Mr Gs Aussie Castable oven.

[mr g]

Hi, I thought I'd put my experiences down in one place for anybody interested in how I made my castable refractory oven. I have posts all over the place but it's hard find them all and fit them all together.
There were a couple of reasons I decided to build the oven out of refractory for in the beginning I was adamant that I would make it out of firebricks.
I thought that it would be time consuming and require me to get a tile saw which I don't own. The other reason is that I wanted some friends of mine to also build one and I knew that I was never going to get them to build one if it was going to be made out of brick since they didn't have the time/skill/inclination. So I made a sand mould at my place and we cast the domes at my house. We made 10 domes in around 5 weeks ie 2 per week.

Firstly I decided that we needed a 40" diameter oven which was a comprimise because a couple of the guys wanted smaller ones. The dome is 470mm high in the centre and has a door opening of 300mm H x 450mm W. I also cast a 25mm rebate for the door to sit against. The other thing that casting allowed me to do easily was make the entrance/landing area taper outwards to allow for easier access and visibility.

I used normal brickies sand to make the sand mould, the sand was damp and it was quite easy to work with and smooth into the right shape.

[mr g]

Re: Mr Gs Aussie Castable oven.

The sand mould had to be able to last for 10 castings so I made it as robust as was practicable. I decided that easiest and quickest way to toughen up the outer part of the mould was to simply apply some old acrylic paint I had lying around. four coats with 30 min drying intervals did the trick. Once it was dry it had quite a tough outer shell. I didn't want to mix cement in with the sand because I have another use for the sand once this job is finished otherwise I would have had a 1/4 cubic mtr of useless stuff that would need to be gotten rid of.

[mr g]

Re: Mr Gs Aussie Castable oven.

Once the mould was finished I tried a couple of bags of castable just on the entrance to see how the process worked and whether the whole project was going to be possible. I used plastic cling wrap as a release mechanism. Mixing the castable was quite easy although it is obvious that it is a different product to normal concrete. I mixed according to the manufacturer's recommendation of 8%-10% water by weight and this was fine. The stuff looks like it is stiffer than it actually is. I belive the term is thixotropic - that is that it is normally quite thick but becomes runny when agitated. It was obvious as soon as I started applying the castable to the vertical walls of the entrance that it was going to slump to much so I quickly made up some forms for the sides. Once this was done I applied the rest to the upper part of the entrance with a trowel and that was fine. I used a vibrating tool on the sides and manually agitated the top part. This took me 45 mins all up. The first entrance was cast without a flue insert but subsequent ones all had a stainless insert that was 6" in diameter by 8" high. Once set and dried I was easily able to lift the entrance off of the mould. Overall I was quite satisfied with how it look although it could have been vibrated a little more thoroughly.

[mr g]

Re: Mr G's Aussie Castable oven.

Next was the experiment with the dome proper. There was lots of talk about casting in one whole piece, two pieces, even five pieces. In the end we decided four equeally sized pieces for the dome would be best both for the dome strength and ease of moving the pieces around (they weigh about 95kgs each). We cast the first two pieces of the dome by using a quarter dome form. This was tedious because we had to wait for the castable to harden sufficiently before we could take it off and move it around to the next quadrant. It took five hours to make the first two pieces.
I then covered the castable in plastic and left it to harden for a few days. The first two pieces looked pretty good but expected better results as the process was refined.
The pieces were quite easy to remove and they formed quite nicely.

[mr g]

Re: Mr Gs Aussie Castable oven.

I separated the entrance from the dome to make for much easier casting. I'm not sure why I didn't think of this from the beginning. There was no need to make the sand mould in one piece. I also settled on a clever little idea to keep the form in place and with the correct slope. The side wall had to be cut to allow for an inward slope and rather than using timber stakes to keep it from bulging out once the concrete is poured we used a tie down strap that goes all the way round the form. Hopefully the pictures clarify what I'm babbling on about.

[mr g]

Re: Mr Gs Aussie Castable oven.

Here are some photos of a completed dome and entrance. I tended not to take photos during the puring process because there was a lot of dust and I didn't want to subject my camera to this. There wasn't that much to see though. We mixed the stuff in the mixer, poured it into the barrow and shovelled into the form. The vibrating tool (which was just a kango hammer drill with a spade type bit on it) worked extremely well.

[mr g]

Re: Mr Gs Aussie Castable oven.

Finished castable dome pictured. Overall there are not many things I would do differently apart from perhaps making the side walls verical as Joey Valderain has done for ease of making form work. Other than that I/we are very satisfied with the way they came out. Although I don't think I would cast ten domes at my place again.

One of the guys got busy and set his oven up on a steel frame in a few days and has got to the point where he is cooking pizzas in two minutes. He lit around half a dozen small curing fires beforehand though and there was lots of steam and water coming out of it. Now that his confidence is high with the oven he fired it so hard that his pizzas caught fire in about 15 seconds (a lesson in what not to do) but it shows that the oven can get to high temps with no advers effects. Currently it is taking him around 1 1/2 hours for the oven to get to pizza temps but we need more firings to see how quickly it can be brought up to temperature but I suspect it won't be much quicker than this.

Note, I haven't mentioned it but fyi the walls are around 70mm-75mm thick and there are 12 1/2 - 13 bags of 25kg castable in each oven.

Cheers
Mr G

[ecoscapes]

Re: Mr Gs Aussie Castable oven.

Did you design the shape of the oven yourself or did you copy another design. How thick is the a profile of the oven. Did you pour the hearth as well? I am hoping to make one of these and put it on a trailer or truck. I'm wondering if I could or should put a metal mesh or grid in the casting of the oven to make it more durable, if anyone know?

[mr g]

Re: Mr Gs Aussie Castable oven.

Did you design the shape of the oven yourself or did you copy another design. How thick is the a profile of the oven. Did you pour the hearth as well? I am hoping to make one of these and put it on a trailer or truck. I'm wondering if I could or should put a metal mesh or grid in the casting of the oven to make it more durable, if anyone know?

Hi, the dome design is my own although I modeled it on some pre-cast ones I've seen either in person or on the net. The thickness of the oven is 70mm-75mm thick. You wouldn't really want it any thicker because of the longer heat up times. As far as strength goes I don't think there'll be a problem. The castable is extremely strong. The stuff I used has small fibres in it to add extra strength. There are also stainless steel needles available that can be added to the castable mix for extra strength but I don't think it's necessary.
Check out Joey Valderrain's project, he also made his out of castable and has it on a trailer.
http://www.fornobravo.com/forum/f43/...uild-7866.html

[mr g]

Re: Mr Gs Aussie Castable oven.

Hi ecoscapes, I forgot to mention that I have poured a hearth slab. Photos to come soon.

[david s]

Re: Mr Gs Aussie Castable oven.

Mr G,
The fibres in the castable refractory don't actually add any strength at all. They are designed to burn out at around 160 C leaving minute "pipes" in the castable which water can find its way to and then travel to the outside. It reduces the incidence of steam explosions and cracking. If you want to increase its strength you need to add stainless steel needles (about 2% by weight)

[mr g]

Re: Mr Gs Aussie Castable oven.

HI David, thanks for that. Do you know what the fibres are made of?

[Johnny the oven man]

Re: Mr Gs Aussie Castable oven.

Fibres are made from Polypropalene Mr G.This is ,as you would know ,"plastic" so it burns out at a low'ish temp.

[mr g]

Re: Mr Gs Aussie Castable oven.

Just an update on my castable oven :

I set the vermiculite based of about 1.5" plus two layers of 1" cal-sil board.
I then put the oven together insulated it and tried a small test fire. There are two layers of ceramic blanket and in the end there should be around 2" of vermiculite. All seems fine so far.

[ecoscapes]

Re: Mr Gs Aussie Castable oven.

Mr. G
Thanks for the info! I am hoping to get started on my oven in a couple of weeks. I am going to cast the oven first, then mount it to a trailer later. If anyone else has any experience with castable WFO on a trailer and would be willing to and any input I would very much appreciate it! I do landscaping and usually get a odd job in the winter when everything is frozen here, but no one has any work so I going to build a WFO trailer instead!!