Hello - new oven build in France

[thargog]

Bonjour everybody...

I live in the sunniest part of France - I'm tall and hungry. After looking at the cost of buying a WFO, I have changed tack and am beginning to investigate what is required to build one. I'd quite like it to be 'mobile' (1 - just to roll on sturdy casters across the terrace and 2 - maybe to be able to lift the base and oven off the frame with the help of friends should I ever move, or even want/need to build a replacement oven).

So my current thinking is a sturdy wood frame, then a base of marine ply, vermiculite concrete and then firebricks. Top this with an oven cast over either a gym ball or a sand dome, and make it with layers of refractory clay, vermiculite and some outer coating that provides some weatherproofing and aesthetic qualities.

I am the opposite of Mr DIY (whatever that is) - I have a load of questions and so will begin browsing these forums - and post if I am unable to work it out myself!

Cheers

[UtahBeehiver]

David S from Australia is our forum casting expert. I suggest you look at this threads and post for the best and most effective cast oven techniques. In general he suggests a dense refractory concrete with stainless steel needles for the dome followed by insulation coat (either ceramic fiber blanket or a vermiculite or perlite/concrete layer, then a render layers coated with a diluted 100% acrylic coating. But look at his threads for more detail. I also believe he suggest a sand dome form over a gym ball. There have been a few wood bases done but they have to be very study and cross braced well. These ovens are heavy.

[SableSprings]

Welcome Thargog! As Russell noted above, there are several threads on this site that describe building a cast oven with David S (and others) giving fabulous suggestions and explanations of the process. As to Russell's comments on the need for a sturdy wood base, here's a link to a very good build that should be of interest to you.

https://community.fornobravo.com/for...andpoint-idaho

If you have trouble finding threads/posts here that involve casting ovens, let us know and we can probably point you in the right direction.

[thargog]

Hey - thank you guys for the welcome and the pointers. Loads to read up on! That wooden stand certainly looks sturdy!

Cheers

Rik

[david s]

Hi Rik,

A few pointers regarding you oven design.
1. I don't like wooden stands as they weather and move, although my own mobile oven sits on a wooden trolley that rolls on and off my trailer. I'd prefer masonry, however that's out if you want the oven to be mobile. Have you considered steel? If the oven is to be movable then it needs to be small as the mathematics of volume increase the weight exponentially. Also you need to consider the surface on which the oven will be moved. It really needs to be pretty smooth and the wheels not too small.

2. Using a gym ball as a mould may appear at first to be an ideal solution, but carries with it the problem of only requiring the upper hemisphere. This means extensive formwork required to support the ball and the casting to be formed over the upper hemisphere as well as the problem of relocating and bedding the casting after de-moulding. It is far easier to form the casting where it is required. A simple one piece casting over a sand mould is far easier if only doing a single oven.

3, Regarding the material required for the inner oven there are a number of mixtures that can be used. The major requirement is that the material needs to be strong and dense. From your initial post it is clear that you want to make this a budget build. The cheapest mix is to use cob, i.e. a mix of 2 parts sand to 1 part clay, by volume and reinforced with a little straw. While this is cheap it can be fairly labour intensive and while the clay mixture will get pretty hard it will never be sintered so will be subject to abrasion damage and moisture degradation should it ever get wet. Going up a step you could use the "home-brew mix 3 parts sand,1part portland cement, 1 part hydrated or hydraulic lime, one part powdered clay, by volume with some burn out fibres added. The next step up and starting to get more expensive is to replace the lime and portland cement with CAC (calcium aluminate cement) a common brand in France is "Ciment Fondu". Going further up the scale and more expensive again is Castable Refractory which contains CAC and stable, high temperature aggregates already in the mix and also contains the burnout fibres in the mix, just add water, mix and trowel into position and vibrate if possible.. Regarding the stainless steel needles (correct name is Melt Extract Fibres) in any of these mixes, they are not mandatory but will increase strength. Most manufacturers of wood fired ovens (I think Forno Bravo included) do not include them, probably because of cost (they are expensive) and difficulty in handling (they're not called needles for nothing) A minimum of 2% by weight is required to expect strength improvement. Exceeding 4% makes handling and placement more difficult.

There are a number of home cast builds under "Pizza Oven Design and Installation" then the "Other Oven Types" section of the forum, but here is a well documented one. Read the whole thread.

#1

Do plenty of reading of this forum before starting. Beware of the vermiculite cement mix brew over the gym ball idea as the material you'd end up with is both weak and subject to abrasion and degradation as well as containing little thermal mass, an important requirement for a versatile wood fired oven.I've not seen any reports of how these ovens stand the test of time and suspect they don't.

[thargog]

david s - thank you so much for taking the time to write your post.

1 - Yes I understand the limitations of building the stand in wood, but the advantages (aesthetically as well as functionally) for me are strong. The oven will indeed be small - I'm thinking just a 60cm interior. Most threads I have read and videos I have watched indicate it's pretty unnecessary to be able to cook more than one pizza at a time. Even at a pizza 'party' it's pretty fun to throw them in one after the other! For the most part, I'm only looking to be able to wheel it from one side of my terrace to the other, so the ground is indeed pretty flat. Heavy duty lockable 4-inch casters seem to be readily available, supporting c1200lbs. Should do the job and make rolling the thing a bit easier.

2 - Ok I take your points. Whilst the gym ball idea was the first I came across and was (I thought) really clever, I have read in various places that the sand dome offers several advantages. Cheers - sand it is.

3 - Hmm whilst it may appear that I require a 'budget' build, I'm actually quite happy to spend some money to make the job easier or the oven stronger or whatever. The main comment about spending money was that at this time I'm not prepared to spend e.g.€5000 on a pre-built one! So for instance your comments about castable refractory are extremely useful - ready mixed, just add water, all music to my ears!

A couple of things (for now!) - you talk about burnout fibres and melt extract fibres; 2 different things, yes? I note elsewhere you indicate burnout fibres are to do with water elimination and are typically e.g. polypropylene. So - burnout fibres do what exactly? Do they offer any strengthening properties as well?

The other question is about a ceramic blanket - I get the impression I don't need one but it may limit performance such as heat retention for long slow cooking. I was just thinkinjg of doing 2 inches inner (castable refractory) and then 2 inches vermiculite (plus outer to smooth, for appearance, waterproofing etc.) - do you have any comments on that combination and likely performance pluses or minuses?

Thanks again

Rik

[UtahBeehiver]

Looks like David gave you the full run down. IMHO, you do need to insulate the dome but vcrete or pcrete is an option for the dome as well as under the firebrick floor. 5 to 1 ratio for the floor and up to 8 or 10 to 1 for the dome. I would make the dome insulation more than 2" though. Attached is a thermal value chart of vcrete that David has posted before and if you compare the K values of vcrete to ceramic blanket you will see that you need more vcrete to be as effective as ceramic blanket.

[david s]

david s - thank you so much for taking the time to write your post.

1 - Yes I understand the limitations of building the stand in wood, but the advantages (aesthetically as well as functionally) for me are strong. The oven will indeed be small - I'm thinking just a 60cm interior. Most threads I have read and videos I have watched indicate it's pretty unnecessary to be able to cook more than one pizza at a time. Even at a pizza 'party' it's pretty fun to throw them in one after the other! For the most part, I'm only looking to be able to wheel it from one side of my terrace to the other, so the ground is indeed pretty flat. Heavy duty lockable 4-inch casters seem to be readily available, supporting c1200lbs. Should do the job and make rolling the thing a bit easier.

2 - Ok I take your points. Whilst the gym ball idea was the first I came across and was (I thought) really clever, I have read in various places that the sand dome offers several advantages. Cheers - sand it is.

3 - Hmm whilst it may appear that I require a 'budget' build, I'm actually quite happy to spend some money to make the job easier or the oven stronger or whatever. The main comment about spending money was that at this time I'm not prepared to spend e.g.€5000 on a pre-built one! So for instance your comments about castable refractory are extremely useful - ready mixed, just add water, all music to my ears!

A couple of things (for now!) - you talk about burnout fibres and melt extract fibres; 2 different things, yes? I note elsewhere you indicate burnout fibres are to do with water elimination and are typically e.g. polypropylene. So - burnout fibres do what exactly? Do they offer any strengthening properties as well?

The other question is about a ceramic blanket - I get the impression I don't need one but it may limit performance such as heat retention for long slow cooking. I was just thinkinjg of doing 2 inches inner (castable refractory) and then 2 inches vermiculite (plus outer to smooth, for appearance, waterproofing etc.) - do you have any comments on that combination and likely performance pluses or minuses?

Thanks again

Rik

3. €5K is a lot for a small oven, I understand your reticence. It is possible to buy a kit which contains the inner castings that you assemble and insulate and finish. This reduces the cost enormously.and should be a consideration for you. Most ready mixed castable refractories already have the burnout fibres added, but if you make up your own mix using either homebrew or one using CAC then you need to add them.They are designed to increase the compressive strength in standard concrete mixes, but in our case because they melt at 160 C impart no strength, but leave a network of tiny pipes through which steam can pass. This gives the material some protection against steam spalling which is common if the material without them is heated too rapidly. Because they are finer than human hair they do not affect the density of the castable.and only a very small amount is required, they do need thorough mixing though to achieve decent dispersal.

The use of ceramic fibre blanket is not mandatory, but highly recommended. A good insulator is perlite or vermiculite mixed with water and just enough cement to hold it together. The table that Utah gave you shows insulation values. The more cement you use the less insulating the material becomes. I find 10:1 is about as lean as I dare go without it becoming extremely difficult to apply and expect it to stand up vertically. Because of the large volume of water required in the mix you then have the problem of eliminating the free water once it has set. Using at least one layer (25 mm) of blanket whilst being more expensive, has the advantage of being a slightly superior insulator as well as not adding additional moisture. The blanket does leave you with a slightly lumpy and springy surface to render against so covering it with a layer of vermicrete or perlcrete regains the spherical form and provides a firmer surface to render against for the final outer shell. 3 inches (I don't know why a Frenchman would be talking inches) of 10:1 vermicrete would be the minimum thickness required. One layer (25mm) blanket plus 50 mm better and two layers of blanket plus 25 mm vermicrete better again. I use one layer of blanket plus 70 mm vermicrete which gives adequate insulation for a small oven. Some say you can't have enough insulation, but of course there is a limit.

[thargog]

"I don't know why a Frenchman would be talking inches" - Ha that made me chuckle. It's because I'm not a Frenchman; I'm a pre-decimalisation Englishman who just happens to live in France! So I'm happy with mm or imperial

Again, thank you for the information and suggestions. I will look again at blankets.

Rik

[thargog]

Bonjour encore. After a busy summer I'm ready to start assembling the materials for my build. It's going to be a slow project. Not being completely confident that I can find the right materials in France easily, I am taking advantage of a trip back to the UK in October to bring home a few items that seem more readily available (or significantly cheaper) in England.

Thank you to all who have commented so far, in particular david s - I have incorporated many of the suggestions into my plans, e.g. metal trolley, sand mould, castable refractory, ceramic blanket etc etc

I have found (and had tested) an old stick-welder in my garage, left by the previous owners. It's ancient, but the local metal-working guy seemed quite envious - perhaps an example of things being built to last 'back then'... so I have lengthened my build by deciding to teach myself welding, with the idea of making a metal base on castors for my oven. We shall see if this comes to pass. Maybe paying a local 'ferroniere' to do it for me instead will occur later...!

A few more questions as I really welcome peoples' observations or experiences...

I would quite like to make the supporting base and oven removable from the trolley. I can't see myself wanting to do this often, and appreciate it may require three friends, but it will make transportation easier should I ever want to move it to another property. So should I make it on a piece of sheet steel? or marine ply? or something else?

What thickness of insulation do I need (i.e. how little can I get away with) under the 6cm firebrick base?

Please could someone confirm my maths - O-levels were a long time ago for me... I am going to cast a 60cm internal diameter oven with 5cm of castable refractory (suitable thickness?). Given the volumes of a 70cm sphere (179594cm3) and a 60cm sphere (113097cm3) this gives a volume for a spherical shell of 66497cm3, or 33249cm3 (or 0.033249m3) for my half shell. Given that the castable refractory weighs in at 2000kg per m3, i need .033249 x 2000 kg of the stuff, i.e. 66.58kg. So that's 3 x 25kg sacks. Make it 4 as they are cheap and I also have the entrance to cast... Does that sound right? It didn't sound very heavy but obv I have other layers of weight to add to that...

I am then going to add two layers of 25mm body soluble fire blanket.

Then at least 25mm (maybe 40 or 50) of vermiculite cement, followed by a final waterproofing layer.

As I type I realise how many things there are to do that I have never ever attempted before..!! <gulp!>

Cheers,

Rik

[Bentham7]

I am super excited to meet with you here dude. Thanks for your thread.