Alan Scott Hearth Slab Materiel (Help!)

[MattDamon4Life]

Alright Folks! I've just about lost my mind... trying to source a concrete for the hearth slab of my bread oven. I'm pretty much following the plans exactly, except that I plan on making the slab and dome a little thicker for longer heat retention.

In the slightly dated book, they recommend using a Calcium Aluminate cement (around 40%)- specifically something like Ciment Fondu (http://www.kerneos.com/content/en/Ou.../CIMENT-FONDU/). However, this company does not sell small quantities and according to them, there are no retailers that carry it.

Kerneos pointed me toward Harbison Walker- they carry a sack mix castable refractory concrete specifically for bread and pizza ovens called Ovenzz castable. The only problem is that It costs $38 for a 55lb bag.. putting the slab alone around $500..

Who has built this oven? Did you open to build the hearth slab with just Portland? Did you buy a refractory? Is there some sort of cheaper Portland based high heat concrete that I should be looking at? How much did you spend? Has your slab already split in two? Help me out! Please

I'm based in South Jersey, close to Philadelphia.



-Charlie

[Fizz]

I think the trick is to use insulation btw the floor bricks and the slab... There are different ways of doing this, from low tech vermiculit concrete mix to high tech calcium silicate (spelling?) boards... Insulating fire bricks is another way of separating the fire brick floor and concrete slab...

[RandyJ]

I have not seen the plans for the oven that you are planning to build but I do know that there are some flaws in its design. I do not believe there is much if any insulation between the cooking floor and support slab. You need it there to maintain temperature. One question is how much bread are you planning to make? Several loads a day or several a week? I just have a standard highly insulated Pompeii oven and after pizza I can easily do bread the next day probably several loads and then roast a chicken the next. If you insulate a lot it makes up for some mass. What ever you do make sure you put plenty of insulation below the oven you can losses a lot of heat there.

Randy

[MattDamon4Life]

The actual oven is suspended from the foundation with rebar. There is and insulating Slab of vemeculite and Portland, and then a Hearth Slab through which the rebar runs. And on top of this open faced sandwich there are the firebrick. So there is no heat loss, this Hearth slab is part of the actually mass that is retaining heat! I'm just trying to find a material to use for this hearrh sllab that will be able to handle the thermocycling- from what I'm told, portland won't last too long! And
Refractory is very expensive

[JRPizza]

I've got a copy of "The Bread Builders" and it helped me learn how to manage my starter, and originally had planned on using the design they share, but based on how happy folks were with their pompeii ovens convinced me to go a different way. I believe Scott's plan has about a 3.5" slab in direct contact with firebrick laid on edge giving a thermal floor thickness of around 7.5". There is no doubt that once saturated the floor will allow sustained baking, but it seems like it will take quite a bit of time and wood to heat soak all that mass. I have seen recommendations to use brick on edge if multiple loads of bread are desired, but not too many where heating a large slab below the bricks is recommended. If that is the way you decide to go we all wish you good luck and hope you post your progress here so we can follow along and maybe learn from your experiences.

[Fizz]

Ok, then I understand the design a little bit better...

I'm no engineer, but have studied many of the oven designs available online and on YouTube, and I think that you have to consider what you will use the oven for.

Is it bread baking on a commercial scale, go for a larger mass oven, since you will keep it warm 24/7 more or less.

For any kind of home usage I think the general principle of fire bricks on top of insulation is just fine... If there's pizza involved in the plan, then a larger mass means more wood consumed, and more time waiting for the oven to reach the higher temperatures required...

[RandyJ]

I guess that if you were going for a small scale bakery then a better insulated version of that oven might be a good idea. If you are doing for yourself and family or friends then probably overkill. It would be very hard to heat to pizza Temps and keep it there..

I might be tempted to stand the floor bricks on their sides for a 4.5"floor and then do a render of a inch or two on a Pompeii and highly insulate it. At least 4" of calsil under and at least 3 " of blanket over the top. More insulation is always better to a point. I am sure you could do this as a 42" and bake almost commercial amounts of bread.

Randy

[Tscarborough]

Unless you are going to be baking massive amounts of bread, I do not think you will use an Allen Scott design oven very often. It is truly not that good a design, even for bread, and for the casual user it is a disaster.

[MattDamon4Life]

I appreciate everyone's input! But I'm set on this design, I'm looking to cook a lot of sourdough bread. I have no questions on mass, design, etc! I'm just trying to source the proper material. The hearth slab (sandwiched between an insulating layer and the fire brick hearth)- any recommendations of a high heat/thermocycling resistant concrete? Refractory is the right material for the job, but it's very expensive.. so maybe Portland and fire clay?

[4Shelby]

I have built an Alan Scott oven from the Bread Builders book. The book is missing many design issue's that took me months of searching the internet for answers. This was a few years back but let me summarize how I built mine: (NOTE: Some of my measurements for thickness may be off. If the book has measurements, please use them)
- The hearth is suspended (as noted above) by rebar. I took lots of pictures to document my build.
- Bolt 2x4 to the side walls to support the hearth while wet. The 2x4 are bolted 3in from the top of the block wall. I separated the base into two sections; a front section (firewood storage) and a back section (not used. I built a concrete block wall in the middle to help with hearth support (pic1)
- Next I placed sheets of durarock on top of the supports to form a bottom of the hearth. I did not attach the durarock.
- Next I built a 5x4 matrix of double 3/8" rebar. I could not locate 1/2 or larger in my area so I doubled slightly smaller rebar and tied the pieces together with wire (pic 2)
- Then i added some additional supports underneath the hearth to support the weight of the concrete mix going in next.
- Add a border of 1x6" pine notching sections for the rebar. NOTE: Its important this outer frame rest on the duraroock and goes to the concrete block walls. Once dry, this wood frame is removed but leaves a 5/8" gap between the hearth and the block wall. This gap is needed for thermal expansion which keeps the hearth from cracking or buckling. (pic 3)
- If you want an ash drop, you need to make sure to frame a spot for it before the concrete pour. While I never used it as an ash drop, it was a big help for pulling cool air from underneath the oven to feed the fire in the hearth. (pic 4)
- The pour: I scribbed a line 1" from the top of the block walls and poured a mix of vermiculite, portland cement, sand up to this mark. About 2".
- Next I poured portland cement, fireclay and silica layer all the way to the top of the 1x6 frame. Make sure the finish here is very smooth and very level. This forms the base of the hearth that will support the fire brick. (pic 5)
- After a couple of days, remove the 1x6 frame and extra support from under the hearth. You now have a solid, thick thermal mass which is floating suspended by the rebar.
- I used firebrick for my hearth. It worked extremely well but must be installed perfectly flat and level!!
- Layer a small base of paver sand on top of the concrete hearth. Take your time on this step as its critical in getting the firebricks to be perfectly flat and level. Pound each brick in with a rubber hammer checking level and flat in all directions. Firebrick is installed on its edge. (pic 6)
- NOTE: If you end up with an edge of firebrick sticking up, it will catch your pizza peel and make it more difficult to get pizza off the hearth!

I hope this helps!!!
~Ray