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.

Wednesday, 20 February 2008

Summerhouse wall & dome mould

I've finished one wall of the summerhouse now, using some 4"x1" tanalised timber from a recently demolished fence. The fence was put up by a chap whose neighbours complained about it, and eventually had to take it down again. I bought all the wood for a few pounds as he wanted rid of the whole miserable affair. There's probably enough to do half a dozen summer houses...
Here's the first wall completed - overlapping each piece by 1":

The 100W bulb did a good job in seeing the frost off, so I made the mould for the clay dome. I made a central box out of bricks with my lightbulb in, then surrounded the whole lot with masonry sand. The bulb in the bricks should keep the sand warm and stop it (and the clay) from freezing.

This took 3 bags of builders sand (25kg each) but I'll scoop all this out later and reuse it for mortar. I could really have done with more sand, but I used all my broken bricks in the middle of the pile to bulk it out a bit. This mould will give me ~32 inches inside diameter of the oven.

Tuesday, 19 February 2008

The oven floor, inner arch

The next stage was to get the oven floor down, made of the cheap firebricks I got from ebay. I've decided to use them laid on their thin side, but because some of them were a bit worn, I've used some nearer the edge of the oven that are the 'wrong' way up with the the brick mark uppermost. I used about 1/2 inch of sand between them and the insulating block, whacked down hard with a mallet to make a solid and (relatively) level base. I drilled the middle brick to about half way and fitted a thermocouple, so I can tell what temperature the floor is at when I use it.

Next came the inner arch - the one that will join to the clay dome. I split the free bricks I'd been given in half (these are solid bricks, with about two inches of one corner cut off) and used a mix of 50% fireclay, 50% grog as a 'mortar'. This wasn't that easy to work with, and was more like moulding a clay arch with bricks in it rather than building a brick arch with mortar. I used some of the chopped corner bricks for the top of the arch to encourage the smoke upwards and up the chimney. Here's the result:

The gap between this inner arch and chimney will be filled up with my insulating mix later on to seal all these gaps. It's below freezing tonight, so I've put a 100W light bulb on a cable below the wet clay and brick arch to stop it from freezing solid. Hope this works.

Monday, 18 February 2008

Finishing the chimney

The chimney's finished off now - more bricks on top of the angle iron, then a section of double walled stainless chimney (6" internal diameter). This was definitely the best ebay find yet - £30 for about 5 foot of sections, with a nice chimney cap too. It's all pretty well used, but in good shape. The stainless chimney rests on the chimney bricks, and was surrounded by a rough wooden form that I filled with concrete. I plan to skin the inside of the bricks and concrete with an insulating mix (probably perlite/fireclay mix) so hopefully they shouldn't get really hot.It wasn't easy getting the chimney through the bitumen roof, as it needs 2" clearance around it to stop it setting fire to anything. I cut the hole in the roof using a stanley knife, using a cardboard template to work from. For flashing I used some aluminium sheet I had lying around, and sealed all the gaps with some heat resistant mastic I had left over from fixing a car windscreen a long time ago. All this took a very long time, as it was very cold and windy. The next day was lovely, so I finished the rear bit of the roof off.The chimney is held in place by the concrete at the bottom, and an excellent bodge at the top using 4 bits of builders metalwork (don't know what they're called, but they were in the same section as joist hangers in Wickes) nailed across the roof beams. The chimney was then held tightly to these using a tumble drier hose clamp. Not pretty, but functional, and nobody's going to be looking at this bit...