Thursday, 21 February 2008

Finally, the oven dome

Finally, it's time to build to build the oven. I'd decided to build it out of a 50% clay, 50% grog mix - for speed, and due to the expense of refractory mortar and firebricks in the UK. I also don't own a brick saw, and didn't want to buy one just for this project.
Here's the grog I'm using - its a 0.5 mm + 1.0 mm firebrick grog, more like a fine grit than the powdery grog I used in pottery class years ago:
Before I started I covered my sand mould with a couple of layers of wet newspaper to make it easier to get the sand out after I'd finished, as per Kiko Denzer's book.
I mixed 25kg of dry, powdered fireclay with 25kg of grog, added 1kg of ballclay (had some left over from another project) then mixed well when it was dry. Next I added boiling water - it was about 3C outside today and the grog and fireclay were about the same temperature. Adding boiling water got the whole mix up to a nice warm temperature, and allowed me to work with it without freezing my fingers solid. Oh, to live somewhere warm...
My cunning plan to mix the wet clay/grog failed impressively. I mixed the dry mix up in a large plastic tub (supposed to be a small garden pool, bought as an end of line item from the garden centre, and usually used to bath the dog in - good news for the dog). I planned to mix the wet mix up in this using my feet, but stepped in and got stuck within a few seconds. I managed to finally extricate myself after about 10 minutes of pulling - this stuff is sticky! - and after that, used a spade to mix it. If you're going to tread your clay - make sure it's a thin layer, not a foot deep! This was the finished mix, a good stiff but mouldable consistency that you could pat into balls after 20-30 pats.

I moulded this around the clay in a roughly 4 inch thick layer, adding balls of clay about 6" diameter each time like bricks in a wall, then moulding them to shape. Here's what it looked like when it was about 3/4 finished:

Once I'd finished it a gave it a good hard whack all round with a 4x2 bit of timber to compact the mix, then smoothed it all down with my wet hands. Finally - the oven dome was finished! This was a definite milestone for the project.

The wires you can see on the left side of the last pictures are three thermocouples. all embedded in the dome clay. One is actually inside the oven (in the sand right now), the other 1 inch from the inside surface , and the last one is on the outside of the dome (under the pimple you can see sticking out).

The clay dome took about 4h from start to finish to build, and used just under 75kg clay and 75kg grog. The internal diameter of the oven is 32", with 15" height (9.5" door height), and the oven has 4" thick clay walls. Now we have to see how this dries - I'm expecting some cracks, which I'll fill in later if I have to.

1 comment:

Anonymous said...

Looks fantastic - i'm very jealous!