I would like to talk about here. I think these qualities greatly affect what heat does to dough, and thus what type of bread you get when the dough is baked.
Convective heat is defined as heat that travels in gas. In our case, then, the gas it travels in is air. Convective heat, by nature, is fickle. I say this because in a gas, the heat can move from one place to another very quickly. When you open the oven door, the heat you feel is the convective heat moving into the room, and out of the oven. The heat touches you, and simultaneously it dissipates rapidly into the room. After a few minutes, the heat has been completely dissipated, and to all intents and purposes it is gone. Thus, fickle.
Heat also can be conducted directly from substance to substance. When you put a chunk of dough onto a hot baking stone, the heat contained in the stone is conducted directly into the dough. It is captured by the dough, which then expands as the latent gases inside the gluten network are excited. Because of this direct transfer of heat, I like to say that this type of heat has momentum. The thicker the stone, the more heat it can contain, and thus the more momentum it has.
Radiant heat is the source of the heat. In a baking context, it's the flame of the fire; it's the electric element in your oven; it's the hot stones in turn giving out heat. Radiant heat, when it comes through stones, is also referred to as thermal heat, or thermal mass that has been heated so much that the stones themselves become heat sources. Radiant heat transforms gas to become convective heat, and it transforms thermal mass to also become radiant heat. So it's transformational heat - it's not at all fickle, like convective heat. It's powerful, with so much energy that it can turn liquids to gases. If you have enough of it, it can melt solids. Thus, it has a transformative quality.
These three forms of heat are the things that cause your dough to become bread. When you bake in a convective oven, in a tin, convective heat does most of the work. Thus, your dough cooks from the outside in. The crust forms first, and later the dough inside cooks. You have to tap the bread to see if it is cooked. Looks are deceptive when it comes to convective heat.
When you use a baking stone in a convective oven, with or without a bread tin, conductive heat does a bigger share of the work in baking your loaf of bread. Convective heat still plays a big part, but the momentum of the heat from the baking stone (especially when you don't use a bread tin) cooks the bread at a more even pace than when you just pop a loaf in a tin into the oven. What I mean by this is that the direct heat from the stone enters the dough and cooks it all the way through pretty much at the same time, rather than from the outside in.
When you have thermal mass, and by this I mean more thermal mass than a baking stone can provide (more like what you have in a woodfired oven, which has a layer of heated brick around the baking chamber), you get a much larger proportion of radiant heat doing the work of baking the bread. In this case, the dough is cooked by all three forms of heat - convective in the air, conductive from the dough resting on the stones, and radiant from the thermal mass all around the baking chamber.
A similar effect can be achieved when using a dutch oven, or when you create a 'hearth' inside your domestic oven (this is a technique I teach at workshops, if you are interested in learning how to do this).
When we bake in a thermal mass powered oven, the dough cooks from the inside out. The last thing to occur is the crust colour. We don't tap the bread to see if it sounds hollow - it sounds hollow soon after we put the dough into the oven. We wait for the crust to colour before taking the bread out. The effect of this type of heat is a demonstration of the momentum of heat I was talking about earlier.
So heat is the transformative ingredient in breadmaking, and the qualities of different forms of heat can make an enourmous difference to the breadyou make.
As always, Happy Sourdough Baking!