Soapstone vs Firebrick ??

[SCChris]

It looks like the Soapstone and Firebrick weight is almost identical per volume. The thermal properties of Soapstone and Firebrick differ in two ways. First the Thermal Capacity of brick, not specifically firebrick, is about 85% of Soapstone. Secondly; Brick transfers heat slower than Soapstone, seemingly at 1/6 the speed. This higher transfer rate can be thought of like the R-Value rating of insulation. My guess is that what this means for the WFO and Pizza Stones is that the Soapstone will heat faster, cook pizza faster, even out temperatures within the oven faster and cool faster. I would guess that an oven with a Soapstone floor would benefit from more sub-floor insulation than a typical Firebrick floor oven.

Thermal Capacity

Brick 0.840 Cp J/(gK)
Soapstone 0.98

This information was found at; Wikipedia "Specific heat capacity of building materials" and Tulikivi.com "The characteristics of soapstone"

Thermal Conductivity

Brick between 0.98 and 1.13 W/m.K
SStone 6.4 W/mK
This information was found at; Traditionaloven.com "Firebricks – heavy dense fire clay bricks" and Tulikivi.com "The characteristics of soapstone"

Material in kg/cu.m

Brick, fire clay 2403
Soapstone talc 2400

Chris

[SCChris]

Re: Soapstone vs Firebrick ??

Other common materials:

Thermal Capacity

Iron 0.450
Aluminum 0.890
Water 4.181
Concrete 0.880


Thermal Conductivity - k - (W/mK

Calcium Silicate insulating Board at 600F 0.097
Iron 60
Aluminum 250
Water 0.58
Concrete 0.42 to 1.7
Gold 310
Silver 429


Chris

[scott123]

Re: Soapstone vs Firebrick ??

One other huge difference when comparing the two is durability. Soapstone is very soft and scratches easily. This pretty much rules out metal utensils. I'm not even certain soapstone could handle a typical brass brush.

Soapstone's superior conductivity and thermal indestructibility make it the Rolls Royce of home oven pizza stones (using wooden peels), but, for the potentially more abrasive environment of a WFO, at this point, there's a few logistical kinks to work out.

I'm presently building a WF soapstone oven, so I'm in the process of working these kinks out as we speak.

[SCChris]

Re: Soapstone vs Firebrick ??

Scott,
I think you'll find that the brush isn't a problem with your Soapstone. As it's been presented to me, the darker more homogeneous stone is what you want in high temp environments, like ovens. Soapstone is not a single mineral but a mixture of minerals and it varies greatly in hardness because of the Serpentine and Talc that are the greatest components of Soapstone. The reason you want the darker more homogeneous stone is because although the lighter component of Soapstone, the Talc, can handle the heat, the expansion difference between the Serpentine and the Talc will ultimately crack the softer Talc, and because of the hardness difference of the darker Serpentine I don't think you'll find that the brass brush is any problem at all. I also think one of the reasons we use wire brushes is the because of the texture and absorbency of the firebrick. Soapstone is much less prone to holding onto spills, ambers and ash.

I think that the heat conductivity of Soapstone makes some design changes to the typical WFO beneficial. My opinion is that the sub-floor insulation should be beefed up and that some sort of thermal break should also be designed in at the entry door area.

Additionally, the amount of thermal capacity, heat storage, that the Soapstone offers is of course related to the thickness of the stone installed. Soapstone is available in firebrick dimensions and in larger slabs but most commonly it’s available in 2 and 3cm thicknesses. These most common sizes provide approximately ? the amount of thermal storage of firebrick. My question is, if the 3cm material is installed rather than firebrick, how important is this heat storage difference? I haven’t mulled this through completely at this point.

I have to say that firebrick isn’t the ideal material for the entry area because of the inevitable staining that happens here. Stone is a much better match for the entry in my opinion. Granite or Soapstone or some other nonporous material seems to make better sense in the entry area.

Chris

[lwood]

Re: Soapstone vs Firebrick ??

Great bit of research on soapstone vs firebrick. I don't think you have to worry about stains on the inside of your oven. At the temps we are working the oven is self cleaning fire brick or soapstone. I think the smooth texture of the soapstone would be a nice working surface inside the oven. The thermal properties all seem to work in the soapstone's favor except the added insulation required (good observation). That is a big difference in thermal conductivity.....6X. That should really add to your oven spring. Watch out for charring on the bottom. You can always hold the pizza closer to the dome to finish it off.

[SCChris]

Re: Soapstone vs Firebrick ??

I had a stone cut to size to put in my gas range and can attest to the “oven spring” aspect. I replaced an aged Trader Joe’s pizza stone and cleared out some “jury rigged” porcelain tile. Most of this junk was meant to provide mass and buffer the internal oven temps, and they did. I have to say that the 3cm stone really changes the personality of this oven for the better. It seems like the oven has a quicker recovery to temp time and that seems logical given the greater T. mass and higher T. conductivity.

I wouldn’t want to say that a standard oven could compete with the WFO, these are very different creatures. What I will say is that today’s standard ovens have been diluted over time to be barely acceptable and the additional mass that this stone goes a long way to bringing my standard oven back in line.

As for the above numbers, it’s hard to divine exactly what they mean. How fast can the pizza dough or bread or whatever take up the additional heat transfer the Soapstone can provide? How much difference does the thermal capacity of 3cm SStone vs Firebrick really make in our WFOs? What exact material was measured and how does that relate to what we’re using?

Chris

[lwood]

Re: Soapstone vs Firebrick ??

I think those numbers are close enough for our mental gymnastics. The heat capacity of soapstone is 14% higher than firebrick. So you are able to store more energy (by weight) in soapstone than you are in firebrick. That's a good thing.

Were you planning to put firebrick under the soapstone then insulation? If installed that way you reduce the thermal conductivity to that of the firebrick. Cause the soapstone cannot loose heat, to the outside, faster than the firebrick. The firebrick is a heat stop of sorts. I think you get the best of both worlds with that configuration.

[SCChris]

Re: Soapstone vs Firebrick ??

At this point I have SStone in the entry area of the WFO. I really like the non-stick and stainless properties of the material. I'm not so wild about the blue cast. This is really an aesthetic rather than functional thing. If the area had some complementary color in the area, I'd be fine with it, but my area colors really run toward the reds and browns. I have felt since I put the SStone in that the efficiency of the oven was somehow less than the measurements I had early on, but I couldn't, didn't have the measurements.

I'll swap the entry for the same Granite I'm using on the surrounding work surfaces. I would have gone directly to Granite, but I didn't have a good feel for the ability of Granite to hold up in the entry. At this point I feel like, what do I have to lose?

In the oven, if I go to SStone, this is not a given, I'd add an inch of rigid Calcium Silicate insulation board and then put 3cm SStone directly on this. I need to think about the entry / oven transition. I don't think putting an exposed inch of insulation board is really the way to go and going back to Fbrick as a thermal break here could be the answer.


Chris

[lwood]

Re: Soapstone vs Firebrick ??

You should be fine with the granite entryway. Mine doesn't get that hot. It's kinda interesting, there is like a wall of heat as you put your hand inside the oven. It's so hot you can't put your bare hand in past it. That occurs, in my oven, right where the dome begins (not surprising). Out board of that heat wall you can easily put your hand (maybe <300F). That is where I will begin my granite entryway slab. It's actually a great place to keep things warm. The granite, is a good choice for that area and it comes in lots of colors.

[SCChris]

Re: Soapstone vs Firebrick ??

It's come to my attention that Ovencrafters doesn't recommend Soapstone for bread ovens because of the higher heat conductivity when compared to the standard Firebrick floors.

I can see how this additional conductivity can really help in a commercial pizza oven where you're moving pizzas as quick as you can build them. Even in our ovens this can be an advantage.

I don't know if Soapstone is a real disadvantage when we're baking bread in our Pompeii style ovens. I would expect that the crust would cook faster. Is it too fast? Can this be mitigated somehow? I bet it can be mitigated. Foil or ceramic tile or a pizza stone on top of the stone and under the bread loaf or whatever else might be all it takes to be just right.

I would also expect that the oven deck would heat faster and the overall temps would even out faster. This would seem to be an advantage.

When I have baked on the Soapstone "pizza stone" in my general "inside" oven I have adjusted the baking by not running the oven to 550f and turning it down to 450f after putting the bread in to bake but just running it to 450f and leaving it there. The additional mass and conductivity of the stone seemed to make this a prudent change and it turned out being just right.

I think the idea of running the oven to 550 and backing it down goes along with the poor performance of standard ovens. Once the oven door is opened and by the time you get the bread in and steam going you lose a lot of heat.

I?ll be trying a 3cm 20 by 16 inch piece of stone in the oven to bake on and see what happens.


Chris

[mrchipster]

Re: Soapstone vs Firebrick ??

I found some 1/2 inch thick X 12 inch X 12 inch soapstone tiles and am considering putting this down on top of my 2.5 inch thick brick floor.

I have 2" of vermicrete under 2" FB insulation Board so I am well insulated in the floor area.

I have not built my arch yet and want to compensate the inner opening to compensate for the added thickness should I decided to add the soapstone.

Does anyone have any comments on using this approach?

Chip

[SCChris]

Re: Soapstone vs Firebrick ??

A thicker material will hold up better to the firewood in and out cycles. Also take note of the consistency of the product. The more homogenious the material the less stress fracturing due to dissimilar heat expansion you'll experience.

Chris

[GianniFocaccia]

Re: Soapstone vs Firebrick ??

Chip,

What originally attracted me to soapstone was the seamlessnessness of it plus I was able to acquire polished, clear(?) slabs for $10/sq'. I am also intrigued with the effectiveness of the stone as a heating element apart from purportedly being optimum for cooking pizzas and flatbreads.

My concerns with soapstone long term are its ability to withstand cracking as seen in photos of another oven here on the FB site. I don't know if cracking is a function of talc veins in the stone or something else. My gut feeling is that the thinner the stone is, the higher its propensity to crack sooner. I am also curious to find out if bricks or tiles like you have would resist cracking more than a whole slab would.

Depending on the cost of your tiles I think your configuration is ideal. First off, 1/2" is not going to alter your entryway-to-dome-height ratio significantly. Realistically, anything between 60-65% is workable, IMO, and if you want, you could design your door height a tad lower in the event you decide you want to take your tiles out. Kudos for thinking of this prior to constructing your oven. Second, your floor insulation is fine. Third, you have the ability to easily customize your floor for whatever you're cooking that session (especially bread) by simply placing the tiles wherever you need them. I think a 50/50 firebrick/soapstone floor would make for a very versatile oven.

If it was me, I would cut the tiles to fit 100% of the floor leaving a 3/16" gap on the perimeter for expansion, build out your oven and give it a go. If any of your tiles eventually need replacing, reach for a spare and drop it in. Much easier to replace than a 100lb slab like mine.

What you gotta try is placing a soapstone tile horizontally on top of a couple of firebricks standing tall in your oven. I'm guessing this would make for a custom pizza pedestal and mimick a low-dome pizza oven beautifully.

John

[SCChris]

Re: Soapstone vs Firebrick ??

John, I was hoping you'd chime in. To date aside from the massive commercial oven, I think you'll be the first with SS as a dedicated oven floor. Regarding the mentioned cracking of SS in another oven, I agree, without more information it's hard to know why. I have a SS stone in my Kitchen Gas Oven and although I see a couple of cracks the stone still has structural integrity and it's been cracked for many months. The crack I have is right along a small talc vein. I know it's only a matter of time before my oven stone gives way. It's really important to point out that this stone creates a bridge over the kitchen oven rack and it doesn't have any support except at the right and left edges where it contacts the rack. In an WFO with a nice flat floor I think the SS will work out. Even if a stone cracks because it's not going to move much, I don't think you're worse off than the supporting floor. I agree with Dmun "see below" 1/2 an inch is to light to work well and I don't know that you'd get all of the advantages of SS.

Chris

PS The 3cm slab that I have sitting in my kitchen oven has made a world of difference in functionality of this oven.. Recomended!

[dmun]

Re: Soapstone vs Firebrick ??

I'd be concerned about a half inch of anything as the cooking floor. Remember, you're going to be throwing logs in, and levering them around.