Invitations to all and sundry, ended up with the perfect number of enthused helpers.
Consult with Gary and Emily - decided to follow their plan of infilling a shower tray with concrete for the top base.
Borrowed a concrete mixer off a friend - orginally to turn the cob mix but this is better done by feet! so used the concrete mixer to make the base (in the ground) and the plinth (upon which the the oven was to be constructed). This could have been done by hand as it was relatively small quantities.
Purchased:
(We live rurally so I buy to ensure I have enough and save what is left for future projects)
Paid to:
What I got for free or very, very cheap!!
Looked up Trade Me and found free clay fill within 20 minutes away. Went and tested this and it balled really easily so took trailer loads and sieved that to get fine grade clay (I sieved 4x100 l barrels and we used most of this)
Calculations you need to make:
Saturday 26th Jan - Mike put down the concrete base in the ground, to the depth of one brick - possibly could have been thicker?. Made a timber surround around the stainless steel tray - much cursing to drill the holes through the steel for the bolts that attach the timber frame to the steel tray!
Wed 30 - Nick came to lay the brickwork stand / wood storage area
Sat 2nd - Friends arrived to make the cob oven.
Steps to making our Oven:
Step One - a little trade mortar laid on top of the brickwork
Step Three - panicked to find the ratio of sand to clay - looked online which suggested three sand to one clay - we ended up with three sand and two clay plus enough water to make a stiff mixture. This was for the ' heat proof' cob layer'.
Step Four - laid the 'heat proof cob layer' as a base all over the concrete platform - about two inches thick and tamped it down well and evened up the edges by using hands and boards.
Step Five - laid a large, thick but flexible peice of plastic over the base (need enough to wrap up and over the whole dome form). We didn't but would be good to have a layer of sand over the plastic before putting heavy infill on so you can wiggle with a bit more ease when removing them later.
Step Six - used bricks to make most of the infill shape (could be tin cans filled with sand), then used sand sausages ie sand wrapped up in bags or bits or plastic of varying sizes to fill in gaps andmake a uniform dome over the brick infill.
Step Seven - this was then topped off with loose sand to form a smooth dome and the whole wrapped in plastic. I think dampen the sand a bit to help it form well.
Step Eight - chimney tin put in position, 4 " back centrally from the door
Step Nine - balls of cob placed over entire dome starting at the base and working upwards. The important point here was to pack them in flat to ensure you maintained a two inch platform all the way up and to also ensure that a dome shape was maintained - important for structural integrity!
As we got near the top we just placed balls all over and then worked them in.
Step Ten - Finely chop the straw (scissors worked fine as you do not actually need much).
Then mix the clay and straw together and follow the same process as above. Just mix handfuls of the finely chopped straw, but do not overpower the amount of clay that you have.
Occasionally, when placing it on the oven you would get a bit much straw clumped together and this needs to teased out a little.
Step Eleven - wet hands a bit and you can smooth over the surface of this layer, evening out the surface by adding more as you need to - to get the final shape you want. Ours is the more traditional beehive shape, actually felt more like a big pregnant tummy but you can be very innovative!
Step Twelve - To weatherproo it we used a render of hydrated lime (which we had presoaked) with added clay to give colour (you can use up to 50% clay). Smooth a .5 - 1 cm layer of this all over. Again, you can vary the finish but we preferred mostly smooth though you can see the curved lines in it where we smoothed it out. Yet to test the effectiveness of the weatherproofing out!
Step Thirteen - decorate. This is individual - I have seen pumice, paua, rocks, shells etc. I came across these old coins and keys in the spring-clean shortly beforehand and decided they would be cool - they glint very attractively in the sun and of course we mostly arranged them in spiral /whirl sort of patterns.
Step Fourteen - admire and have a drink. This is the point at which after being very self congratulatory a huge chunk fell out of the door arch. At this point we cut a band of copper and placed in the doorway to hold the arch in place. I kind of think this is a good idea anyways but ultimately I guess good construction should do it. Our door way was quite generous as discussed previously. At this point a bit of patching of the interior roof went on using lime render as a kind of glue to stick the clay layer back up.
Step Fifteen - set the fire and light!
Step Sixteen - maintain a quiet, steady fire with solid burning wood ie macrocarpa for a slow burn. We had a bit of poplar in our mix which burnt off a bit fast and furious hence the cracks. Stay up really late and when you have to go to bed you have to get up every 45 minutes to tend the fire
Step Seventeen - in your shattered, early morning state cook breakfast pizzas. We used wraps (tortillas) which make a nice, light, easy, cook quick base. Our toppings were:
Test baking:
The next day I lit a fire which burnt for about one and half hours, having placed a grate inthe fire box and cooked bread and muffins and then put in a slow bake casserole. Bread was great, muffins completely scorched as needed to have waited and slowbake casserole was done in two hours but I left it in there and it stayed warm till dinner at 6.30 (went in at 1.oo pm). Have acquired a cast iron muffin tin and cast iron circle cake tin off a friend so will try those next. Casserole was cooked in cast iron (Dutch oven) and bread was in a quality bread tin with a lightweight tin over the top to prevent scorching.
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