sand under firebricks?

[Poetryart]

We're pouring cement for the hearth this afternoon on top of concrete backer board, I've ordered FB board from Forno which should be here soon. I'm seeing different options of putting a thin layer of sand between the FB board and the firebricks versus using a sticky mortar. Which is best? I'd like to be able to remove damaged bricks over time and think the mortar wouldn't allow me to do that. We're planning an igloo with 2 layers of FB Blanket and concrete. Do I need to use refractory concrete on this? or will regular work?
Will post photos if I can figure out how...
Thanks - I'm really excited about this and am so grateful to the Forno website!

[christo]

I put dried fire clay under mine... wow 4 years ago. And the bricks are still quite flat. It seemed to take up for the irregularity in the fire brick. I was lucky, my bricks were quite uniform to begin with.

[Poetryart]

This may sound nuts, but... I am a sculptor and have lots of bags of clay around, including some that is too dry to work with. Traditional oven says I can use that, let it dry and crush to a powder to be my fire clay. Do you see any problem with that? It sounds so logical to me, I guess that's what makes it sound nuts. I really like your oven, cristo, good job. Now that you've had it 4years, how often would you say you use it? I'm curious if the novelty will wear off, or if it becomes even more of an obsession over time! My husband and I are already drooling over the possibilities...

[cobblerdave]

Hi
Use the fireclay.... its fired and will not soak up moisture and return to soft clay. It still has the properties of clay which allows slip betwwen the particles and helps in the expantion/contraction during heating/cooling of a WFO. I have just found this out and am thankfully that I did use fireclay to set my hearth bricks and as a component of the home brew fireproof cement.

Regards Dave

[Les]

Some clay can fire at low temps (that would be a bad thing). I am sure someone here has measured the temps below the hearth bricks. I know the dome will go off the chart. What temp is recommended to fire your clay?

[david s]

Originally Posted by cobblerdave

Hi
Use the fireclay.... its fired and will not soak up moisture and return to soft clay. It still has the properties of clay which allows slip betwwen the particles and helps in the expantion/contraction during heating/cooling of a WFO. I have just found this out and am thankfully that I did use fireclay to set my hearth bricks and as a component of the home brew fireproof cement.

Regards Dave

Dave,
I think you are confusing fireclay with grog or chamotte, which is crushed and fired clay. Fireclay (unfired) will certainly turn back into mud if it gets wet enough.
Any unfired clay will not become permanent until it reaches the quartz inversion stage (573 C) too high for a WFO.
Dave

[Les]

David,

I may be off base but 573c is only 1063 f. If I am cooking at 750 - 800 f after I let it cool down, are you sure we don't hit that temp? My laser only goes to a few clicks below 1000 but it pegs every time I fire the oven. If I recall, I thought someone fired some clay a few years back in their oven. I only bring it up because sitting your brick in glass has got to be worse than mortar.

[david s]

There's no way you will get near melting glass. That occurs at around 900C depending on the glass type, unless you throw in some small beer bottles some of which I'm told are partly plastic. They will melt in a fire. The surface of the oven crown may go over 573 C but not the rest of the oven.
I have fired clay many times in a campfire, directly in the coals. That is sufficient to fire the clay. The tricky part is to increase the temp slow enough not to crack the pots. Sometimes there are portions of the pots that did not get hot enough and you can wash the clay away there.

[Poetryart]

My clay is a low fire terra cotta, so it won't work, I fire it at about cone 06. Does anyone ever just set the firebricks directly on the FB board? Can I put a layer of vermiculite down? Regular sand is probably not a good idea, as one really good fire could melt it into glass? I've melted bottles in my kiln before at 06.
I really appreciate the advice - I would have never attempted building a WFO before this website. If I get the refractory/ fireplace mortar I won't need the homebrew? Is that correct?

[david s]

Usually the mix for under the floor is a 50/50 sand, fireclay. This is only required to get the bricks level. Place them directly on the insulating board if you want. Do not be concerned about sand melting, it won't. It needs the presence of fluxes to lower it's melting point anyway and you are not adding any to the mix.