Brick or clay?

[mrtbenn]

Hello, I'm building an oven in my garden.  I can't decide whether to use clay and sand mixture or whether i should use fire bricks.  I know that clay will be cheaper, but I haven't been able to find a summary of the pros and cons of clay/cob vs brick ovens.  

Please help!

[oblertone]

Bricks are made of clay, so you'll be using clay whatever; most folk find using pre-formed lumps easier, but there is a certain allure in using the raw form. As to firebricks, they are expensive and can be fragile, so consider using recycled bricks for the dome at least.

[downunderdave]

Jul 11, 2016 9:59:48 GMT oblertone said:

Bricks are made of clay, so you'll be using clay whatever; most folk find using pre-formed lumps easier, but there is a certain allure in using the raw form. As to firebricks, they are expensive and can be fragile, so consider using recycled bricks for the dome at least.

Also a clay/sand mix (cob) is far weaker than a fired brick, so it is subject to damage and abrasion wear to a far greater extent than a brick or cast oven. The cob mix will get a little harder from the firing but it will never be sintered (rendered permanent) because that occurs at a much higher temperature than is achievable in a WFO.

[pizzafanwolves]

I went through the same dilemma myself recently and decided to go for a cob oven as I've got a local source of free clay, compared to expensive fire bricks, or trying to find suitable clay house bricks. Plus for me there is something a bit more awesome about building a pizza oven from locally sourced eco friendly materials.

As far as pros and cons. I believe that a cob oven will have more of a maintenance overhead as the cob mixture has a tendency to crack over time, although it can be fixed easily with adding more clay into the cracks. Also cob ovens need to be protected from the rain as the clay is porous and getting it wet is a bad idea. You can waterproof it with an final cover of waterproof render (more cost) but I understand that that can be prone to cracking too as the clay underneath expands when hot...but I've not fully investigated this yet...

The brick build is stronger, but will cost more and you will need to cut the bricks in half for the main roof structure, as well as buy special heat proof mortar which is a little pricey too.

I believe that a brick shell with fire blanket insulation and outside render will be the best at retaining heat (hence getting to high temperature quicker and then staying there) and will need little maintenance in the long term.

Don't be tempted to use cheap paving bricks as they are usually concrete which will crumble after being heated a couple of times.

Ultimately it's more of a cost / aesthetics / ease of sourcing materials balance.

[downunderdave]

There are a couple of other alternatives. You can make your own cheap castable using home brew which is 3:1:1:1 sand, Portland cement, lime, powdered clay. Also you can use clay brick pavers (not concrete ones) for the dome, but use firebricks for the floor. Some builders in the uk have used heat storage bricks with success, but they are a bit of an unknown quantity as they are designed for a different purpose, so while some may be successful others may fail.

[mrtbenn]

Thanks all, I've decided to go with clay/cob for my first attempt. Will use fire bricks for the hearth and will build a roof i think, as my understanding from Kiko Denzer's great book is that if you try to waterproof a cob/clay oven then it traps moisture and can damage the oven. clay needs to breathe. building my base has been half the battle, so I guess i can always knock down the clay one when it reaches its life's end and build a brick one on the same base.