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cecilB 08-11-2009 03:50 PM

uneven hearth
Hi everyone,
We have laid the insulated hearth, but we lined the oven area with firebrick, to hold in heat longer for bread baking and stuff.
The problem is that when "we" laid the firebrick, it was not exactly even. I thought it could be resolved by using the fireclay as a leveler before adding the oven floor.
Can we just put another form around the hearth, with it about an inch higher than the hearth and pour the portland cement/vermiculite mixture over it and level it, over the firebrick?

Also, I tried to get high heat mortar today, which is what is called for for the making of the dome. Is that the same thing as the fireclay?
Are there brand names? They don't carry, but can get in, stuff in bags that you mix yourself. Otherwise the mortar they sell, which is for's in buckets of different sizes. But the lady helping me didn't really know, and I was sort of in a rush so I didn't think to ask for someone knowledgeable in the field (this is Capitol Concrete in Topeka, KS) - They do sell oven kits - I didn't think to ask for a brochure, either.

I'm the one with the 17year old boy doing this - he doesn't like to admit mistakes. But he does know that he made some and he tries to resolve them after pretending he wasn't wrong! heh heh heh.

Thanks for your help,


Gromit 08-11-2009 04:13 PM

Re: uneven hearth
Here in these parts, "fireclay" is what they call high-heat mortar in pre-mixed pails. BUT, that is not what you want; that stuff is quite water soluble. "Heatstop 50" is the brand name of drymix high-heat mortar you would want to ask for, or you could order RefMix from FornoBravo. If you are on budget restraints, don't be afraid of making the homebrew mortar mix described in the Pompeii plans. In that case, you will need real fireclay. I got something called Missouri fireclay from a local potter's supply; I would imagine that is something you could come by easy in KS.

You don't want the concrete/vermiculite as your oven floor--you need a much harder surface like firebrick. If your floor is THAT uneven, can you just pry it up and try again? What did you set it with?

If you can post pictures of your progress so far, that will help folks around here give you pointers.

Happy building!

Les 08-11-2009 04:43 PM

Re: uneven hearth

Originally Posted by cecilB (Post 61563)

Also, I tried to get high heat mortar today, which is what is called for for the making of the dome. Is that the same thing as the fireclay?

No - file clay is just that, clay. It has no portland in it at all. You can get it wet, let it dry out, and it's pretty much the same as it was.

As gromit suggested, can you lift the bricks and reset them? Also, uneven seems to be a relative term. I took great pains to belt sand my hearth to get it "baby butt" smooth. That was probably not needed. And pic's would help here.


cecilB 08-11-2009 05:57 PM

Re: uneven hearth
Ok, The actual oven floor wasn't laid. It was the extra mass under the oven floor, instead of just the insulated hearth, which is just a layer of firebrick. Kinda like the picture on pg. 22, 3.1 in the plans. Only not THAT neat looking! So I was thinking about adding an inch thick layer of the insulated hearth over the extra thermal mass, and then laying the oven floor.

I would like to post pictures...unfortunately I am not at all computer saavy. I have had my kids taking the pictures and at least downloading them onto the computer. My son, the one actually doing the oven, has a much greater talent for computer stuff so why he isn't into the posting of pictures and creating a blog for me, I don't know!

But back to the subject...The stuff he tried to use today to lay the oven floor is called "Alsey air-set refractory mortar, medium duty". It came in a 15 bucket that looks like it would hold a gallon of liquid.
But he took the bricks up, rinsed it all away, and now we will put a small layer of insulated hearth on, and then lay the real oven floor.

I was thinking that the Asley stuff was the stuff we would also use to build the dome. Am I wrong?

I hope this isn't as confusing to you as it is to me!

Les 08-11-2009 06:22 PM

Re: uneven hearth

Originally Posted by cecilB (Post 61574)
It was the extra mass under the oven floor, instead of just the insulated hearth, which is just a layer of firebrick.

This may be a problem - you NEED insulation under the fire brick hearth. Fire brick is not your insulation. Tell your son to put down the pipe and read the PDF again. ;) In regard to posting photos, it's quite simple. When you post just pick the paper clip icon and go from there.

good luck with your build,


Dutchoven 08-11-2009 06:46 PM

Re: uneven hearth
Refractory mortar in buckets is usually very in not something you could trowel should see if they can get Heatstop in the bag as a dry then add water and reach the desired consistency...your brickyard should be able to get it...they use the bucket stuff in fireplaces nowadays because they can lay them up quickly and have verrrry thin mortar joints...but fireboxes also tend to have many more bricks laid flat
Good luck!

RTflorida 08-11-2009 09:22 PM

Re: uneven hearth
I'm with Les, there is some confusion here.
Lets start at the top:
The firebrick is your cooking hearth. Yes you can add more thermal mass directly under it. I don't see the need for home use; many, many members are able to bake plenty of bread without it. If you choose to add mass there are several options - a second layer of firebrick probably being the simplest, or turning the firebricks on their edge. Another option would be an island hearth which would be and additional slab under the firebrick hearth but ONLY covering the area directly under the hearth and dome - no need to add extra mass to the entire support slab, it will wick away too much heat and you will use too much wood.
UNDER these options will be your insulation - vermicrete, perlcrete, or FB boards (or equivelent refractory insulation boards).
UNDER the insulation will be your reinforced support slab, which bares all of the weight - you REALLY don't want to heat insulate well, above it.

I would reread the plans agains to fully understand (I'm certainly not the best at conveying things properly).
Again, I don't think the extra mass is necessary, hopefully some of our breadbakers who have built their ovens similar to the plans will pile on and agree. More mass means longer heat ups and more wood....unless you are baking dozens of loaves at a time, you won't need the mass.


RTflorida 08-11-2009 09:30 PM

Re: uneven hearth
Stay away from the wet air set mortars in a bucket...they usually are not rated for outdoor use (they don't even stand up well to high humidity) and are intended for very thin mortar joints. You need a dry refractory mortar (Heatstop 50, RefMix, or equivilent) or the homebrew made from fireclay.


RTflorida 08-11-2009 09:42 PM

Re: uneven hearth
I just checked, Capital Concrete is an authorized Rumford dealer......which means they either stock or can order Heatstop 50. The oven kits you saw are probably those from Superior Clay, another partner of Rumford.


cecilB 08-11-2009 09:54 PM

Re: uneven hearth
Ok, this is what we have from the ground up: the foundation slab, the stand, the support slab, the insulated hearth, with firebrick sunk in leaving about 1 to 1.5 inches of insulating (vermiculite/portland cement mix) cement between supporting slab and firebrick.

If we were to level the spots that need leveling - they are in the actual area of the extra thermal mass firebrick - should we use the vermiculite/portland cement mixture, or should we use the heatstop 50?

I do plan on baking dozens of loaves - I bake about 2 - 2.5 dozen in a day most days when I'm preparing for the market!

He promises to get me pictures tomorrow! Or maybe even tonight! woohoo!

I will call about the Heatstop 50.


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