30" cast pizza oven - scotland

[Daveholdn]

Hi Team,

I took the plunge a few weeks ago after watching a you tube video series on building a cheap pizza oven, and basically thought i was an expert!! How wrong i was after stumbling across this forum and got the vibe, if your going to do it, why not go the extra mile and do it properly!

Attached are the pics of where i am at;

I laid a 4" thick, 39" x 47" concrete foundation and built a small block base, initially with 2 legs, but added the 3rd, middle leg in for support after reading forums on here.
I've then placed 4 x 23.5"x 23.5" (2" thick) paving slabs on top as part of my sub-base.

Thats where i am at, i plan, unless the expertise on this forum say otherwise, to:

Build a 47" x 47" form and pour a, 47" x 47", 5:1 vermiculite/portland cement mix slab, at a thickness of 2" on top of the slabs to complete the sub-base.

Then lay 2 x 1" thick ceramic fibre boards as insulation directly on top of the vermiculite concrete.

Then place my fire bricks on top in herringbone formation.

Use a gym ball to cast a 2" thick, refractory cement mould, appox 29-30" internal diameter, and then place this on top of the bricks, ensuring no bricks are touching the vermiculite sub-base.

Insulation around the dome will be, as mentioned, 2" refractory cement, 2" fire blanket, 2" refractory cement and then 1.5" render to finish.

The flue gallery, i'll have to get my head around, more reading required.

If that sounds like it would blow up in my face then please let me know, i'm extracting info from this site all the time and trying to put it into my own build.

My first question is; as per the pics, I've placed 4 x 23.5"x 23.5" (2" thick) paving slabs on top as part of my sub-base, these where bought in haste after watching the youtube stuff but i'm now worried these wont cut the mustard with the potential amount of weight of the oven going through them? Is there anything i can put into the vermiculite slab to reinforce or does this defeat the purpose of the insulation slab?

Also, what is the best way to mix the vermiculite/portland mix for the slab? i'm going to use a shovel and a mixing board as i've read using a mixer can break the vermiculite down too much. I've also read you need to add a bit more water to the mix as the vermiculite is thirsty, but what kind of consistency should i be looking?

Any info or direction would be much appreciated, and thanks, great forum.

Dave.

[UtahBeehiver]

Welcome to the forum. I am glad you visited before you got too far along. These YouTube videos can be really bad and one offs (there are some good ones though). There are a number of cast ovens going on right now especially in the UK area. So look in the Other Type of Oven section and also lean heavily on David S of Australia threads, he is out resident cast expert.

5 to 1 vcrete is correct for under the fire brick.. Is there a reason for the 2" refractory over the ceramic blanket (shape?) consider 8-10 vcrete for shaping and added insulation as base for render. A in situ sand mold is recommended over a gym ball, See current builds

[Daveholdn]

Hi utah, thanks very much for the welcome and the information. Apologies, I meant a 2" vermiculite mix over the ceramic blanket. When you say 8-10 vercrete, is that an 8-10 part vermiculite, 1 part portland cement mix to shape around the the blanket?

I'll have a look at the other ovens being conctructed and already was half thinking about an in situ mould over sand. I take it with the sand mould, builders sand will do and just a case of shaping the sand to as best a dome shape to the dimensions you can? There's no special way of doing it?

I've read a lot of the stuff from David S, and a multi part cast seems to make for a more effective dome, rather than an all in one cast, but again is there any special way of doing this over sand?

Thanks again and sorry for all the questions, cheers.
Dave

[Hattori-Hanzo]

Reading this with interest.

For the insulation layer over the ceramic blanket an 8 or 10 parts vermiculite to 1 part cement.
As I understand an 8-1 mix should be slightly easier to work with but reduced insulation.

10-1 is a bit harder to work but slightly better insulator.
The more cement you add to the vermiculite mix the worse the insulation properties get.

I would also look into casting over sand. The problem with an oven your size is a gym ball can deflect and sag under the weight of the cast.
Another problem is you have to build a lot of form work under the gym ball and some how remove it all once the cast is done increasing the risk and chance of cracking the dome.

Casting sand is fairly straight forward, to get a good shape cut the desired half round from a scrap of wood and use that as a former to smooth the sand to a nearly perfect dome.

Good luck with the build, keep us updated.

[Daveholdn]

Hattori-Hanzo Thanks very much, I'm more than certain I'll do a sand mould now. I can get the circumference of the sand mould by marking it out on the firebricks, but how do you get the height of the sand mould? Just a case of measure and guesstimate from there?
Thanks for the info

[david s]

Just cut a thin stick to the required length and place it in the centre. Then build the sand around it. To reduce the sand quantity you can put some empty plastic bottles near the centre at the base and build the sand over them.

[Chach]

Glad you stumbled across this great forum. This is where you want to be when your building your oven as there is a plethora of information here. If your not happy with your paver "slab" Now is the time to think about a reinforced concrete slab for your insulation and oven to sit on. If your not happy with the way things are going my opinion is to stop think about it and fix or change it right then and there before it's too late and you regret it. Never settle because you probably won't be happy. You're going to do a great job so congrats on your start.

Ricky

[Daveholdn]

Thanks david s, I take it any builders sand is good to shape, you don't have to buy like a fine plasterers sand or anything like that? and Chach thanks also, only reason I was concerned was for strength of paving slabs but I've been assured because of the middle supporting leg, the slabs will be fine.
I'm going to lay my vermiculite 5:1 slab this weekend, I have a form measured out but has anyone had any experience of framing a vermiculite slab with bricks? I have a lot of bricks in the garden and thought I could lay and mortar a border of bricks on their side (4" tall border) which would act as a form for the slab. Would the slab be too much for the bricks and just simply force them off?

Thanks again,

Dave

[david s]

Any sand should do, but builders sand is probably the best as it is graded, ie it has a range of grain size which apparently results in a stronger bond between cementious materials (cement, lime) and aggregate (clay and sand). The high clay content and lime give the mix great workability so some larger sand grain size doesn’t matter. Make sure you wear rubber gloves as handling the stuff for more than about 15 mins will damage your skin.

[Daveholdn]

Thanks david s, I've ordered a load of sand and a bit more just in case as I heard it takes a bit more than you think!

I poured/crumbled my vermiculite slab layer today, 5:1, weird stuff to work with but seems all good, I know it takes a while to cure but with reference to taking the form/shutter sides off, how long should these be left on, and at what point has anyone started building on the slab? I've read quite a bit and understand it's a waiting game now for the slab to get rid as much water as possible, and can take up to 3 weeks?

Also, I'm getting to my flue gallery and oven entrance, I'm going to set the flue back as far as possible to make the gallery as short as possible, but for the entrance for the oven itself, I understand height is most efficient at 62% of the internal oven height, but is there any such rules for the width of the oven entrance?

Thanks and any info is really appreciated.

Cheers Dave

[david s]

Hi Dave,
I usually wait 48 hrs. Because the stuff is pretty crumbly sometimes the formwork will pull some of it away, but it’s easy enough to repair. Hope you oiled your formwork.
The 63% rule is really not as important as most have you to believe. The larger the mouth the greater the heat loss. Conversely the smaller the oven mouth the more difficult it is to work the oven. So it’s like boat hull design V , flat bottom or a compromise between the two, totally your choice.

Dave

[Hattori-Hanzo]

david s I went with around 63 % high for my door opening but looking at it now I think I may have gone a little too wide. Have you even seen or do you think it's possible to make a sort of half door that would shut the mouth down to retain more heat while at the same time still being able to operate the oven?

A sort of door on stilts at the sides, so you could slide the peel under it.

[Daveholdn]

Thanks again david s, I'll mess around with the gallery forms and see what looks and feels right.

[david s]

david s I went with around 63 % high for my door opening but looking at it now I think I may have gone a little too wide. Have you even seen or do you think it's possible to make a sort of half door that would shut the mouth down to retain more heat while at the same time still being able to operate the oven?

A sort of door on stilts at the sides, so you could slide the peel under it.

Maybe wait until you've built and fired it, see how it goes, then start tinkering.

[Daveholdn]

Hi Team,

So the form/shutter came off the vermiculite slab nicely leaving a big, square, 2 inch flapjack/granola bar! Now looking forward to the dome and gallery, I've decided against using refractory cement as I'd approx need 6 x 25kg bags which would kill the bank, so have decided to go for the home-brew option with added SS needles and polypropylene fibres!

Researching the forum, particularly the excel spreadsheet available, I have the following quantities calculated for a 30" internal dome, 2" thick, gallery height 9", length, 9.3: (all include 10% wastage factor)

Sand = 229lbs/104kg
Fireclay = 55lbs/25kg
Portland = 60lbs/28kg
Lime - 20lbs/9kg
SS needles - 10lbs/4.5kg
Polypropolene fibres - 5 handfuls?
CF board, 40" x 40" x 1" thick
Firebricks 230mmx114mmx76mm = 40

(superwool insulation, vermiculite layer and render I have separately)

Can anyone who's built the same or with experience please give this a quick once over to ensure I am not completely off the mark and actually ordering a stupid amount of material!

I don't know when the build will start as the forecast is ridiculous for rain up here for the next week or so, but does anyone have any sort of timeline for building on the vermiculite slab? Its been 3 days now, is 2-3 weeks about the norm for curing?

Thanks again, cheers.

Dave