Heat Dynamics and Woodfired Ovens

It’s taken me about ten years, but I’m beginning to understand heat dynamics. I guess I’m a slow learner - it’s just that I keep asking ‘Why?’ A great many things about thermal engineering are counter intuitive; and lots of things seem to make sense in theory, but in the real world they no longer do.

A blurry prototype Barrel oven is fired up for the first time.

A blurry prototype Barrel oven is fired up for the first time.

For this reason, I am going to try to explain heat dynamics to you - kinda like a brain dump, so that I can nail what I know down for a bit where I can see it - on the page. This has nothing to do with providing education for you, dear reader - and everything to do with maintaining some degree of sanity. Sometimes a bear with very little brain (like me) just needs to write stuff down to make sense of it. So here it is. My brain dump on how heat works, in layman’s terms. Enjoy!

More art than science

Thermal engineering, as they say, is more art than science. Art has a way of messing with one's mind, for the sake of anyone who has ever brushed up against it. I guess that’s it’s function - art causes us to look at things differently. Science is very much a part of it too, though. At school, my weakest subject was physics, followed by chemistry. Thermal engineering is all about physics. Sourdough bread is all about chemistry. I guess I’m a sucker for punishment, ‘cos these two subjects occupy a great deal of my time these days!

The Artisan Woodfired Oven Firebox in action.

The Artisan Woodfired Oven Firebox in action.

Heat is heat - plain and simple - but it sometimes seems to behave in unexpected ways. This can be quite a problem if, like me, you are untrained in the principles of heat exchange, and wish to build an oven or three. In my quest to create the perfect oven, I’ve learned about heat the hard way. I didn’t get it from a book, I can tell you. I’ve also learned that even people who work in the furnace industries are often mistaken in their beliefs. It sounds arrogant, I know, but nothing tells you about the nature of heat better than bread. So I’m going to use it to try to explain what’s happening when heat passes through different types of substances.

So far, I’ve built quite a few ovens, and assisted in the design of quite a few more, with my friend Craig MIller, from Aromatic Embers. About a decade ago, I found Craig’s ovens online, and contacted him to see if he could make me a baker’s oven. We’ve been trying to make a decent one ever since.

Firing up the Artisan 3000 for the first time.

Firing up the Artisan 3000 for the first time.

The thermal engineers, in their scientific way, classify heat according to how it moves - conduction, convection and radiance. Physical transfer, gas transfer and heat emission, or radiant heat. This last one  is when something fills up with heat and then passes the heat out. The other two are fairly self explanatory.

I find these pieces of information as useless as teats on a bull, in practical terms. And kinda stupid, as they tend to overlap - radiant heat, for example, fuels both conductive and convective heats. In turn, when you have a fire contained by a brick firebox, conductive heat promotes radiant heat; convective heat is the transfer medium for more conductive heat, which becomes radiant heat in the bricks of the oven. This radiant heat transforms again to convective and conductive in the baking chambers. The convective and conductive heats transform once again when they pass through dough, which is loaded into the baking chamber. This heat gradually is diffused by the baked bread into convective (gas; it’s stored in carbon dioxide now), or re absorbed back into the baking stones to return to being radiant heat again. On and on it goes, swapping from one thing to another all the time. It gets rather complex, in my mind. It seems to be the way scientists like things.

I don’t. I just want to know how to make heat do what I want it to.

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Loading Luna

Lets have a look at a lump of dough for a minute, to explain a certain type of insulation principle.

Dough trapping carbon dioxide via heat

Dough trapping carbon dioxide via heat

Edible insulation?

Imagine, for a moment, that you are a nice, ripe lump of dough. You have been cultured and shaped many times to become a finely woven mesh made up of sheets of gluten, all intersecting in a kind of 3D web, all of this is then wrapped up in a smooth layer of stretched gluten.

You have had your gluten skin pierced by a knife, to expose your delicate inner layers to the air. This ‘scoring’ process will assist in the transformation you are about to be involved in.

The baker dumps you on the hot stone floor of his (or her) woodfired oven. Instantly, conductive heat finds it's way into you through the hot stones in the baking chamber. Soon after,  you feel the heat all around you, convectively, as the hot air in the oven wraps you up. It’s very steamy, as the baker has created a moist environment inside the baking chamber. All of this heat is being radiated throughout the baking chamber via the hot stones surrounding it, and moisture is carrying the heat through your skin and into your gluten network.

In spite of all this heat absorption, your surface would be only warm to touch, at least initially. You have the capacity to absorb lots more heat before you too must begin to give the heat back out.

Continued exposure to heat will burn you, of course. Ultimately, you too must ‘catch fire’ if you continue to absorb heat. The baker knows this, and rescues you before you do.

As a result of this sudden heat, your gluten bubbles are forced to expand quite rapidly. The very last bit of fermentation energy inside them undergoes a chemical transformation as a result of all this heat. It literally explodes, but it is trapped by the gluten network.

Your gluten network is very pliable, and it can grow and expand as you absorb all the heat. So the dough grows.

After a while, you solidify. You would now be quite hot to touch, as anyone who handles crusty hot bread straight out of the oven would testify. Hot bread burns skin - quickly - via heat conduction.

Some Seed + Grain breads transferring heat from the oven into the bench.

Some Seed + Grain breads transferring heat from the oven into the bench.

The baker flips you out if his oven onto the cold steel bench. You take your heat with you, and it passes into the bench via conduction, and some passes back into the air via convective heat.

You have become a kind of edible insulator, and, inadvertently, a heat transfer device. The convective heat is now in your gluten network as carbon dioxide gas. It filled up quite rapidly with heat energy, and this convective heat will now pass back out into the atmosphere. The CO2 simply leaves the bread as gas.

While you are still hot, you have become a radiant heat source. And the bench you are sitting on is instantly hot, as conductive heat energy has made it so. It feels hot because it is quite dense, and instantly reflects this heat.  If it was airy, like the dough, it would absorb the heat, so it wouldn’t be as hot to the touch.

Light, Water and Sound, all rolled into one

The thing is, heat is like light, water and sound.

It reflects instantly when it comes up against a dense substance (like steel or brick). Over time, these substances absorb the heat, and then begin to pass this heat on. So they reflect, then absorb, then pass on the heat.

Dough is an excellent insulator!

Dough is an excellent insulator!

When heat comes up against a substance filled with gas or bubbles, like dough, it is absorbed. Thus, the substance remains cool to the touch until it fills up. Then it passes the heat back out. So spongy, porous substances absorb heat. They eventually fill up and then pass the heat out slowly when they are full. They pass the heat out for a long time, not all at once, rather than ‘reflect’ heat.

People say that insulators (like dough, or pink batts, or ceramic blanket) ‘reflect’ heat, due to this effect of absorption then gradual passing out. I disagree with this view. If you want to ‘reflect’ light, you use a super dense substance, like a mirror. If you want to reflect heat, you use brick or steel. These dense substances reflect heat very quickly, and continue to reflect it as they absorb more. They are better conductors than anything filled with gas. Gas absorbs heat, and tends to absorb more than it gives off.

The best insulator, it is said, is a vacuum. By its very nature, a vacuum is an ‘absence of gas’ - a place where gas has been removed completely. A vacuum will actually reflect heat, simply because it does not absorb it. This is very different in nature to a pink batt - yet both are considered insulators.

And they are. Dense things become a good radiant heat sources over time, whereas light, airy things don't. Dense things reflect heat, it’s true. They also absorb it, but more slowly than a porous substance.

Up to this point you can see that heat is like water. Spongy substances soak up heat, while solid substances cause heat to bounce off. Like washing the car - the sponge absorbs water, while the steel bonnet bounces it off. 

Heat is also part of light energy. So a dense, dark coloured substance will tend to absorb heat - or even, I suppose, a substance that is in the dark (though I'm only guessing, as it was in the dark and I didn't see it).  Similarly, a light coloured substance will tend to reflect heat.

The substance of brick also has a part to play. The more silica, the higher temperatures the brick can survive. Similarly, in porous insulation, we start with glass based fibre for lower temperature insulation, and move to carbon fibre and mineral compounds for higher temperature absorption.

All of these principles come into play in my world all the time; as a baker, as a Furnier (one who runs an oven or furnace), and as an oven designer. It's only recently that I feel I have achieved some sort of mastery over them. Understanding how they work has been revelatory. As a baker who uses the sole of the oven to bake on, I have learned how heat flows into dough. As an oven user and designer, I have learned how to get the best from a fire.

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Designing ovens for others has shown me that different heat dynamics can be employed for different applications. Some bakers want their oven to heat up quickly for occasional use; others want it to hold heat for long periods of baking, day in and day out. Some bakeries have access to certain kinds of fuel and not others. Some bakeries require the ability to fluctuate temperature for different products, while others prefer a steady heat. Some require more heat than others - for example a pizza place - and this needs a different set of design tricks. All these things play into how an oven is designed.

So that’s my brain dump. I welcome input from those further down the track than me; there are still so many mysteries to solve. Even though I feel as though I have a degree of understanding, I’m also prepared to be educated!