Oven floor not hot enough

[Wooden Piper]

I've made an oven using a exercise ball and vermiculite. The oven is 800mm diameter, internal height 515mm, internal height of door opening 325mm. The fire burns really well with temps well over 400 degrees C however the floor never seems to get hot enough to cook the dough as it should be. The floor is 25mm vermiculite board set into 50mm of vermiculite concrete. Any ideas would be gratefully absorbed????

[fox]

Yes it is quite simple, you need a high mass floor to hold the heat generated by the fire.
The most Successful pizza ovens are made from high mass material like fire brick or refractory cement and then the outer side of the shell is covered in insulating material like vermiculite.
So your oven ‘will’ get hot inside but ‘not’ hold any heat for length of time.
However if you line the bottom with fire bricks then you will at least get some success …..

[UtahBeehiver]

In addition to Fox's comment, it is also possible that the vcrete is still wet as well. What is the was the ratio of the vcrte? 50mm is roughing equal to about 1" of CaSi if ratio is 5 to 1. A min of 2" of CaSi is the recommended min. floor insulation thickness (or abt 4" vcrete at 5 to 1). Just to be certain, the oven floor is vermiculite board??

[Wooden Piper]

Thanks guys. the oven is dry, it's been made for about 4 months. The floor is 25mm vermiculite board on top of 30mm vermiculite crete

[david s]

Sounds to me like inadequate floor insulation. 25mm is minimal. Is it possible to remove the floor bricks and add another 25 mm layer?

[NCMan]

I'm still a bit confused what the oven is actually made of and just how much insulation there is over and under it. Also, the OP says the insulation was "set into" the vermiculite. Photos and/or a better description would help. There are lots of very knowledgeable and helpful builders here that I'm sure can help, if all was known.

[MarkJerling]

Thanks guys. the oven is dry, it's been made for about 4 months. The floor is 25mm vermiculite board on top of 30mm vermiculite crete

How thick are your floor bricks?

[Gulf]

Mark, from what I read, there is no high density floor in this build. Just vermiculite board set on top of vecrete.

[MarkJerling]

Mark, from what I read, there is no high density floor in this build. Just vermiculite board set on top of vecrete.

That's my understanding too Gulf, but I figured I'd ask just in case the OP is talking about what's under the bricks. I've never heard of a cooking floor of vermiculite board (only).
It would seem a relatively easy fix would be to add floor bricks if there's none.

[Pizzarotic]

Any ideas would be gratefully absorbed????

Buy the largest and thickest pizza stone you can find. Alternatively tile the floor with 1" fire brick tiles.

[cleverdick]

The European contingent seems quiet, as ever, so I thought I'd write something!

TBH I''m not familiar with vermiculite board. (Calcium silicate board, yes.) And I'm assuming you're cooking your pizzas, on this, in a metal pan or something? Way not to go, I'm afraid!

Your cooking floor, onto which you place the pizza directly, must be of firebrick or refractory concrete (as per certain kits you can buy).

In your case, having already built the dome, I'd suggest a 2" cold-pour of refractory concrete mix inside (obviously after sealing the edges and damming the doorway) to form a cooking floor. Then a very long, gentle cure after it's set. I've no idea where you'd buy the stuff; you could try Four Ephrem in Manosque - given that it's what they make their kits out of.

Photos, please!

[Sixto]

Adding another option off the top of my head, New pottery Kiln-shelves could make great cooking surfaces, they come in different shapes and sizes, sometimes are about 1" thick and are made of very dense firebrick material so they should hold more heat than vermiculite board which is essentially an isulator... A 25 cm kiln shelf won't hold as much heat as 60cm of firebrick, but it may improve things. (you don't want to get used ones 'cause you never know what glazes or oxides have been fired on those shelves before, and they could become volatile at pizza temps.)

[david s]

Yes that is an option. I’ve been down that road too. Unfortunately they are usually a more expensive option. Many kiln shelves these days are not solid and have holes in the centre in order to reduce thermal mass in an effort to save fuel consumption. This is the opposite to what we want in a WFO. The shelves are usually only 20mm thick, about half what is usually used for an oven floor.

[cleverdick]

I think the greater problem would be cutting the shelf to size (and sections) and getting it in there, flat.
A refractory concrete cake-pour really is the only solution I can think of for a dome that's already built.

[david s]

Kiln shelves can be cut with a wet saw and diamond blade. The problem with a castable refractory floor is twofold. Firstly, any large refractory section is more prone to cracking, particularly a one piece cast floor. Not that it is a huge problem as it’s just deciding itself where the joints should be. Secondly, castable refractory should really be kiln fired in a slow and controlled rise to achieve permanent vitrification. When only relying on oven temperatures this does not occur. The typical hairline cracks in cast domes are not a worry, but the higher and more uneven floor temperatures can lead to spalling. For this reason a vitrified firebrick (or kiln shelf) is a more durable solution.

Because you used a vermicrete dome, it will have minimal thermal mass and poor strength. Most builders of these types of ovens find they don't last too long. You are probably better to play with what you have while researching a new build here for a much improved version.