Clay Pizza Oven - Part 4: The Hearth

The hearth is the floor of the oven - it is the surface on which the fire will reside and on which pizza and bread will be cooked. It needs to be capable of soaking with high heat (500 degrees c or so), and it needs to be quite flat.

The two main choices tend to be fire brick or clay pavers. Clay pavers (but definitely NOT concrete pavers) are said to last OK, so I bought some second-hand through eBay. I'd much prefer to re-use existing materials, and clay pavers are way cheaper than fire bricks in this country.

Because the Hebel insulating layer was nice and flat, it was easy to lay the hearth pavers directly over the Hebel blocks without any sand in between.

In this oven design, there is a course of pavers sitting upright on their long edge (that is, upright but lengthwise) forming the bottom part of the dome. The dome sits on top of the hearth pavers, so the layout of the hearth pavers will roughly determine the size of the dome. This photo should help illustrate how I laid things out:

My hearth is basically 5 pavers long, by 7 wide and narrows down to 6 wide at the front tunnel. I cut a couple of the hearth pavers at angles so the corners wouldn't project out past the curves of the dome. The photo shows the layer of upright pavers as well; they have been mortared on using high-temperature mortar that can be purchased at places like BBQ shops - I only needed a 1kg tub.

In laying out the pavers and sizing the dome, remember that the dome will be covered in insulation when it's finished - you will have to allow for that thickness.

Clay Pizza Oven - Part 3: Floor Insulation

One of the most important aspects of this kind of oven is insulation. If you don't insulate the oven it will leak heat, so it will never heat up properly, you will burn immense amounts of wood, and it will become dangerously hot outside. A really well-insulated oven stays cool to the touch outside for hours, and heats up inside quickly.

There are two main kinds of insulation required:

• Insulation under the hearth, which is a flat bed of insulation that needs to also bear the weight of the hearth and dome.
• Insulation over the dome.

In this post we're talking about the under-hearth insulation. The main options for this are:

• Most commonly, a layer about 10cm thick of "vermicrete", which is vermiculite with some cement in around a 5:1 ratio by volume.
• A more high-tech option is a board of calcium silicate ("CalSil").
• Really low-tech options such as empty bottles embedded in sand or clay.
• AAC blocks, marketed in Australia as "Hebel".

I used the Hebel AAC blocks, and totally recommend them for these reasons:

• They are made from cement which is aerated; the final block is only about 20% cement (and 80% air).
• They are comparatively cheap - $4 each at Bunnings for 600mm x 200mm x 100mm blocks. Since 100mm is a good thickness for the insulation layer, I only needed $64 worth of blocks.
• So much quicker to lay than pouring a vermicrete slab - just stick them onto the FC sheet with adhesive, the whole job done in a few minutes.

Here's what they looked like when I put them down:

The Hebel just needs to be treaded with some care: until it's rendered or otherwise covered up, it is very fragile and can chip very, very easily.

Clay Pizza Oven - Part 2: The Base

Part 2 of the "How to Build a Clay Pizza Oven" series - I built the base from wood for several reasons:

• My original location was uneven ground, and it is much easier to make a wooden base with different leg lengths than to pour a new concrete slab.
• I had some large timber sleepers lying around, so it represented good re-use.
• Timber sleepers are comparatively cheap and can be picked up in the car.
• A timber base is quite light - important since my oven ended up on an upstairs deck!

Here's what the base looked like (before it was moved upstairs):

The external size of this is 1600mm long by 1200mm wide. This was originally to accommodate a slightly larger oven dome than I finally went with and also a planned 30cm tiled landing in the front; in retrospect I should really have reduced the base a little. Since I used Hebel blocks on it that were 600x200, a sensible minimum would have been perhaps 1200x1000.

The base is held together with long outdoor screws, and each leg also has a galvanised coach bolt for good measure. It needs to support almost 300kg of oven, so it needs to be very solid. Make sure you can walk around on top of it and not feel any movement at all.

The flat surface on top of which the subsequent layers will be built is simply fibrous cement sheeting (specifically here 6mm tile underlay). This kind of sheeting comes in 1800mm x 1200mm sheets and is easily cut using a "score and snap" technique:

I didn't actually attach the FC sheet to the timber base: the weight of the oven will hold it down, and this way there is less chance of cracking etc. by leaving the oven to "float" on the base.

The costs for this stage were very small; I think the FC sheet is about $20 and I needed about $25 worth of timber. There's probably another $20 worth of fasteners required too.

Clay Pizza Oven - Part 1: Background

I'm just finishing building my clay pizza oven (AKA cob oven); there's been a fair bit of interest in how it was done, so I'm going to try to document it on here. Here's a shot of my oven, almost complete, for starters:

The basic features are:

• Oven dome is made from a clay mix that is formed in place.
• Oven is well insulated, both on top and underneath.
• The enclosure allows the clay to breathe while being protected from the elements.

I based it on some instructions that were originally featured on Better Homes and Gardens. There is a pdf here, and a video here. If you're planning on building something like this, it's worth getting familiar with those resources. I made the following primary modifications:

• One major drawback of the Better Homes and Gardens oven was the location of the chimney inside of the door - meaning you cannot close off the oven for baking unless you set up a mechanism to close off the flue pipe too. I created an entry tunnel and located the flue pipe outside the door area.
• A couple of stands were proposed; I used timber (which was cheap, and allowed me to re-use something that was lying around). But whatever base you use, I highly recommend using the Hebel/AAC blocks under the hearth floor for insulation. The decomposed granite/sand combination doesn't seem to be a good idea.
• I insulated using a rockwool insulating blanket - more on this later, but it is a great way to insulate.

In subsequent posts I'll try to give some details on each step of the process, along with dimensions and an idea of the costs of things.

Cooking and Making Biochar

I recently made an EverythingNice Stove - see WorldStove for the free plans.

Ant the Potato King

The results are in, and Ant has won our potato grow-off by a mile!

Here he is in the photo with his "potato king" crown on, holding his most excellent crop of Kipfler potatoes.

These were the 'taters that we grew under mulch, on top of the old dead grass; now we've got some great Kipflers and an awesome garden bed to start growing lots of other veggies in.

Biochar in Central Africa

In a comment on my blog, Erich Knight pointed me to the work of "Biochar Fund", who are working in Cameroon with subsistence farmers (apparently trying to survive on under 50c a day).

It seems that they operate out of Belgium, and they are reporting some excellent results. It really does seem that the win/win/win cycle is very feasible - efficient stoves for cooking which also produce char which is used as a valuable agricultural input.

One piece of research that was linked from the Fund website is this trial here in Australia:

We actually harvested that plot this week and we've seen some amazing differences. Its a bit early to give you the exact data at this point in time, but we've seen probably double the biomass production and double the sweet corn yield where we've had high rates of biochar application in the soil. That's somewhere between 10 and 20 tonnes per hectare of biochar. And, yeah, we've had some very, very significant differences in corn production there.

That's really amazing - doubling the corn yield through the application of biochar!