First time builder. Strange/stupid oven design, advice required.

[Hattori-Hanzo]

Hello all.

I've been wanting to build a pizza oven for the last few years and having some time off recently I discovered this forum and I'm very glad I did, its a wealth of information.

While researching how to make a simple pizza oven I, like many others it seems have been duped into thinking the vermiculite cement mix packed around an exercise ball was the way to go.
After spending the last couple of days reading threads on this forum it seems it's not the recommended thing to do.


However after a little more reading it seems it may not be a terrible idea depending on what you want the oven to do?

I would love to build a large firebrick dome oven but my garden is tiny and it is just not feasible. Also cost is going to play a big part in my build, being in the UK from what I've found materials are expensive.

My idea was to make a lightweight smaller sized oven with its own base (probably made from wood) on castors that could be wheeled out into position when needed, then pushed back out of the way after use.

Heavy bricks and mortar are probably out of the question but a lighter vermiculite mix may be the answer?

From what I've read so far it seems a vermiculite mix main disadvantage is its's poor strength and thermals.
However I only plan to use this oven for pizza, bread and the odd bbq. I'm not planning on using it for long roasting or slow cooking etc.


I had an idea that I just wanted some feed back on to see if you think it might be a viable option before I go ahead.

I've done a very quick sketch in paint.

The idea is to make a wooden mould in the shape of a ring roughly 3" wide, 16" tall, 25" round
I'd then fill the mould with a vermiculite, perlite, sand and cement mix.

I'd then make another mould to form a round top which again would be a 3" thick vermiculite mix with wire mesh in the middle for added strength.

The base would be made in the same way with a vermiculite mix on top of a bed of sand then 1" fire bricks layed on top.

I would used refractory cement to fix the top to the ring and the ring to the vermiculite base.

I had an idea to cover the inside of the ring and the underside of the top with 1.5mm mild steel sheet.
The idea of the steel was to protect the vermiculite mix from direct heat from flames and to add strength.
Not sure how I could secure this though, could I bed it onto a 1/2" layer of refractory cement?

Here's the sketch




Once the 3 sections where secured together I was planning to cover every thing in 1" fire blanket then chicken wire and finally rendering over the top and add a mosaic detail to finish.

I'm hoping the finished oven will be light enough to be move onto a finished wooden base which I've yet to decide on the design of.

One of my main concerns is the internal roof of the oven will be flat and not domed. Is this a major disadvantage?

This is all just an idea from a complete newbie so I would really appreciate any input to its design, if you think it would work or if its just a stupid idea all together

Cheers.

[UtahBeehiver]

Read the links from David S that I just posted today. They are cast oven that are either commercial dense refractory or homebrew dense refractory with a sand mold and a fire brick floor. Either or would be a candidate for what you are wanting to do. I highly suggest you do not go the v or pcrete for the oven dome.

[david s]

Hello all.

I've been wanting to build a pizza oven for the last few years and having some time off recently I discovered this forum and I'm very glad I did, its a wealth of information.

While researching how to make a simple pizza oven I, like many others it seems have been duped into thinking the vermiculite cement mix packed around an exercise ball was the way to go.
After spending the last couple of days reading threads on this forum it seems it's not the recommended thing to do.


However after a little more reading it seems it may not be a terrible idea depending on what you want the oven to do?

I would love to build a large firebrick dome oven but my garden is tiny and it is just not feasible. Also cost is going to play a big part in my build, being in the UK from what I've found materials are expensive.

My idea was to make a lightweight smaller sized oven with its own base (probably made from wood) on castors that could be wheeled out into position when needed, then pushed back out of the way after use.

Heavy bricks and mortar are probably out of the question but a lighter vermiculite mix may be the answer?

From what I've read so far it seems a vermiculite mix main disadvantage is its's poor strength and thermals.
However I only plan to use this oven for pizza, bread and the odd bbq. I'm not planning on using it for long roasting or slow cooking etc.


I had an idea that I just wanted some feed back on to see if you think it might be a viable option before I go ahead.

I've done a very quick sketch in paint.

The idea is to make a wooden mould in the shape of a ring roughly 3" wide, 16" tall, 25" round
I'd then fill the mould with a vermiculite, perlite, sand and cement mix.

I'd then make another mould to form a round top which again would be a 3" thick vermiculite mix with wire mesh in the middle for added strength.

The base would be made in the same way with a vermiculite mix on top of a bed of sand then 1" fire bricks layed on top.

I would used refractory cement to fix the top to the ring and the ring to the vermiculite base.

I had an idea to cover the inside of the ring and the underside of the top with 1.5mm mild steel sheet.
The idea of the steel was to protect the vermiculite mix from direct heat from flames and to add strength.
Not sure how I could secure this though, could I bed it onto a 1/2" layer of refractory cement?

Here's the sketch




Once the 3 sections where secured together I was planning to cover every thing in 1" fire blanket then chicken wire and finally rendering over the top and add a mosaic detail to finish.

I'm hoping the finished oven will be light enough to be move onto a finished wooden base which I've yet to decide on the design of.

One of my main concerns is the internal roof of the oven will be flat and not domed. Is this a major disadvantage?

This is all just an idea from a complete newbie so I would really appreciate any input to its design, if you think it would work or if its just a stupid idea all together

Cheers.

Whilst lining the inner face of the oven with steel may seem like a good solution, remember that steel being way more conductive than the vermicrete mix means it will expand considerably before any heat reaches the vermicrete. This will set up stresses on the vermicrete layer, probably cracking it considerably as well as issues conducting heat because of poor contact between the two materials when the steel begins to cool. There are lightweight stainless steel ovens commercially available that work quite well, but being low in thermal mass are unable to store much heat. The stainless overcoming the corrosion issues and insulation with blanket doing a good job of retaining what little thermal mass is contained in the stainless sheet. The form of the interior oven is important for good circulation. The hemisphere has proved to be an outstanding form in this regard (think Jaguar hemispherical cylinder heads for example). Also it is no accident that the hemisphere with the front flue, as developed by the Ancient Italians over 2000 years, has become the accepted best design, . Italian ingenuity would have attempted millions of major and minor departures from this design yet it still remains the preferred design.Unfortunately the hemisphere is not an ideal form for say a kiln or bakers oven because it leads to loading problems and ideal utilisation of the volume. Kilns in the form of a cube are notorious for having cool spots in the corners. For an oven which operates at a much lower temperature than a kiln, the circulation in the chamber is more of a problem because circulation and draw are reduced the lower the temperature in the chamber.
Having said all this I'd love to see you progress with your idea because I'm curious as to how it will turn out. Who knows, you may strike a great solution that might work really well.
Being in the UK you may also be interested in the UK wood fired oven forum.
https://ukwoodfiredovenforum.proboards.com

[Hattori-Hanzo]

Thanks for the replies chaps.

I had seen a few threads regarding casting the dome with refractory cement over a sand mould which sounded a much better method.
After some brief research trying to find some refractory cement it seems it's elusive stuff with a hefty price tag. Cheapest I could find it was £25 for 25kg bag plus £60 for shipping to my area.

I'm guessing even for a smaller oven I would need several bags of it? is there a way to calculate how much I would need?

My other concern was weight, I read it was as heavy as Portland cement?
Do you mix it just with water or do you need to add sand too?

Not a problem if going on top of a fixed based but maybe to heavy for a portable base.

I understand there is going to be a compromise with wanting a portable oven.

After some more searching I found Scottish Al Thread were he made his dome with a homebrew cement using 1:1:1:3 mix of fire clay, cement, hydrated lime and sand. Which could be a cheaper alternative but again I'm find it hard to source the fire clay (maybe just due to the current lock down) and again there is still the weight concern.


Thank you for the detailed reply David.
I had wondered about the expansion of the steel, my thought was that it being thin it would not put to much pressure on the vermiculite layer when it expands.
I wondered if bedding it onto a thick layer or refractory cement would help? probably not.

Initially I had the idea of using an old shipping barrel cut in half then clad with a vermiculite mix but again this would have a flat internal roof.
Also I'm not sure if a shipping barrel is safe to use? Guessing if its made from mild steel it would be okay. Am I right in saying you want to avoid galvanised steel at all costs?

I did see a thread on here where someone used a very large condemned metal fuel tank and truck wheel to make the dome then insulated in the usual fashion.

I'm open to any ideas and suggestions on the best way forward.

I've still got a load more reading to do and I wont be starting this project for a good while yet giving me plenty of time to suss it all out.

Cheers.

[david s]

Mild steel like an oil drum will corrode. Remember that heat accelerates any reaction. A stainless barrel would be better, but still suffers from the expansion problem and lack of thermal mass.
Dense castable refractory is a proprietary blend of high temperature aggregates, calcium aluminate cement and other goodies. It is expensive and hard to work. The homebrew would be far more suitable for your purposes, cheap and easy to work. Any powdered clay will suffice. Try looking for bricklayers clay. If you can’t find any try pottery suppliers and ask for ball clay.

[Hattori-Hanzo]

Thanks again for your reply David, safe to say you have convinced me to steer clear of trying to incorporate metal into the design.

After some more reading over the past few days and as suggested by Utahbeehiver I will also stay away from using a vermiculite mix for the internal dome, though I think I'll still use it for insulation on the out side.

Doing some more thinking last night how to reduce costs and wondered if you'd ever heard of someone using a combination of brick and casting?

Brick for the walls and casting (either refractory cement or homebrew) for the dome?

This could reduce costs and weight while also potentially being easier to make for a beginner.

Thanks for the tip on the powdered clay. I may have found a source for it close to home so can save on delivery costs.

cheers.

[Hattori-Hanzo]

Also because I'm after a smaller oven what would be the minimum internal diameter you would recommend.

I was hoping to go no bigger than 500-550mm or 20-22" in old money.

cheers.

[david s]

My ovens are 21” internal diam and big enough to cook pizzas for a crowd of 30 or so easily, provided everyone shares whatever comes out of the oven.For roasting the limiting factor is the height of the door. Mine will fit a 5 kg turkey, but nothing larger. For bread, I usually bake one large loaf and two baguettes, but could probably
squeeze three loaves in. The beauty of a small oven is its fuel economy. It’s no trouble to fire it up for a couple of pizzas or a Sunday roast with minimal fuel consumed.
Your brick/castable idea sounds doable, but you should make the castable section the same thickness as the brick, which will give you a pretty thick walled oven in relation to chamber size. Make sure you insulate well.

[Hattori-Hanzo]

Thanks David that's reassuring to know the smaller size still functions well and is more economical. Thinking about it some more I may try and go slightly smaller still with an internal size of around 17-18"
Ideally I don't want the external size to go above 22" as it won't fit through the door where I intend to store it.


Re the homebrew mix is it worth using a portland cement with calcium aluminate in? I read that refractory cements have it but not really sure what benefit it has?

something like this: https://tarmac-bluecircle.co.uk/prod...-rapid-cement/

Or would a standard portland cement be fine

Like this: https://tarmac-bluecircle.co.uk/prod...-cement-trade/


Also I planned to use a vermiculite mix under the fire bricks for the floor of the oven.

I've since read that its a good idea to use a ceramic board under the fire bricks too.

What thickness fire bricks would you recommend for the floor? and what thickness ceramic board?

If I use a ceramic board under the fire bricks do I need to seal it in any way to stop the fibres escaping or can I just lay the bricks directly on top of it.

I still plan to use a vermiculite mix for the sub base of the oven, so it would be constructed as: Sand, vermiculite mix, ceramic board, fire bricks.

I was planning to use a 1" thick ceramic blanket to insulate the dome then a vermiculite mix over the top of that.

Thanks again for taking the time to reply.

[david s]

You need to draw up plans with required thickness of each layer. A 17” diam oven is not workable IMO as it would require a door height of only 10.7”also remember that when cooking pizzas it is essential, to maintain pizza temperature, that you keep an active fire on the side. In that case you won’t have enough room for a pizza and for roasting won’t have sufficient height.. you could skip the bricks and cast the whole thing 2” thick to save some space. An external width of only 22” is not possible once you add insulation layers and an external render.
A combination of calcium aluminate and calcium silicate cements is possible but has the disadvantage of accelerated reaction leading to reduced working time. The product you linked to has too low a proportion of calcium aluminate cement to be useful. A 50/50 calcium silicate (Portland cement)/hydrated lime is adequate, cheaper and provides good working time.
The usual firebrick floor thickness is 2” or if using standard sized bricks laid flat 3” Underfloor insulation usually 2” of calcium silicate board or 4” 5:1 vermicrete.

[Hattori-Hanzo]

Thanks David, all duly noted.
I sketched out a 21" oven today and I think it will just about fit where I intend it to go so that's the decision made on size.

I also think I'll use a mixture of bricks and home brew as this seems the most economical and hopefully will be the simplest method of construction.

Just got to try and source all of the materials now....that's the hard bit

[Hattori-Hanzo]

Hello again.

Starting to get a few materials together and had a couple of questions regarding the insulation layer under the hearth.

I was planning to use a 6-1 vermiculite and cement mix at 4" deep under the hearth, then use 3" thick fire bricks for the hearth itself.

I've since read that ceramic fibre board is a preferred option for insulation under the hearth.

I've found some 10mm thick ceramic fibre board for a reasonable price and wondered if it would be better to use a 3" deep vermiculite cement mix then lay the 10mm ceramic board on top.

Or can I save some money and just go with 4" of vermiculite cement on it's own?

Second question is if the ceramic board is recommended can I mortar my fire bricks directly on top of the ceramic board or do I need to cover the board with something first?

I've seen you can get ceramic board in thicknesses from 10-50mm but the cost for anything over 10mm will be too expensive.

Cheers.

[UtahBeehiver]

I am not sure you really gain anything with 10mm of ceramic fiber. Although CaSI is a higher tech insulation it is also expensive. Vcrete is acceptable, 4" is abt equiv. to 2" CaSi. We normally suggest 5 to 1 but I cannot remember what size over you are looking at. Use 8-10 for over dome for insulation.

[Hattori-Hanzo]

Thank for your help Utah.

I will do the vermiculite mix for the base insulation at 5 to 1, thanks for clearing that up.

I've found some calcium silicate board 25mm thick for around the same price as the 10mm ceramic fibre board.

Would doing 3" of the Vmix and then 25mm calcium silicate board offer any benefit?
I should add this insulation layer is going directly onto a 36mm thick piece of marine plywood so i need to ensure it stays cool enough.


cheers.

[UtahBeehiver]

I read through your post again and it appears that you are doing a smaller diameter oven. So going back to David S comment of post #10 I would agree with him that 4" vcrete is good. Use the money for the dome insulation which can be either p/vcrete or ceramic blanket.