The Cob Oven Saga, part II

29 05 2014

This is a continuation of The Cob Oven Saga started a few days ago……

IMG_0339IMG_0340Having finally finished the brick arch entrance, we then began building the support base for the sand mould that the inner clay dome is supported by.  Without a strong support, it’s impossible to build a dome, and wet sand is what is normally used.  The only trouble is, we could not get the sand to stay vertical for the first 100mm of its rise, and I ended using scraps of zincalume sheet metal lying around to hold it up.  I was going to remove it before stacking the clay onto the mould, and thought the better of it, I reckon it can be removed along with the sand through the door when the time comes…..

One important thing to mention at this stage…….  it seems universally accepted that the height of the entrance door should be 63% of the internal height of the dome (ie, sand mould..), and that the height of the dome should be 63% of the diameter of its base.  For this particular oven, that turned out to be 400mm high, and 635mm in diameter, but if you’re going to build one too, the dictating factor will always be the height of the entrance.  To make sure we got the height right, I used a spare piece of 6mm dowel which I marked with a pencil 400mm from one end and inserted into the sand at the centre point of the oven.

Serge, whose advice on building these things is second to none, thinks that as his technique evolves the dome should be thinner than what he has been doing for the past dozen years or more.  I’ve looked at youtube videos too, and frankly, nobody does exactly the same thing, and really, as long as it works, who cares how it’s done?  As we had started to build the dome at the base some 75mm thick (which is what Serge did nine years ago on the original dome), I decided to try a hybrid method where the wall thickness decreases as one reaches the top…….  Will it work?  Watch this space……

IMG_0344The important thing to understand here is that it has to be thick enough so as not to collapse, but thin enough to dry properly to the middle of the wall.  Clay shrinks when it dries, and it will shrink even more when it will be fired; and if it shrinks unevenly, you could have a collapse on your hands.  At least, this oven, being on the southern side of the house is in total shade all day at this time of year, and even though it is unseasonally warm here for this time of year (another 28°C day today…. in May?) I’m hoping for a good result.

Serge tells me the trick is to watch for the outside to start cracking, whereupon the inner sand mould can be removed to allow the interior to dry.  patience is the word of the day, even if Glenda’s birthday is a mere 13 days away, and the Pizza Chef is chomping at the bit to fire it up and make some pizzas!

IMG_0347IMG_0349Serge also expressed concern that as the clay shrinks it might separate from the brick arch which will not shrink……  so this morning, upon uncovering the beast from the tarp that stops the heavy dew from slowing the drying process, I decided we should cover the bricks with cob, and make this cover overlap the neck of the oven in an attempt to key the oven to the arch.  it also stops the bricks from moving, which they did as we tamped the clay against them.  It’s all experimental, but there you go.

IMG_0350We made the cob mix pretty wet so that it would key into all the brick arch’s irregularities…  the bricks I’m using are pretty rough in texture, as are my cuts!  Now we wait.  it’ll be a few more days before we can finish it, the sand has to come out, and the inner dome must dry before we can cover it with cob, and then the outer render.  In the meantime, Alessandro will just have to keep making pizzas in the AGA…

Continued here……

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