The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

Firebrick vs. Refractory Cement

MNBäcker's picture

Firebrick vs. Refractory Cement

Firebrick vs. Refractory Cement...?


So, here's a question for all you experts out there: I just came back from my oven building class and started thinking about the following: when building the oven, one would definitely want to use firebrick for the hearth surface, but what about the inner dome? Is firebrick really necessary, or could the whole inner dome be constructed from refractory cement? I have heard about companies using large inflated tubes to build bridges, and I'm wondering if I could have a solid "dome" made from plastic, set it on top of my firebrick hearth and then pour refractory cement arond and over it to form the inner dome? Would the characteristics of the refractory cement be similar enough to the firebrick to make this product feasible? Or would there be concerns about cracking in the cement over time? So, inflate dome, pour 6 or so inches of cement around it, let harden, deflate dome and build chimney as usual?

Any input will be greatly appreciated.



ClimbHi's picture

One of the things that brick has going for it is it's natural ability to move with heating cycles due to the joints. Not so much with solid cement (well, technically concrete made with refractory cement). My construction experience has taught me that there are only two kinds of cement. 1) Cement that has cracked; and 2) Cement that is about to crack.

I also note that industrial furnaces for glass & ceramics are generally made of brick rather than solid.

That said, the kits that are available are generally made of castable refractory, (but again, they have joints built into the design) and mud ovens seem to work just fine. However, the straw in the mix may have something to do with that since it may allow it to expand/contract without breaking up.

Me, I wouldn't try -- but only because it's a lot of work & expense for something that's not proven. But if you do it, let us know how it works out! ;-)

Pittsburgh, PA

polo's picture

.......but there are several who have either built or purchased WFO's that were made from castable refractory. Forno Bravo sells several styles of pre-cast ovens, they battle the expansion problem by making the dome in sections. I'm sure that making the form and casting the dome would be challenging if you had never done that sort of thing before. I would try to find someone who has experience in using castable refractory to help guide you on your way.

A company called ANH would be the place to contact regarding the castable refractory material.

I would have to say though, that I am in agreement with Climbhi. We have several pusher style preheats where I work. While these preheats incorporate both brick work and castable refractory, it is the castable areas which need to be repaired most often.

mvilhauer's picture

I have constructed 2 ovens using bricks for the hearth and refractory cement for the dome.  I built a standard oven up to the hearth using conventional firebricks as the hearth.  Onto that, I built the interior mold.  I used construction sand (it's cheap and easy to use) to build a "sand castle" in the shape of the inside of the oven.  I covered the sand with plastic wrap to keep the interior of the oven smooth.  On top of the "sand castle" I formed, by hand, the refractory cement dome.  The cement was mixed in a wheelbarrow and then hand scooped onto the form.  The cement was allowed to cure for 7 days while being lightly coated with water to slow the curing process.  After 7 days, I scooped out the sand, leaving the concrete dome standing.

I built a small fire inside the dome, increasing its size over a couple days.  On the 3rd day, I built a large fire inside for final firing.  That oven was used every weekend for 2 years before I moved away.  The internal cement structure did crack during expansion.  The cracks were very small and did not seem to effect the strength of the dome (it really only supports itself, not a bridge or building, so strength isn't much of a concern).

So, I would suggest that this method of "casting" an oven works and works well.  I was very happy with the results and have built another oven following the same proceedure.  It too has cracked, but seems to be holding up well after 5 years.  This method isn't cheaper than bricks, but it does come together more quickly as there is no creative cutting of firebricks.

dlwilbanks's picture

What was the approx size of your dome? Height to center,interior floor diameter? Approx thickness of dome?



jeads's picture

I am new around here and it is old enough that it might not be read, but here we go:

I have also built a few brick and refractory domes for a pizza business i have.  My first large one had a brick floor and a cast dome.  I built the form with a pile of lumber (lighter than sand) and then piled wet sand around the lumber.  After casting, i let it cured for 5 days or so before removing the form.  I cast 4" of dense cast, 4" of castable insulation and 2" of smooth coat.  I got one nasty crack ( from shrinkage around an immobile form) the thickness of a US nickle.  it ran through the entire 10" thickness and  it was about 4 inches long.  I freaked out at the time, but decided to fire it up.  That was 2 years ago.  I fire to over 900deg F for 16 hours a day, 6 days a week and have had no issues that i can tell.  It has other hairline cracks as well, but no problems.

The next one i built was with brick and i have to say that while the brick seemed more "natural" or "authentic" , it took forever.  The cast oven took about 3 hours to build the form, and 4 hours to mix (with a machine) and put on 8-50# bags of castable.  then another 2 hours to do the insulation and another 2 for the smooth coat.  So 12 hours of work over 2 weekends.  The brick took me a solid week of work, a cut on almost every brick to fit, and enough math and geometry to make my head have hairline cracks.  

So i would say for the first-timer- stick to castable.  find a good dealer in your area and work with them about how to mix it and make it work best.  

BTW my ovens are both about 55" deck width, 20" dome height. more or less.



brpw's picture

Hey Jeads,

I have been baking in a clay oven, 36 x 48 inches on the hearth that I built about seven years ago.  We mainly use it for a small home bakery business.  The oven is showing enough wear I am considering a rebuild on the same foundation and very interested in using refractory this time.  Have experience with internal form of sand so would probably go that rout.  

Couple of questions, during lay up, workable time at about 70 degrees for the product I have been looking at, is advertised at around 45 minutes, talked to the rep for OvenZZ refractory, and he said it is probably more like hour to hour and a half, when laying up your oven did you trowel on or is the mix thick enough to apply in more like handfuls compressing as you go around the oven, either way did you have any issues with drying time.  

What type machine cement or mortar mixer,  about how dry, again trowelable  or hand lay up.  I am thinking for a bread oven maybe adding a couple of inches to the thickness of the of your four inches of thermal mass. What did you use between the refractory and the form, newspaper, plastic etc?

What type insulation and smooth coat did you use.

You said the oven has some hairline cracks, any spalling issues

Thanks BRPW

jeads's picture


We used a product from Harbison Walker called Mizzou Plus Dense Castable.

I believe the work time was only 20 minutes, but honestly that was more than enough.  One person mixing, one or two slopping it on.

We used wet newspaper over the sand form and that worked great.

We used a mortar mixer. And followed the instructions exactly. It resulted in a mix that was dry enough to pick up handfuls and slop it onto the form. There was little to no slump.  And having a working time of only 20 minutes means that the next layer would not slump under the weight of the layers above it. I think we used bamboo skewers to make sure that it was the correct thickness.  you can talk to your supplier, but 4" is sort of overkill, so 6" would be a waste of time and material, in my opinion.  Insulating it well would be my recommendation.  We used 4" of castolite, another castable product, but i would recommend 4" of fiber instead.  That stuff is toxic, so look for the Super Wool brand.  It is considered biosoluable so apparently does not stick around in your lungs, if it somehow gets around your mask.  (you are wearing a mask when doing this work, i hope.)

If you use blanket, do your curing of the castable before you put on the smooth coat (we used 2" of regular mortar mix ) so that the moisture can leave the castable out the top without cracking the smooth coat.

Is your floor well insulated?  We used 4" of soft brick which important to hold the heat in. 

I originally quoted using 8 bags, but i dont think was right.  I think we used way more than that, but you will have to check with your supplier.

One final note: I would have pulled the form out earlier to keep it from cracking from shrinkage.  Check with your supplier again, but 1-2 days should be plenty.  as noted below, you could add SS pins to help with the cracking as well.


Good Luck!



michael_carlin's picture

I've built a couple of clay ovens but aI'm about to build a refractory cement dome and was wondering what recipe you used for your mix? Any other cartable recipe suggestions gratefully recieved





Vdonti's picture

I've been trying to figure out how to follow individual threads.  It seems I have to comment to follow this thread,  Hince, this comment.

sgregory's picture

I guess you can cut every brick, but in reality, you can use straght brick on a form, breaking the joints between each row.  Back fill with a mortor and construct the next row if required by design.  Brtick is more forgiving but not required.

If you are worried about cracking and spalling with castable, use SS needles in the mix.  (watch the hands).  this will allow it to crack but maintain basic integrity during the heating and cooling cycles.  ANH is anker hearth or vietche radix,  which iirc is RHI.  hey merge and change names daily it seems.  In both cases you need to secure the base of the arch for its primary strength.


mikku's picture

I am new to this forum--like today.  But I have a lot of interest in this exact topic.  This year, I built a dome shaped WFO using castable and was very happy with the results.  The finish is as good as the forms that you make.  Now after seeing sgregory semi professional WFO thread, I want to build another oven as soon as I have the time.  But, I want to cast the entire oven and not just the dome.

I live in Japan and the dealer that I buy refractory brick or castable cement sells both at practically the same price.  If you determine the volume of a single brick and the volume price and then determine the volume of a 25kg bag of refractory cement, they are almost identical.  I have no qualms about the refractory cements ability to carry weights.  I am considering lining the cast oven with a firebrick floor and walls, all loose set.  If I desire additional mass to the dome, I think that additional layers of standard brick can be laid loose in additional layers over the cast dome.  The dome can be cast with an inner arch but the casting outside can be level.  I am thinking about adding rebar, like Sgregory used in making the lintel in his brick oven. 

I think that this would work just great.  Again, the quality of the finish is only as good as the formwork going into making it.  But a lot of time can be put into the formwork to get a great finish... It does not require a lot of expensive tools to cut brick and make complicated joints.  Only basic hand tools and a little patience.

This is something that I plan to do this summer.  Any information concerning interior heights, ratios of dome height to door height, size of transition to flue and flue size would greatly be appreciated.  Also any information on oven door type.  I am thinking about one similar to a guilotine (don't know correct spelling) but one that would operate in a track moving vertically, with the assist of some counterweights.

Really glad to have been directed by another forum to this site. 

Hello to all, I hope to learn a lot from my experience here.