Floor for cob oven

Hi, we are in the process of building an outdoor cob pizza oven. We are not planning to buy fire bricks for the base (hearth) of the oven but are going to use old fireplace red bricks. Recently in an article I read they suggest cob for the floor. That would be without straw in it. I guess it would be easy to repair, seamless, and smooth but we wouldn't want sand in our pizza bases.
Has anyone tried this?
Thanks,
Graham

I can say that fire bricks are best.
Here is a link to a wood fired oven forum
wood fired bread ovens

I hope it works for you

Thanks, I'll have a look but we won't use fire bricks as they are too expensive.
Cheers,

Any chance of finding second hand ones?
Many do get lucky and find them.

This may help you
fire bricks for sale in NZ.

To answer you as honestly and helpfully as I can: If you were to go through all the time and effort -- which is considerable --to make a cob pizza oven, and not use firebricks for the base, I would question why you are building a pizza oven in the first place. It's not guaranteed to fail, but there is about a 97% chance that you will be unhappy with the outcome.  If you use firebricks as the base, there is a 3% chance you will be unhappy and a 97% chance you will be thrilled.  Personally, I cannot see any way in which I'd be happy with a cob floor in my pizza oven. Normal bricks will either explode or split into tiny slivers, leaving gaps in the base which will snag the peel.   The oven can get up to 1100 degrees F if you build it right and normal bricks simply cannot withstand that temp, especially if there is any stored water (which there almost certainly is unless you live in Death Valley.)

The only advice I can give if you insist on building an oven without a firebrick base is to get as large and thick a pizza stone as you can.  Which is more expensive than firebricks.

ETA:  It's not just the surface that is at issue.  Firebricks soak up heat and retain it.  So when you take out a pizza and put the next one on, you still get that blister from the stored heat.

Hi, thanks very much for your advice. I've looked at the fire bricks here in NZ and the closest to us are $48 each for a 230x115x40mm brick. No second hand bricks locally as yet.
The cob floor isn't actually our idea and have attached a photo of part of the article. I'm hoping to email the author if I can find their contact details to see if he has revised his opinion over the six years since the article came out. I'm mainly interested if anyone has actually tried a cob floor.
Cheers,

Hi Carol -

Bryan is a colleague and close mate of mine. DM if you're interested in getting in touch with him.

Carol Clark wrote:
i, thanks very much for your advice. I've looked at the fire bricks here in NZ and the closest to us are $48 each for a 230x115x40mm brick. No second hand bricks locally as yet,

Sorry you are in that situation.  In your shoes I would use normal red brick, dry it out thoroughly for a couple weeks in a solar oven or similar. Then dry fit the bricks on the hearth.  That way if any blow up you can replace them.  Good luck!

Maybe you could use a large sheet of steel?
One thick enough to not warp might be hard to source.
I'm assuming most refractory cement would be to pricey for you,  or I would suggest casting your own firebrick.

I find refractory cement to be expensive, best price I have found is about $1.00 a bag.
Because of that I'm experimenting with RapidSet Cement All.
It's a food safe calcium aluminate cement.
My last attempt had  very bad surface spawling after a trip through the oven, but the bulk of it held up fine,  so I attribute the problems to my finishing technique.
Fortunately I cast it in a sheet pan,  so if I can't fix the surface,  I will just flip it over and bake on the other side,  either with or without the pan itself.

How about you get a sheet pan, fill it with cob and let it set up.
When it's dry,  cover it with a stout preice of cardboard and holding that in place,  flip it over.
Using the cardboard, set it in the oven and slide the cardboard out from under it.
The steel bottom should be smooth and transfer heat well,  and the can is there to help the pan resist warping.
Even if it fails,  it's very cheap to try.

I'm in the same situation as you. I'd do a cob floor and put in a 400x400x20mm bluestone slab which will become your oven floor. No sandy residue. Bluestone generally heats well and is cheap to find in NZ at your local Bunnings or any paver shop. They tolerate high temperatures very well. If it cracks, just pull it out and put in a new one. Just make sure your opening is more than 40cm so you can fit it in. Otherwise you may have to cut it down to size with an angle grinder and a diamond cutting disc. It worked well for me.