How to make a refractory cement?

[Pizza eater]

Hi all 

 

I hope you don’t mind helping me gain some knowledge on how to approach a problem which arises as part of a home build pizza oven project?  I want to cast a flat, smooth, single piece floor for my oven, and it should be able to withstand pizza oven temps (up to around 600C /1100F). I want to avoid the use of firebricks, or anything which isn’t perfectly smooth and free from gaps, so would want to understand how to create a mortar or cement mix which I could cast into a mould sized around 20 inches x 23 inches and a couple of inches thick (unless it should be thicker?). This floor would sit on a vermiculite/cement mix plinth for added heat insulation. Since it will be the floor of an oven, with food in contact with it, it should not shed sand or dust when scraped by a pizza peel. 
 

Ideally I want something which, when set, looks like and has the properties of a pizza stone. I’ve heard that it might be as simple as mixing bentonite kitty litter, water and sand in the right ratios, but then other sources online recommend dry lime, cement, all sorts of variations, so I was hoping to tap into some of your experience and see if there’s a simple solution which will work well?  Thank you for any replies!

[Glenn]

What do they use commercially?

[Pizza eater]

Hi,

All I have been able to discover about the product I’m inspired to copy - I mean pay homage to - is that it uses “refractory stone” for its floor. That is basically pizza stone, same as the pizza stone you might put in your home oven to cook a pizza on.  So I am hoping to stumble upon a simple formula to let me mix a cement which will set into more or less the same stuff.

Here is a link to the product I would like to replicate, albeit at less than the $2,000 they charge for this one.

Remove commercial link per TOS

I am thinking a Pilates gym ball mould. A couple of layers of vermiculite/cement concrete to form the dome and entrance way, and then I will need to cast a suitable floor which is the bit which has me puzzled. 
 

I searched extensively, but finding a large enough pre-made pizza stone with the correct dimensions will be impossible. 

Edited May 4, 2020 by Mod30
Commercial link

[Steve Sells]

concrete tends to explode when it gets that hot

[ThomasPowers]

It is probably possible to make a home mix that will work.  It will almost certainly cost several times that of buying a commercial product due to economies of scale. (Most of the component materials will come in 80 pound sacks...)

If you want to experiment on your own I would suggest finding a "pizza oven building forum" and see what they are doing.  Out here in the SW we do a lot of Hornos and generally the fancy ones use firebrick and the traditional ones use adobe plastering or floor tiles (like quarry tiles) and live with the peel/floor issues.

[Latticino]

The pizza stones I've seen look a whole lot like mullite kiln shelves.  These are typically high fired in kilns well beyond the temperatures you would expect to reach in a typical pizza oven.  I would see if you could source one from a ceramics supply house that would work for you.

My concern is similar to Steve's.  Anything that you cast for the enclosure will have the potential to spall over time and leave refractory insulation material in your pizza.  Not really ideal...

[Pizza eater]

Thanks guys, it seems it’s likely more complicated than I had assumed. I’ll find a good diy pizza oven building forum and research it further. There are a few videos on YouTube, but they all seem to use kiln brick floors and presumably trap all kinds of dirt and ash in the gaps. But spalling is not going to be desirable, so maybe I do need to find some suitably sized tiles.  
 

I appreciate your advice!

1 hour ago, Steve Sells said:

concrete tends to explode when it gets that hot

Thanks - the main body of the oven would be a 4:1 ratio of vermiculite to cement, plus 1.5 to 2 parts water as needed to form a paste. This seems pretty widely used amongst diy pizza oven builds from what I could gather on youtube, so hopefully no exploding.

[ThomasPowers]

Do you have any ideas about if those are *GOOD* videos?   We continually get hammered by people watching a video on the 'tube that is really really bad---but it is very popular as a "cheap" way to build a gas forge.  Of course the person advises them to use a material as a refractory that starts to degrade around 1000 degF BELOW forging temps and isn't insulating and so any "savings" get eaten up by increased fuel costs *fast*.

People who don't know enough to look at it and say---"That's bogus!" and move on; fall for bad information.  Unfortunately popularity of a video is not necessarily a good judge of it's worth; neither are good production values as those indicate skill with a camera and not necessarily skill at what the camera is pointed at.

[Kozzy]

Do it once, do it right.  There are no savings in short-cuts.

Johnson forges do (or did) use vermiculite as an insulator--but as a layer below the actual hearth.  I've seen pizza ovens do similar where the hearth and canopy was done in the proper high temperature materials and there was an air gap that was filled with insulating vermiculite..and then standard concrete over that for the canopy and final exterior shape.  Even though the oven temperature is only about 1000F, the fire used to bring it to that operating temperature is going to generate a lot more localized heat.  

You likely want a floor that can eventually be replaced because they do degrade in the long term--so I'd personally go with a tile layout of appropriate materials.  Ovens like this are swept in use anyway so I doubt you'd have ash issues due to tile gaps.  

Set up the opening for more than pizza--they are perfect "beehive" bread ovens also if you make the opening and volume an acceptable height for a rustic loaf.  

[Glenn]

 

Reference https://www.epa.gov/asbestos/protect-your-family-asbestos-contaminated-vermiculite-insulation

What is Vermiculite

Vermiculite is a naturally-occurring mineral composed of shiny flakes, resembling mica. When heated to a high temperature, flakes of vermiculite expand as much as 8-30 times their original size. The expanded vermiculite is a light-weight, fire-resistant, and odorless material and has been used in numerous products, including insulation for attics and walls. Sizes of vermiculite products range from very fine particles to large (coarse) pieces nearly an inch long.

Concerns about Asbestos-contaminated Vermiculite Insulation

A mine near Libby, Montana, was the source of over 70 percent of all vermiculite sold in the United States from 1919 to 1990. There was also a deposit of asbestos at that mine, so the vermiculite from Libby was contaminated with asbestos. Vermiculite from Libby was used in the majority of vermiculite insulation in the United States and was often sold under the brand name Zonolite. If you have vermiculite insulation in your home, you should assume this material may be contaminated with asbestos and be aware of steps you can take to protect yourself and your family from exposure to asbestos.

Risk of Exposure to Asbestos If You Have Vermiculite Insulation

Asbestos causes cancer and other diseases. There is no known safe level of asbestos exposure. Asbestos fibers must be airborne to cause a health risk through inhalation, so the first step is not to disturb the material, which would release more fibers into the air. If you remove or disturb the insulation, it is probable that you may inhale some asbestos fibers - the degree of health risk depends on how much and how often this occurred. If you do not go into your attic, handle, or disturb the insulation, it is likely that you will not be exposed to asbestos fibers from vermiculite insulation.

[Kozzy]

Vermiculite mining from sources that had asbestos content was stopped long ago.  The current sources are tested asbestos free.  Now the old stuff----don't be snorting that dust akuz you never know. 

[Pizza eater]

22 hours ago, ThomasPowers said:

Do you have any ideas about if those are *GOOD* videos? 

Thanks Thomas - in all honesty I’ve got to say I’ve never seen a decent pizza cooked in one!  But there might be a couple of reasons for that. First, I’ve noticed a peculiar inverse law where the better people seem to be at building nice looking pizza ovens, the worse they are at making a pizza. It’s true, I’ve seen immaculately crafted pizza ovens used to cook a dough which I can’t even call pizza dough, it was just not kneaded, balled or risen right, wouldn’t stretch and predictably produced something more like chapatti than pizza. Probably people get too carried away with having finished their oven they put together a demo video before they even learned how to make a pizza or get good results with it, and I can’t fault them for that.
 

But there’s another reason, and I think a lot of the diy pizza oven builders are all copying each other’s designs and missing a trick. They’re all making hemispherical ovens which are too tall. The apex of the roof is so high the heat of the fire isn’t channelled low enough over the cooking space.  If you contrast their design with the commercial example, the commercial one has the same hemisphere or igloo shape, but the floor is much higher. The angle of incidence at the edges between the floor and roof will be much smaller, directing the flame across the roof and lower across the pizza. The other thing is the entrance tunnel is relatively wider and arranged so the cooking stone area is rectangular all the way to the back, not a thin entrance tunnel opening out into a hemispherical chamber inside.  Finally the lack of a flue on the commercial one. The air supply and exhaust is via the entrance tunnel. 
 

So in summary, I want to take the construction techniques from the DIY pizza ovens all over YouTube and design inspiration from the delivita oven and other commercial products like ooni and roccbox, all of which keep the oven roof kinda low, and hopefully make something which works ok and has a nice smooth stone floor, sized at least 20 inches wide.

And I do take your point, I might end up with something which doesn’t work well, but I will have learned along the way and will know how to fix it “next time”! 

21 hours ago, Kozzy said:

Do it once, do it right.  There are no savings in short-cuts.

Thanks, that is an important point about the heat of the fire being higher than the 1000F I’m targeting as an oven temp. Probably I do need to abandon my plan to cast a stone floor and go back to fire bricks or kiln tiles, as others are using in their home built pizza ovens. 
 

The floor of the oven should have a characteristic that accumulates and retains heat, not too thermally insulating, so I will have to check some specs and figure out what might be the best material. Long term maintenance and replacement is a good point too. I’ll try to consider that. 
 

the oven entrance and roof should be high enough to accommodate a rustic loaf.  I’ll be going with a somewhat raised floor,  but not crazy high, and the domed shape means there will be height available in the middle. Thanks for your comments (and to all others too). 

[Marc1]

If you want to make a pizza oven that works, ask about how to make a pizza oven, without your own premises.

If you want to make a refractory slab for a kiln, you probably need an industrial chemist. 

Because you don't need any of that for a pizza oven, even the most sophisticated one.

20"x23" is a puny surface for a pizza oven. You need about 40" diameter and not higher than 20". The door dimensions are also crucial but most of all, the efforts into insulation are not on the floor, but the dome. Your concerns about smooth surface and clean, no sand, are noted but misdirected. There is such thing as steel brush to clean the oven floor, and a mop or vacuum cleaner when it is cold. 

I suggest you ask your local italian pizzeria to allow you in the kitchen to observe how a wood fired oven is used and pizza is made. 

Don't try to reinvent the wheel. Pompey ovens are thousands of years old, made with clay without refractory materials, perlite render or any other modern day technical additions.

Oh, and if you intend to heat your oven to 600C and keep it at that temperature to cook pizza ... better call the fire brigade in advance

As for youtube "how to make a pizza oven" they are as plentiful as they are wrong and will mislead you. The number of people who pontificate about how to, after building ONE oven is staggering. 

 

 

[Irondragon Forge ClayWorks]

9 hours ago, Marc1 said:

call the fire brigade in advance

I thought the same thing. When I was a puppy I worked in an Italian restaurant that  made wood fired pizzas and the oven was kept between 370°-400° F.

[Marc1]

If you intend to make lots of pizzas like in a restaurant and need them to come out nice and crisp in 2 to 3 minutes, you will have your oven at 350C or 660F, however ... you can cook pizzas at a slower pace with your oven at 400f or 200C

If you want to incinerate you pizzas and your utensils you heat it up to 1100F and the incineration process will be instantaneous ,,, I am sure the above is a mistake and the OP meant to say 600F not 600C.

Furthermore the temperatures people state their oven is at, must be adjusted according to how it was measured. I have a fixed gage on the side of the oven some 6" off the floor, yet is is more for decoration purposes. There is a wide fluctuation of temperature in a wood fired oven, that can go from say 400C in the back and 200C in the front with the door open. The dome can be up to 450C and the floor as low as 150. Then there is the convection movement that adds to the complexity of measuring the temperature inside a cavity that has air circulating, radiated heat from hot coals, from floor and dome. 

There is one youtube channel I found to be helpful when it comes to cooking, it is "The wood fired pizza oven chef". As for building one, if you want to get a headache and listen to the cacophony of  one-oven-experts, go to "Forno Bravo", the noise is indescribable yet a lot of good information if you can find it among the chaff.   Otherwise some good books around. I built a few ovens following the instructions of Russell Jeavons book, only because he has built a dozen or so himself and uses one in his restaurant. My previous built made with house bricks and mud from the river, mixed with lawn clippings in a pit by my kids stomping on it worked a treat too and my brother is still using it 30 years later.  

No refractory slab.

[Marc1]

I believe that I hold the record in highest number of thread ending post 

Not sure why ... do I intimidate others?  or are my post so rotten that they stop anyone else from replying? 

[JHCC]

"Meditate on this, I will." -- Yoda

[Glenn]

Why try to reinvent the wheel?  Find a pizza oven that works.  Measure and copy the dimensions from several pizza ovens that work and see if there is a pattern.  Be sure and realize that pizza ovens are somewhat like cooking on a wood stove, it is the person cooking that has THAT oven figured out, and makes THAT oven work for them.  

Next eat a lot of oven cooked pizza.  It may be cheaper to place an order and enjoy a delicious meal.   Keeps the pizza shop in business and there are no dirty dishes when you finish.  Not saying to give up on the project,  just to enjoy the chase. 

[Marc1]

3 hours ago, JHCC said:

"Meditate on this, I will." -- Yoda