Drying the dome

I left the clay dome overnight, then scooped out all the sand and broken bricks the next day. Although the broken bricks were only there because I'd run out of sand, it made the job quicker as they were easy to remove. I put my 100W light bulb in the oven, and left it for 5 days to dry. The bulb kept the temperature above freezing, but the clay never really got warm. The newspaper soon dried though, and I peeled it off after a few days. The next few pictures were all taken after 5 days drying with the 100W bulb - about 5-8 C temperature outside

You can see the lumps of clay that I used to build the dome on the inside face - it was impossible to get this smooth, because I can't reach in that far and the oven entrance is only 13"...

Notice that as the dome has dried it's pulled the arch and cracked it. Around the base of the oven dome I could see the whole dome was shrinking inwards, but staying as a single intact piece. The whole dome could contract over the flat firebrick base as it dried. The cracks between the base of the dome and bricks show about 1/4" movement after 5 days.
The cracking was worst around the outisde of the brick arch. Stupid of me - because the arch couldn't shrink as the clay dried, it cracked instead. Oh well, I'll fill these in later.

Not known for my patience, after 5 days drying I decided to substitute the 100W bulb for a 500W halogen work lamp. After another 5 days the dome was very dry, and felt warm to the touch all the way round (even at the base, away from the light). The clay was a very different, almost white colour now, with no darker regions (although the photos don't show this well). The arch had cracked a lot now with the extra drying, although all the bricks were still very firmly fixed, and I couldn't extract any with a lot of wiggling about.

The cracks around the arch (there were three) had opened up to about 1/2" on the outside, although hardly showed at all on the inside. A couple of small surface cracks had opened up near them - but no sign of these on the inside at all.

The base had probably moved about 1/2" inwards all round - so that's about 1" shrinkage in total across the dome. The dome was intact apart from the cracks around the arch - no other cracks anywhere else. I'm pretty pleased with this.

I filled all the cracks with the spare clay/grog mix that I'd bagged up from when I made the dome - a very quick and easy repair that soon left things looking good again. I wet the dome around the cracks before doing the repairs to make sure they all stick. The repair's obvious right now - you can see the darker fresh clay.

I'm pleased with that - far less cracking than I had dreaded. The 500W lamp has gone back in, and I'll add some insulation to fill the chimney gaps in the next few days.

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.

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.

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.

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...

Infuriating arch

So, building an arch should be really easy if you plan it right. So, I mocked it all up on fibre board, marked all the bricks, cut it to size using a saw, and went to use the cut board as a prop to hold all the bricks in place. Did it work? No.After I couldn't get the last brick to fit (three assembly-disassembly cycles, two hours work...) I lowered my support by about 1/2 an inch, then used bits of wood to hold everything in place. Jamming lots of broken brick into the big mortar gaps helped too. The arch itself took 4h, and was the single most infuriating bit of building I've ever done.The arch is 1/2 a brick wide, and is separated from the brick ledge behing it by a gap of 1/2 inch, which I'll fill with an insulator later on. The ledge behind will support the chimney, and sits on two sections of angle iron which rest on the surrounding brickwork. A second angle iron supported ledge sits about 8 inches behind this first ledge, which you can see best from behind...

That gap in the middle will form the chimney, hopefully. I'll fill the second ledge in next time.
You can see the front ledge is lower - hopefully this will encourage the smoke to go up the chimney, not out the front arch. The angle iron was the only recycled bit here - from an old bed frame which I salvaged a few lengths from. All that surrounding brickwork will vanish soon, so I've only bothered cleaning up the bricks on the front.

In the dark...

It's unseasonably mild right now - about 12 C during the day and only 7-8 C at night. This is letting me get some masonry work done, albeit at night with the halogen light on. I've started on the oven entry, using engineering bricks. Much head scratching is going into this - I want the chimney to be supported on 4 lengths of angle iron (on the right side of the picture above). I want the oven dome to be completely separate from this masonry with a thermal break, which I'll probably make out of clay/sawdust mix or vermiculite. I reckon some of the ovens I've seen on the web leak a lot of heat up the chimney through the masonry joining the dome to the chimney, plus I'm worried about the thermal expansion of the dome cracking the masonry.
The plan is to have an arch at the front (on the right hand side above), with a span of angle iron and bricks behind it. Another span of angle iron and bricks will sit on the left hand side, above the entry to the oven dome. I'll fill all the gaps in the insulation mix afterwards. This plan may change - it's changed three times so far...Eventually all the messy masonry on the sides will be hidden by the cladding - the brick entry is set 2" forward from the insulation blocks to allow room for it. I still have no idea and no plan how to do this (render? more stones? pebbles?), but ignoring the problem is working fine for me at the moment.