New Build - got the base/stand, but now some options!

[WoodywWun]

I've almost completed the stand for my oven, using reclaimed clay bricks. Next is the standard concrete base layer that the hearth/oven will sit on. I've got the form-work in place for casting the base, which will be strengthened with 10mm steel reinforcing bar and is 75mm thick. I've added a 65mm x 150mm lintel for additional as well. Some of the base will sit on a rubble layer over part of a raised area of the garden.

Budget, as ever, will have to control my choices and I will have to make some compromises. I want to use the oven for quicky stuff like pizza but will also hope to extend to bread and slow-roast options so some heat retention will be good to have!

It looks like the insulation is a key element so I'm pricing up Calcium Silicate board (at 50mm thick) and insulation blankets. Thermalite (aerated concrete) blocks might provide an alternative but they don't seem to get mentioned much! They are cheap and have quite a low U value for thermal conduction. Vermiculite concrete is an option but it doesn't match the U value of Thermalite. For the cooking floor, I'm going to blow some of my budget on proper fire bricks and may go for something thicker than the standard 64mm. A real sticking point is the cost of making the dome with fire brick. Obviously the best but if there's a good alternative then that's the way I will go. I've heard tell of the David S threads on casting so I'll check these out. I also have a pile of clay bricks, which (despite the risk of heating/cooling cracking) I may have to go for.

I'm going to attempt to add some photos. I've had some really helpful Forum responses to other threads. I'd be pleased to hear of any experience, ideas (UK) material sources etc.

[WoodywWun]

Moving on, I now have the concrete base in place. It was 75mm thick and made of a standard concrete mix, including some coarser ballast, with 10mm steel reinforcing bars in it. Part of it rests on an earth bank, which is part of the garden. I was able to include a brick arch, and what will be handy wood storage underneath. Calculating the overall height was important at this stage as I found it all too easy to either exceed a comfortable working height or get it too low. The advice for the final cooking floor was "the elbow height of the main person that's going to load the oven".

On the basis of price, I've opted for a Vermiculite concrete insulation layer on top of the concrete base. This is 100mm thick and a 5 parts Vermiculite to 1 part Vermiculite mix. I was strange stuff to mix and work with and, following advice, it's going to cure for a minimum of two weeks. Three if it looks like it needs it.

[WoodywWun]

..sorry folks. There's a mistake in my post above. My Vermicrete base was of course 5 parts Vermiculite to one part cement!

[WoodywWun]

Having left the standard concrete base to cure well, I constructed some shuttering out of plywood to give me a mould for a 100mm thick Vermiculite base to provide an insulation layer under the cooking layer in a morning and is good to go.

I'm going to go for a cast dome on top of my cooking floor. A whole bunch cheaper than fire bricks and provided you get the correct thickness for the cast (Mine will be 50mm (2") of castable homebrew) then I understand that this will provide some good heat storage and give me a versatile oven. So the next step is to order up the materials. I used the excellent Forno Bravo calculation Excel spreadsheet drawn up by Mark to calculate volumes. So a stack of things will arrive (for the 3:1:1:1 homebrew mix) of builder's sand, Portland cement, lime and fire-clay. I'm adding melt extract fibres (stainless steel needles) and polypropylene fibres to the mix to try and reduce the chance of cracks appearing.

There's a picture attached to show the Vcrete insulation layer, once the shuttering came off.

[wilgaburra]

Hi Woody,
I am in much the same boat although haven't yet completed my base. Look forward to seeing your progress.
I will be using perlite instead of vermiculite - not sure if either is the better and pumice where insulation is required. Fire bricks are too dear for me too and also considering red solids, which is all our local brick factory used to line their kilns. Currently investigating recipes for homebrew castable made from scratch (fireclay hard to get round here). Did some trial mixes of sand and wild clay this morning to measure shrinkage, Good luck with it and keep us posted

[WoodywWun]

OK so I've taken the next steps:

As I mentioned I'm going to go for a combination of 'proper' fire bricks at a thickness of 76mm (3"). They were the same price as the 65mm (2.5") fire-bricks but I thought that if I want to do longer cooking and keep the heat in the oven, then these will have a greater 'thermal mass'. (I hope I'm starting to learn some of the terminologies!) I think it also means more fuel needed to heat them up but you can't have it both ways! Although they are only a little thicker, the height of the cooking floor keeps creeping up so I've got to watch it.

The Vermicrete layer has cured really well now, with it standing almost three weeks. It's is a very different layer to the standard concrete below it and doesn't tolerate being knocked. I've got to think of some way of protecting the edges ...probably some tiling. Perlite or Vermiculite? I really am no expert on this. They are very different things. My choice went with the number of times Vermiculite cropped up on WFO forums. I've successfully used it now and I'm hoping for good things! Maybe Perlite makes a tougher brew as it's hard, volcanic in origin? Just guessing.

My fire bricks arrived along with the melt extract fibres. To start with, I laid out the bricks, uncut just to check that I had got my calculations on numbers right. What with the carriage charge, they aren't cheap! I would have been OK using clay solids, upturned, had I not been able to stretch my budget. Clay solids, I understand, were the traditional material in many ovens over the years and will withstand the sort of temperatures in a WFO. Maybe a little more care needs to be taken when heating and cooling to ensure gradual changes in temperature so as not to crack them. Doing the dome in high-grade fire-brick was out of my budget, given the number needed but I hear good things about a home-brew castable dome for thermal mass so I'm confident that it's going to work. I was able to get of fire-clay here in the UK so all my ingredients are ready for the dome to be constructed.

I've thought hard about the options for either a one-part or two-part cast (with a thermal break.) Mine is a relatively small oven with an internal diameter of 690mm (27") so I've opted for a single cast to do away with the need to marry up two casts well. I'm going to put my faith in a well-mixed home-brew, with the necessary additives and to give it a good week, at least, to cure before I attempt any firing.

Setting out the uncut bricks (on a thin layer of builder's sand to get the level spot-on) was a good idea. I've done lots of drawing on paper to calculate measurements etc. but to actually start visualizing it on the stand was very useful. It gave me an opportunity to think through how the chimney/door gallery would link in with the dome. I also chalked out the different layers and it allowed me to fine-tune the positioning of the dome on the stand to optimise the use of space and to see if loading the oven would be practical.

I've been constructing an internal mould for the gallery, which I'll post some pictures of soon. I've spent some time checking my calculations, the order of the various layers and things like the internal height of the gallery and its overall width. The advice is, I understand, to have the gallery height at 63% (or thereabouts) of the internal height of the dome as this is the best way of utilizing the heat from the fire as it circulates around the dome. Also, I calculated (and tested!) the actual aperture of the door so as to be sure I could get a pizza in and things like the cooking pots etc. that I thought I might use. Embarrassing, if they wouldn't fit in! I'm turning my thoughts to door design as well at the moment.

I've had a first attempt at cutting the fire-bricks today, which I've done with a diamond disc in my grinder. It won't cut all the way through but attacking it from two sides and then cracking the last bit has worked. If you have a full-sized circular cutter and a nice big blade, then this is going to be easier.

I'll attempt to put up some pictures of the initial brick layout to show how things looked at the close of play today. This is all 'learned' stuff and several forum members have been really helpful in guiding the project so as ever, thanks to them and if you spot a big boo-boo in what I'm doing then now is the time to tell me!

[Mullster]

Hi WoodywWun looking like you’ve made some great progress so far. Just one thing I’ve noticed in your last post - you mention casting and then giving a week before curing fires. My understanding is you need to give it longer than that, and certainly need to get some of the insulation layers on before the curing starts. I think it’s fairly common to get the blanket and vermicrete on too before the fires start.

[david s]

Good pick up. Yes you are correct. The inner homebrew casting should be kept damp for at least a week to enhance strength. 28 days even better. The moisture then needs to be driven out slowly or risk damage. Allowing sun and wind to do much of the drying depending on the weather is preferable to lighting fires. Adding the insulation layers after the casting has dried is also preferable. Then once they have dried you can start the 7 fires in 7 days more safely. I know all this waiting is frustrating when you want to crack on with the job, but it’s far better than permanently damaging the oven because of impetuosity.

[david s]

“I've thought hard about the options for either a one-part or two-part cast (with a thermal break.) Mine is a relatively small oven with an internal diameter of 690mm (27") so I've opted for a single cast to do away with the need to marry up two casts well. I'm going to put my faith in a well-mixed home-brew, with the necessary additives and to give it a good week, at least, to cure before I attempt any fires.”

It is really far easier to cast the oven chamber first, remove the oven mouth mould plate, then form the gallery mould in front of the oven mouth and cast it in place. After the experience of placing the mix on the dome mould you will find doing the gallery an easier job.

[WoodywWun]

Thanks for the replies and advice guys.

I settled for a 'staggered line' arrangement for my fire-bricks rather than a herringbone pattern. I went for this as it seemed to present fewer edges that might catch the edge of a metal peel if any of the bricks moved slightly. I've had to skew the orientation of the dome/gallery to get it on my base.

I should have come back and updated this thread sooner as I've moved on quite a bit and accidentally leap-frogged some of the comments about casting posted above. Curing has yet to be completed so many thanks for the advice above.

To continue with my build...

I eventually invested ina bigger grinder/cutter (It was a great excuse to buy a new tool!) The bigger saw blade was so much better and it allowed me to do the cutting from one side so increasing accuracy. Having cut the fire bricks, the shape of the oven could be finally located to make the best use of space.

Next was to construct the form-work for the gallery (There are some pictures attached.) I mainly used MDF, with some, more flexible, 'hardboard' for the curved parts. The form-work was screwed together, with the idea that it would be easier to deconstruct this piece by piece if it wouldn't release from the home-brew easily. Also, I sealed/primed the MDF as it absorbs water and can become distorted. I did opt for a single casting, which avoided the need to have to join two castings together. Time will tell when the curing fires start if this was the right decision!

The home-brew proved to be very good to work with. Much easier than Vermicrete. Having the clay element, it made it very easy to work. One key learning point for me was avoiding air pockets. I used the sand-dome method and covered it with wet newspaper (Try to avoid a windy day!) When the interior of the dome could be inspected, it showed numerous 'ripple lines' from the newspaper (Not too much of a problem). However, some air pockets had been created during construction that I had nor known about. These needed filling from inside the dome and it was an absolute pain of a job. My advice is to work the home-brew more so as to reduce the chance of any air pockets forming between the sand/newspaper former and the cast. Take your time! The chimney was set in as I worked, with the weight being taken by the former. I've found two clay roof ridge tiles which look quite good! The height may need extending but I'll see how it all behaves before I do any more. The vent is a 110mm diameter hole formed from a drainpipe, now removed and the chimney has an approximate square section of about 140mm (5.75")

I released the cast from its gallery/door former after 36 hours. I didn't expect it to be easy and it wasn't. However, with care, I released it with a combination of unscrewing the parts of the former (the screw-together was a good idea) and drilling/jig-sawing the parts away.

After a few hours work filling missed places and fixing the air pocket issue, the dome and gallery, as one piece, are now air curing, with a tarpaulin over it to control the drying/curing process. I'm continuing planning the door construction and seeing this as an excuse to buy a riveting tool! I'm going to try and make a stainless steel 'box' with a sealable hole in the top so as to be able to fill it with Vermiculite.

More to follow, once the curing process is well underway.

[Mullster]

WoodywWun good progress!
You mentioned about how you might protect the exposed vermicrete insulation layer because it currently extends across the slab and beyond your oven footprint. What about cutting back the vermicrete layer so it is only underneath your oven and then when you add your insulation layers on top of the dome it will extend down the sides and connect with your concrete slab. (Not sure I’m describing this well but you’ll see on my thread I did something similar because I had not managed to frame my vermicrete layer perfectly so used a handsaw to easily cut it afterwards to the keyhole shape)

[WoodywWun]

Hello Mullster. Thanks for the post and your idea. I had a plan but I'll give it another think-through. My plan was to join 'hot' to 'hot' which I've achieved now (Some more pics will be posted when I get a chance.) So the homebrew connects with the fire bricks and I've got hot elements linked all around. I want to insulate in the best way so linking the Vermicrete base layer to the blanket and the dome Vermicrete is important.

My plan was to put all the insulation elements in place and then move to 'finishing' the top, exposed, areas of my base. I've a supply of clay brick solids, which I'm going to use for a final top layer, flipped over so they give a flat, no-frog, surface. I'd have to set these on a standard mortar mix as they are slightly less high than my fire bricks. That is 69mm (2.75") as opposed to 76mm (3"). So I could cut out the 75mm (3") excess Vermicrete and either have a step-down to my concrete base like you suggest or keep the Vermicrete. The Vermicrete, as you'll know, is a lot weaker, and more friable so there is a question about keeping it. However, leaving it will be bound to increase the degree of insulation on my stand.

Stuff to think about, either way, so views would be welcome!

[WoodywWun]

Here's an update on my build....

Having got the homebrew on, I let it cure/dry for about three weeks. This was by just watching the weather and covering it up if there was a lot of rain on the way. Checking it all round, I was relieved to see no cracks appearing. I'm still looking at Mullster's suggestion to look at getting rid of some of the Vermicrete outside of the dome area.

The thermal blanket has gone on now, with some doubling up on the layers over the top area of the dome. Some lower parts of the dome only have a single layer of blanket. Any thoughts? I elected to use chicken wire over the blanket. This was very easy to put on and shape. It was cheap as chips to buy and it holds the blanket down well and I hope will give a good key for the next (Vermicrete) insulation layer.

Going on advice, I decided to start my curing fires (after three weeks of air dry/cure) with just the thermal blanket on. I've achieved six of the seven fires so far and got the upper dome up to a temperature of about 550C (1,000F). The temperature of the middle dome got up to about 300C (570F) and the lower dome/floor up to about 250C (480F). I've set the fires without the availability of a door so I suspect I'm getting quite a cooling effect from the door area. I've had no cracking of the homebrew dome or gallery. My clay tile chimney has held up well, with no cracks. About 75% of the fire's smoke ends up going up the chimney with about 25% coming out of the door.

The door is under construction now and is to be a stainless steel double skinned affaire with a piece of the thermal blanket as the internal insulator. Lots to do yet like making a presentable front face to the oven. I'm going to use clay bricks, cut to size for this, I think. Decisions also on the external finish of the base. ..again, probably clay bricks. I'm still checking out an air vent to go through the outer weatherproof render coat.