Friday, 29 August 2014

The 'Lean' Oven

A while ago, I started a project to build an outdoor kitchen complete with a wood fired oven. It has been a completely frivolous adventure which has kept me busy over the summer. The whole driver for this was the wood fired oven (and the need to build things, obviously).

When I came to the point where I wanted to fit the oven, I kinda stalled. You should know that these things are really expensive, the cheapest was around £500 the most expensive around £1000. I had planned to have a small one since I figured I would only use it a couple of times a year.

It was this thought that made me realise that I was not thinking clearly about the value, or perceived value it would bring me. I had bought into buying the oven because I wanted an oven and that is as far as it went.

So, I started to think how I could test if having an oven would be as valuable as I thought it would be?

So, I found some guides on building a clay oven. This made more sense - it was less money, I could fit it into the space I had created already and it looked fun to do.

Most of guides said how they had 'dug up some free clay' which obviously makes this sort of caper much more attractive. Not much of that around here, so it would be a cost. I found a local company selling clay, the cheapest was around £6 for 12 kgs. A rough calculation suggested I might need about 5-7 bags. This is mixed with sand which is easy to get hold of and relatively cheap so I ignored that for the time being.

It also had a hidden cost in the floor should probably be made using firebricks. It doesn't have to be but if normal bricks crack from the heat then I would loose my entire investment. I would need 15 for my base and they cost about £4 each locally. I can get them cheaper on the Internet but the postage offsets this so much that it probably is not worth it.

This would give me the option to keep it if it worked, which is a fair presumption since I would probably be following instructions from someone else rather than making it up as we go along. So, to test this idea with the option of keeping it would be around £100 - significantly cheaper than buying one that's for certain.

My thinking was a little clearer now. As great as this was sounding, it's still limiting my options and carries significant risk too. It was certainly a do or die operation - if it worked then awesome but if not I would only have a base and a whole load of clay to get rid of.

Then I saw this....

Yes, it's a fire back. I removed some from my house a while back and put them to one side since they are heavy.

I saw an oven however and with some bricks I had laying around I quickly bodged something that 'looks' like an oven:

But does it behave like an oven? The guides all have a ratio between the height of the oven and the height of the opening. You need enough to ensure a good airflow from the bottom to the stoke the fire. Let's test what we have and get some feedback to see if we need to tweak our design or if this is a non-starter.

It lights and keeps going at a steady pace, it never roars but the wood is questionable and probably damp. I stick a BBQ thermometer and just leave it for a while, monitoring the temp every now and again.

The max temp the oven reached was about 150 deg C. Way short of the 600 you would expect from a 'real' oven but this was with dodgy wood and zero insulation, which is key to getting the temperature up and holding it. Whilst it was going, a lot of smoke come out of cracks and gaps. I solved this by simply packing them with some damp sand, which worked a treat and was nicely temporary as well.

What I also observed when I lit the fire was how the smoke came out of the front, which looked like it was causing a bottleneck. I also noted that the heat losses where significant since it got really hot, so insulation would certainly play a part in how well this works for actually cooking things.

So a further improvement would be to create a chimney just after the dip with a column to funnel the hot air up and out a little easier and hopefully improve draw. This is a one time shot since I would have to cut the fire back but I happen to have a spare so this is not as bad as it sounds. If I ruin it then I still have this one as a backup - so I still have options. If I break the first one I would need to be clear on my commitment since if I break the second one too, I have no more options with my current experiment.

Before I do that, however I could try to insulate it and make sure I can get higher temps. Just a covering of sand would probably help, maybe with an oven fire blanket too which has pretty good insulation value. Using some decent wood would be more representative and pretty low cost (or zero cost if my neighbour lends me some from his wood pile).

This is still a work in progress but it is the thought process that I thought was interesting. I was unsure what value my 'feature' would provide me so I wanted to test it and get feedback. I looked around to see what I had that could get me feedback as fast as possible. I then review what I now know and see if it fits with what I thought. I learn from it, tweak it and iterate to get more feedback.

I still have to prove that the oven will provide me with 'value' a.k.a yummy pizza - but a pile of wood and a few friends will answer that. So my BBQ 'showcase' will get me buy in to create a production version of my prototype.

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