Wood-fired oven in Dubbo
Well after a long and drawn out build my oven is just about finished. I ran out of vermiculite when completing the insulation over the top of the oven. Two weeks until the local hydroponics shop will get more in so I have to wait to fully complete the insulation, but it was thick enough to still light up a fire.
I thought I'd make this post mainly as a way to share what I had learnt. To start with, I don't have a huge income and so bought bits and pieces for the oven as funds became available. I'm kind of glad this was how it was built because it saved me rushing in and making a mistake.
I built the oven in several separate stages. To start with I partly demolished an old bbq to make room and lay the brickwork for the base. I built a brick pillar in the middle of the base to provide additional support for the oven. I felt this was necessary to begin with but looking at the way the dome sits closer to the edges of the base I now don't believe it was worth it.
Next up was the base and this is where I used the first tip I'd learnt from this forum- pre mixing the concrete and water before adding it to the vermiculite insulation. This is the best tip I've used, but it must be done right. I used a drill and paint mixer attachment and thoroughly mixed it up. It's important to really make sure all the cement is mixed in properly. I also found that wetting down the vermiculite a bit first and mixing that made for a better final mix.
With the base done I laid the oven floor over a thin layer of fireclay. While the bricks in the base have not yet moved I think they may a little as given the porosity of the vermicrete I think the fireclay may move down through it over time. To stop this I would suggest perhaps a simple layer or two of heavy duty alfoil.
I built the sides of the barrel/dome hybrid over this. I originally planned a barrel vault design and during the construction turned the bricks in a little at either end of the dome to remove the dead space in the corners. A little hard to explain I know but I'll put pictures up shortly and you'll get the idea. Now in the past I've seen various ideas by different builders about supporting the sides while building the dome but I just built some formwork around the edge of the oven and poured in insulation, which was enough to support the walls and stop them moving while I built the dome.
The dome was fun to make, easily the best part about building the entire oven. I used premixed sand,cement and lime and mixed it with fireclay at a ratio of 2:1 and so far it has held up fine to the heat I've given it (the dome was 460C tonight). To make the dome I used part of an old benchtop as a template. It was perfect for the job as it was thick enough that I only need one piece to rest the bricks on. As soon as the dome was finished I removed the small wedges from underneath the dome template and it stayed standing (phew). A few cracks opened up as it dried indicating I'd used too much water in the mix but once it was dry I mixed up a dry mix fireclay mortar in the ratio described and swept into the cracks with a fine dustpan brush. I kept the dome wet for a week after this to allow for a really good cure. No more cracks appeared and the next step was to insulate the top.
I wanted to dry the bricks out as much as possible before laying down alfoil and then insulating the dome so I bought some heat beads and lit a small fire. I managed to keep a constant small fire going with little or no flames for close to 72hrs. The temperature wasn't too hot over this time, and it drove a lot of moisture out of the dome bricks. My only worry was the moisture in the base escaping- the base of the base is four sheets of corrugated iron (obviously non-porous). So far this seems to be fine.
Next I fitted the chimney and foiled up the dome before laying down the vermicrete insulation over the top. I ran about 40 litres short and so all except a small area on top is 200mm thick. This area is about 100mm thick and you can feel a small amount of heat in this area after having the roof of the dome up to 460C for well over an hour- impressive, and it will get better as more moisture is driven out.
The only thing left to do is add the last bit of insulation and extend the chimney a little more. The position of the oven means the front of it is under a pergola. This means angling the chimney back and up around the outside of the structure. For now it rises a metre out of the dome but I'm waiting on to 45 degree elbows and some more stainless flue to complete this. As a result I've frazzled a few leaves on the ornamental grape and blackened a little bit of timber- oh well. I', looking forward to having a higher chimney as I'm hopeful it will draw a little better than the current set up which allows a bit of smoke out the front.
I just ordered the pizza peel online and should have it by Wednesday- plenty of time ahead of my first planned cook up on Sunday night
Now, a few lessons.
1. Take your time.
2. Once you have a design try to stick to it, but at the same time if problems come up, don't be afraid to adjust accordingly.
3. Use a drill and paint mixer attachment to mix your cement and water very,very thoroughly.
4. Use gloves when mixing cement in with the vermiculite and for that matter any time you're dealing with cement. I have the missing skin and have gone through some pretty extreme pain to prove it is not a good idea to use your bare hands.
5. Light small fires with little or no flame- get heat beads they are the best thing for this I reckon. And once the bricks are heat soaked to your desired temp (I went for 75-80C) trap the heat by blocking the oven door. By doing this I did not have any cracks form in my dome and had a good steady heat that forced moisture out of the dome bricks.
I'll post some photos of the oven shortly.
Last edited by Capt. Toddy; 04-20-2009 at 03:06 PM. Reason: Left out some important info
Re: Wood-fired oven in Dubbo
your chimney venting directly from your vault concerns me with huge loss of heat directly up your chimney. You don't seem to have a vault entrance or door access apart from the front, so much of your heat will be lost up your chimney.
Not so much of a problem when cooking pizzas with a fire still burning within your vault but when trying to bake!!!! well how do you keep the heat in the chamber?
Re: Wood-fired oven in Dubbo
I was wondering if someone was going to ask about this, Rastys.
There is a refrectory tile that forms the back of the chimney leaving the top of the chamber to collect heat (This wasn't in the photos). This keeps the front arch to oven height ratio correct.
As for stopping the heat escaping when cooking I originally planned to put a butterfly in the chimney to stop the chimney from working and also putting a door on the front. Now I am getting a door made that will be about 50mm thick and will have a collar around the sides and top. To seal the oven, the door will first go inside the oven chamber and then be pulled back flush with the front of the oven chamber. That's about the best I can explain it without photos. Once I have the door made I will take some photos and post to show you what I mean.
But do not fear- the door will totally seal the oven so I can do more than just cook pizza
Re: Wood-fired oven in Dubbo
when you put your door into place for baking, I would also consider making a 4 to 6" thick slightly tapered plug made from insulation material and plug the chimney at dome level. Even by having a baffle (or butterfly) will not prevent considerable heat from escaping and prematurely cooling your oven.
Re: Wood-fired oven in Dubbo
By putting the door into the oven and then pulling it back to seal it, the chimney will not be in play so a plug will be pointless, which is also why I didn't follow through with the butterfly idea. Good idea though.
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