In the two-odd months since we last checked in with an oven update, we have been learning how to use our beautiful new oven. It is most definitely a learning-by-doing kind of process; there is no manual for this sort of thing. I had some previous experience with other so-called “black” ovens in Toronto and a clay oven we built last year, but this one is bigger, and way, way more massive.
At its basic level, a black oven couldn’t be simpler to operate. There is a single chamber that holds both the fire and whatever you wish to bake. You build a fire first and heat up the masonry walls, floor and ceiling of your chamber, like charging a battery. Then you remove the coals and put in your bread; it is the heat stored in the masonry “battery” that bakes the bread not the fire itself. In other words: fire heats masonry, masonry bakes bread.
In practice, building a hot fire is challenging. It takes a lot of labor to cut and haul firewood, so the more heat you can coax out of a given amount of wood, the better. The first thing our oven taught us was that we needed to use top quality firewood; seasoned, split, and stacked oak or locust stored in a shed. As it happens we had a handy abandoned goat shed not too far from the oven.
Apart from wood, the other ingredient fire needs is oxygen, and this is a function of air supply, or draft, how air moves through and around the physical structure of the fire, and the outlet, or flue. I find it helpful to imagine the air flowing through the oven like water in a brook–you want a well-contained and directed swift and smooth flow with no dams, stagnant pools or disruptive cross-flows. All parts of the fire should have access to air, so I try to think about the bottom and back and sides of the fire–all the parts that I can’t see.
This oven draws its air supply through the ash drop, which is a gap in the masonry in the front of the oven that opens into a cavity below the oven where ash is stored. There is an access door on the side of the oven that I crack to allow fresh air to be drawn in by the fire. I have discovered that removing the door completely is counter-productive, especially on windy days, when the wind blows up into the oven and scrambles the flow of air. The flue (chimney) is opened with a simple handle. It has two positions–open and closed, so there’s not much nuance there.
Where I have the most control is how I build the fire itself. In the morning, I load the oven with a single layer of wood, trying to loosely cover the whole hearth. I close up the oven to let this wood heat up for an hour or two, and so that any residual moisture from the night steams off. I then put two brush bundles on top of the wood and stuff some paper underneath them. One match lights both pieces of paper, which lights the brush bundles, which lights the wood. In a minute or two, there is a hot fire burning across the front of the oven.
I close the blast doors, which protect the fire from wind while still allowing smoke to escape up the chimney, and which will eventually keep most of the heat of the fire out of the bakery, and wait. It is so tempting to mess with the fire, but (a) you’re doing more work, and (b) the doors are open and you’re poking at it so your fire isn’t establishing a smooth draft. In an hour or two, I come back and find that the fire has burned itself part-way back into the oven, and it is burning cooler–you can tell by the smoke. I want it to burn hot and clean, and to continue moving back, so I push the fire back to the middle of the oven to consolidate the heat. I move the fire back one more time to get it in the back of the oven. I still don’t put any more wood on because I know that there is fresh wood behind the fire. At this point, after the fire has been burning for 3-4 hours, I see the temperature of the firebrick start to budge.
Temperature is measured in the middle of the oven by six thermocouples embedded at three depths in the firebrick hearth and in the vault, or ceiling. When there is fire in the oven, the thermocouples closest to the surface are hottest, while the deeper ones–the ones that tell me that the battery is heating up–take longer to clue in to the fact that there is a blazing hot fire in the oven. This property of being slow to take on heat (and therefore release it) is one of the things people mean when they talk about thermal mass. We built a massive oven because we wanted it to release heat slowly, so that I could eventually bake all day, but the cost of that is that it heats up just as slowly.
Back to the fire, which we left burning away in the back of the oven. By the time the base layers of masonry start heating up, I start adding more wood. Rather than shoving the wood back to the fire, I put it up front and walk away. In a few minutes, I know it will start burning, bringing the fire back up to the front of the oven again. In an hour, I move those coals back and add more wood to the front. Because moving fire around is hot work, I try to get the fire to move itself around the hearth. When I first started firing this oven, I kept the fire in the back of the oven for most of the firing, but in this new oven that resulted in over-baked bread in the back and under-baked in the front; the oven’s way of reminding me that it has a front-to-back dimension that also needs attention if I want even heat, which I definitely do.
This back-to-front-to-back dance is repeated until the base layers surpass 450 deg F, usually 1-2 more rounds. At this point, I spread the coals and remaining wood over the front 2/3 of the hearth to distribute the remaining heat. I have found that by the time it has burned down to ash, the deepest bricks will be between 550 and 600 deg F. Before I go to bed, I close up the oven, putting in insulated “plug” doors that block the flue and ash drop, so that the oven heat can equilibrate. As I sleep, the heat flows through the masonry into cool areas so that by the next morning, if I did a good job, all thermocouples register around 575 deg F–the ideal temperature to start baking bread.