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Pizza Party

The goal of this build was to get a basic feel for double-decker ovens--how they perform and how they are built.  The design was done by Jon Santiago of Hearth + Timber, and informed by the work of John Fisher in Europe and Jeremiah Church/Turtlerock in the US.  Potential advantages include 1)increased capacity in the same footprint, 2) increased baking per unit of firewood used 3) diverse baking environments allowing different foods to be cooked simultaneously 4) the ability to cook on the top deck while firing the bottom deck allows for temperature management and continuous cooking (almost "white" oven).

The oven was designed for a very straightforward and fast build, with good results on both counts.  The width of the oven was laid out specifically to match a real-time layout of 12 #1 Arch bricks so that no ripping of tapered bricks would be necessary.  The length was likewise set so that only fulls and halfs were used along the length.  The oven was roughly close to 2' x 3' (inside hearth dimensions).  The vaults were identical in form, so that the same form could be used twice.  The lintels were cast onsite with castable refractory.  The oven core was finished in about 12 hours with a regular crew of 3-5 people (many coming and going).  

Oven drafted right away and had no trouble with spillage except in one circumstance, with both doors wide open and a large fire starting in the bottom deck. Once the fire and draft was well established there was almost no spillage, and even better when the bottom deck was closed up to a minimal air entry.  The top could be left open without spillage as long as the bottom deck opening was reduced as it would be with a firing door. All of this done with a chimney that went near horizontal for 6 ft before entering the masonry chimney.   oven was connected to a 7" chimney.  Height considerations prevented us from creating a smooth, tapered transition from top deck exit to chimney connection, a further disadvantage that makes low/no spillage more impressive.

The oven was fired very heavily to dry it out for the pizza party.  It was fairly easy to get the bottom deck clean of soot, but with fire only from the bottom deck, the top deck would only clean off in about the back 1/3 of its length.  readings with an IR gun showed that with temps in the 900-1100F range on bottom hearth/walls/vault, the top seemed to linger in a lower range, roughly (and on average) 500-800 front to back, although temps around the throat would approach the 900+ range.

Before the pizza party we fired both decks at once, which thankfully caused no problems with draft.  With dual firing we were able to more or less equalize the two decks, although the top deck hearth always lagged behind because it is almost 10" thick in some parts.

The pizza baking experience was a learning one!  We found that while the bottom deck was plenty hot for standard thin-crust pizza baking, it lacked a bit of "top heat" which you get when flames like across a dome in a standard pizza oven.  Even with a large fire in the bottom deck, the flames were immediately sucked into the throat and away from the pizzas.  This was a very subtle problem as the pizzas were still coming out beautifully, but the top broiling wasn't as easy as in a standard pizza oven.  We then switched to baking pizzas in the top deck with a massive fire in the bottom deck, so that large flames were licking into the back 1/3 of the top deck.  This was great for top-broiling, but the upper deck hearth was a little cool for 90-second pizzas.  The top hearth was so thick that it was sluggish to reach high temps and hard to recharge.

A couple design notes:  

1) My initial design included a "reduction lintel" on the top deck so that gases would be forced to duck under it to exit the oven.  It was identical to the other two lintels and would create a "bell" effect in the top deck.  We decided to scratch it for simplicity and time concerns in this workshop setting, though, and built in a central exit right into the vault.  As such gases were exiting at the highest point of the top vault. you could feel the gas flow with your hand and it was obviously a strong single current.  As such the front part of the top deck struggled to gain temps when only bottom firing was used, especially the front corners.  Also, the flames had less residency along the top vault and a less disperse flow path.  I would consider including this reduction lintel in the future, although it presents its own disadvantage in that it will result in a cool spot of its own directly below it, and thus reduces the usable hearth size anyway.  Another option that was considered would be to exit through two square holes built right into the vault at the front corners of the top vault.  Each of these designs requires a more complex and larger throat complex on top of the oven.

2) The throat which vents the bottom deck into the top deck was built large out of caution.  It was roughly 5" by 16".  It was originally drawn around 4"x12".  While firing we reduced it in width by putting a couple splits over it, reducing the width down to as little as 7".  This resulted in a spectacular flame-show, with the flames created tandem spirals or "rams horns" as they follow the curve of the vault and careen downwards.    More importantly though, I think it creates more residency time for flames in the bottom deck, and a more vigorous flaming of the top deck.  I'm not sure that starting the oven would be as easy with a smaller throat, but depending on the chimney size and height it may not be an issue.  At any rate, I feel some sort of adjustable damper would be a nice addition so that the throat size could be "tuned" according the firing and cooking desires, as well as close off the bottom deck.  Max Edleson mentioned that pottery kilns use a removable kiln shelf or thin refractory tile that can be slid in and out of the brickwork.

3) I would consider building the bottom vault with arch bricks cut into "soaps", so that they have a vertical thickness of 2 1/8".  Once this vault is leveled off and the top hearth in place, it would have a more manageable thickness with regards to heating up.

4)Another avenue to explore would be having the gases exit each deck, or at least the bottom deck, through a long, thin throat that is running lengthwise along one side of the oven.  This would allow the baker to place the fire on the opposite side and thus get flames licking across the full width of the vault during live-fire cooking, potentially increased the "broiling" capabilities.  It might also make fire management easier for pizza baking.

Ultimately I think a double deck oven has great potential for someone who is willing to learn it and able to take advantage of its versatility.  It is probably not ideal for a "purist" baking either pizza or bread, but for someone with limited space, a diverse menu, or a culinary-pyromania complex, it could be a fantastic baking machine.

Jon Santiago