Friday, 13 August 2010

Phase 3 - Optional Extras

This section will be written very soon and will basically cover:
(i) the insulation (a necessity really),
(ii) the chimney,
(iii) a brick arch, and
(iv) a rendered weather-proof shell.

Tuesday, 10 August 2010

Phase 2 - Build the Clay Oven

1. Marking out & planning
I wanted to ensure (1) the dome of the oven sat exactly over the arch and (2) that I sited it in the correct position on the sub floor. So I marked it out first with a simple nail-and-string compass.

From the centre nail I first marked out: (i) the 27" diameter oven floor, then (ii) a further 4" of structural clay wall, then (iii) a 4" insulation layer, and finally (iv) a 1" rendered shell.

2. Structural clay over floor
This was my first attempt a mixing the structural clay mix and bedding it together. I followed the book to the letter. Every clay is different and the book helps you adjust your mix to the correct consistency.

The easiest way to mix it is on a tarp with your feet. You want to be working the sand, clay and water into each other. A cement mixer might seem like a good idea here, but its more about pushing the elements together with pressure than mixing and rolling them which will basically tear them apart. There is something in the squeezing and stomping that works the mix into a flexible clay-like mud that is really easy to work with.

One of the tests it mentions to ensure you have the correct consistency is the 'Drop Test'. Basically work a handful of the mix by pushing it into itself over and over until it feels almost plasticine-like. Then drop it from shoulder height on to a hard surface. If is pancakes or crumbles the mix is either too wet or dry respectively. If it plops into a neat cow-pat-like mount then its about right.

So once I was happy with the mix, I just took handfuls of it, worked it on my hands for a short while to get the plasticine-like feel the pushed it into the sub floor (or other clay bits I'd already done) to form the structural clay floor.

3. Lay the fire bricks forming the oven floor
The next step involved laying the fire brick on the (still pliable) clay floor I'd just made and bedding them in around their edge with more of the structural clay mix. This also allowed me to position and cement in the two base bricks for my oven's brick arch entrance.

I left this base to dry out and harden as I knew the next stages would require a solid base. There was some cracking of the clay mix at the corners of the fire brick area, but I just patched this up with more mix.

4. Building the sand form of the oven interior
Now this is where it gets really interesting ! And you have to do this step and the next in the same day - so check the forecast and pick a guaranteed dry day.

Basically you have to build a giant sand castle dome in the shape of your oven's interior. This is done with just damp sharp sand.

IMPORTANT NOTE: You have to use sharp sand for this and the structural clay. Its grain shape locks it together with other grains. Building sand is not suitable as its grain shape is too rounded.

From memory I think the height is about 60-70% the width of the oven. I did wonder if I would need a template for this part, but I managed to get a really good dome just with the hand and eye.

Don't under estimate how much sand is needed for this shape. For my 27" oven I reckon I used about 30-40% of the entire cubic meter/tonne bag.

5. Building the structural clay ovens walls
You need to do this straight after the dome if finished - on the same day. I did these two steps on my own and it took about 9-10 hours in total. I had no idea it would take that long and wish I'd enlisted a mate or two to help.

Basically, you mix up a large amount of the same structural clay mix you built the floor with. Then little by little to work handfuls of mix into each other around the sand form. Do not work the handfuls into the sand dome form. Make the walls about 4" thick and eventually when your hands are numb and your back aching your have a perfect, beautiful clay dome - and its worth all that work!

6. Cut out the door and empty the dome
I left the clay dome for about a day and a half before cutting the door out. I wanted it to settle and harden a little but not to the point where it would become brittle.

According to Kiko Denzer's book Building Your own Earth Oven the ideal height of the door is about 63% of the oven's interior height. So with my calculations in my head, and a large knife I cut the door shape, with some trepidation I can tell you ! But that all went well so bit-by-bit I dug out the door shape to reveal the sand done interior.

Once this was done, and I could see the sand dome inside the clay oven wall shell, I dug into the sand a little around the door edges to see how the clay interior felt. Feeling solid enough I proceeded with emptying the oven of all the sand inside. Again, I felt a little nervous after all the hours I'd put in, but nevertheless confident that it would hold.

... and it did. Fantastic!

So I let it sit for a few hours, then lit a small fire inside to help dry it out.

And that is basically it - Everything from here on is optional - the insulation, chimney, brick arch, exterior render. When I get a mo, I'll write these up as a Phase 3.

What I should have done at this point, once the oven was fully dried out, was light a full size fire inside to ensure it got to the temperatures I needed (i.e about 500+ Celsius). But my judgment was clouded by my achievements and I wanted to finish every off and get it just right. So after a week or so I move on the the chimney and brick arch.

Phase 1 - Build the Wood Bunker

1. Foundations and first courses
After measuring the plot, I dug 6" deep foundations a little wider than two-brick's width and filled them with concrete.

Once dry, I laid two courses only where they would be visible, and allowed just a single course for the proposed arched entrance to the bunker. For the far side and back I choose to opt for breeze block straight on to the foundation, so save money.

2. The walls
I choose to breeze block and render the walls for economy, simplicity and speed.

Notice, the notches I cut out of the end walls to sit the concrete lintels in. Working with, cutting and shaping the breeze blocks was, as they say, a breeze!

3. Lintel supports for bunker roof/oven floor
After then deciding to build two further posts for each lintel, I cemented them in.

I gave it a good few days to dry out, then added the brick arch* and bunker roof/oven floor by laying the concrete slabs. The observant among you will spot the 'deliberate mistake' - I miscalculated the length of the slabs and came up short, so had to finished the very end with breeze blocks - no big deal!

* The build the brick arch I simply cut two bit of wood the the shape I wanted the opening to be. Nailed them together. Placed this on two thin sticks to raise it slightly. Laid the brick arch and remaining wall blocks on top. Let it to dry, then kicked out the brick arch. Voila!

4. The insulated oven sub-floor
As guided by the book Build Your Own Earth Oven by Kiko Denzer and Hannah Field, I had to now make an insulated, structurally sound sub-floor for the oven. After building the edge with more breeze blocks the inside was filled with a combination of clay, sawdust and empty wine and beer bottles. The liver took quite a bashing the night before to empty all of these, I can tell you!

As I still wasn't 100% confident about using only clay (and contrary to the books guidelines), I added a little bit of cement to the insulation mix - but I know now that this really isn't necessary.

5. Render face of breeze blocks

This was the only thing I felt I'd have trouble with, so I enlisted my father-in-law Mick. This was definitely the right thing to do. In the time it took me to render on quarter of the back, he had done all the front and used about a sixth of the materials I had used. I was quite happy at that point to sit back and watch a master at work.

Think the render mix was 5 sand to 1 cement. Each face was battoned first with 2x1" wood before having 2 coats of the render mix applied. Once complete, the finished faces were battoned for the remaining faces to be rendered in the same way.

And that was basically the Wood Bunker FINISHED !!

How long did it take?

  • From start to finish, probably about 3 months - but this was very drawn out with me working only perhaps once a or twice a week, sometimes only for an hour or so at a time
  • If I condensed all the time I worked on it down to one continuous period I reckon, it probably took about 7-10 days (probably 60% bunker, 40% oven).
  • If I did it all again, it would definitely be quicker

Material & costs

I came in a little over my £250-300 budget. Most of the cost involved was in the materials for the wood bunker or base. Here's a rough breakdown:
  • Tonne bag of Sharp Sand
    40 breeze blocks (used about 24, I think)
    4 bags of cement
    2 1800mm concrete lintels
    Total £180 from Wickes
  • 24x 25mm deep Fire bricks
    Total £50 online
  • Cobble stones for decorating top of bunker
    Total £60 from B&Q
  • Most of the footing bricks I had already, so these cost nothing.
  • Various tools and extra materials
    Total £50
  • MOST IMPORTANTLY: The Clay was FREE - dug up from my garden !!

An overview of the build

Basically, I had very little knowledge of Building, or Clay Ovens. So I started out with some very basic requirements:
  1. In addition to the oven I would need somewhere to store wood - so I thought I'd incorporate a bunker into the design. So the main build comprised of two phases:
    Phase 1 - Build the wood bunker

    Phase 2 - Build the Clay Oven on top of the bunker

    Phase 3 - Optional extras such as insulation (a must have really), a chimney. brick arch and rendered weather-proof shell.  Not got round to writing these yet, but will do soon.

  2. That I wanted to cook multiple, decent-sized pizzas - say about 10-12". So I opted for a 27" diameter oven floor rather than the standard 22.5" outlined in the book.
  3. That I didn't want it to fall down so I would (i) plan it out on paper, and (ii) probably over-engineer it, just to be safe
  4. I only wanted to spend about £250-300 total

Where did the inspiration come from?

So many places really ...
  • Lots of Mediterranean and UK holiday nights out in restaurants with wood fired ovens
  • General love of eating and drinking outside
  • Playing with fire, brick, food, you name it !
.. but the point at which I knew I could build my own oven came when I finished reading a book on the subject: Build Your Own Earth Oven by Kiko Denzer and Hannah Field. You can buy it from Amazon for about £15, but its priceless. Its the best reference book on any subject I've read - striking exactly the right tone of what is helpful, optional, essential.