Tuesday, 16 September 2008
Monday, 15 September 2008
It tasted good, once I cut the top off... I need to retard the bread in the fridge until the oven's cooled down enough - it's so well insulated now that it only cools about 9C an hour when it's about 250-300C!
Friday, 27 June 2008
And this was the result - the first pizza! Actually, this was the second pizza, as the first was sadly offered to the fire gods due to circumstances beyond my control (gravity and clumsiness with a pizza peel). It tasted at least as good as it looked - mozzarella, basil & tomato.
The recipe came from the Forno Bravo pizza cookery book - the best one I've seen on wood fired pizza, and better still, it's free. You can download a copy here:
This was a great moment - finally something to eat from my hours of hard work.
Next - bread!
Here's the inside, with some of my firewood stacked up (thanks to my friend the tree surgeon for that - only 2 tons left to split now, and then I can have my front lawn back). The pine table and chairs were second hand from ebay - £25 the lot. I bought the peels, while the rest of the tools were made from scrap landrover aluminium and broom handles. Here's those fancy LED switches that I built into the flint block walls - the red one works the thermometer, while the blue one... ... works a 12V halogen light fitted in the rafters, shown below. It's just the right angle to shine into the oven, so I can see what I'm cooking. The power for these comes courtesy of an old car battery, charged by a 5W solar panel fitted to the roof. The bracket for the 12V light is more scrap aluminium (see - I told you all that junk would be useful one day).
And finally, below, is the instruction manual. Much prettier than my original notes on which thermocouple was placed where. The drawing was burnt into the wood (a salavaged scrap oak floorboard offcut) using a soldering iron, then treated with linseed oil afterwards. The numbers 1-6 remind me where my thermocouples are placed - so I can select the right one using the rotary switch on the front of the oven.
Next - start cooking!
Monday, 23 June 2008
Thursday, 29 May 2008
I was pleased with this, and moved on to insulating the back of the door. I spent a long time moulding a nice plug for the door entry out of my insulating mix (vermiculite, fireclay, cement and sodium silicate), attaching it to the back of the oak with metal ties. The insulating mix dried, cracked a bit, dried some more, then fell apart when I lifted the door up.
Lesson 1: my insulating mix is good for filling cracks, and coating chimney insides. It's not good for making doors.
Door number 2 was made by using the wooden front from door number 1, then carving two thermalite blocks to act as a plug for the oven. They were fixed to the oak door using screws - the carving was very easy thanks to the softness of the blocks. This looked promising.
Pleased with this, I had a nice big fire and stuck the door in place after the coals had cooled off for 10 minutes. By watching the temperature (graphs in C and F here, C top, F below)...
... I could see that this door worked well. Notice that this graph's over 28h, and the dome surface now takes 6 hours to cool from 300C to 200C - twice as long as it did without the door. I thought I'd cracked it, but unfortunately:
- the sodium silicate round the edges of the door had erupted into a white fuzzy mass in the heat, and looked horrible
- the wood around the door had charred badly, causing the aluminium frame to come loose, and in one place, fall off
- the wood on the back of the door had warped with the heat and cracked, causing the thermalite block to move and crack as the wood bent. You can see one of the big cracks in the block below. The bit at the bottom is a loose chunk that fell off when I lifted the door.
The second large fire was earlier in the day, so easier to measure for longer without staying up half the night. The dome surface (red line) got up to 600C, while the hearth brick only got up to 400C. The vermiculite was still steaming on this burn - it's absorbed lots of water from the mortar. You can see how the temperatures rapidly drop after the fire is finished (about 4h on this graph below), then soon even out to a more steady decline. At present, the oven dome surface takes about 4 hours to drop from 300C to 200C. Most of this heat will go out of the open oven front - there's no door on this yet.
Here's the same graph with a Farenheit scale for those who prefer it in old money...
You can see how all the heat is locked up deep in the clay dome - the 2" deep probe is always the hottest, and gives up its heat to the dome surface and outer surface. The temperature below the thermalite blocks never goes above 45C. I need a door!
Monday, 21 April 2008
I got a bit nervy after the first foot or so, as I was concerned about the mortar bonding tightly to the thermalite blocks. To help the bond I screwed 2" screws into the blocks every 6", leaving about 1" sticking out that I embedded into the mortar. All finished on the masonry work now - just need to leave this to cure, then clean everything up.
Looks good in the summerhouse, especially now I've finished the front and side boarding off. Just need to do the back now, then lay the floor.
Two hours later I had this large pile of flints, just from my own garden. I cleaned them all up the lazy way by putting them in my cement mixer with a bucket of gravel, filling it with water, and running it for 10 minutes. It also cleaned the mixer brilliantly, and I'm sure the neighbours will have enjoyed the din it made. I then spent a happy hour bashing them all in half with a hammer. Goggles are a must for this, along with thick gloves.
I fitted two switches to the front of the oven, one for a light and the other for the thermometer power switch. I'll wire these up later. Then it was time for the flint jigsaw - this took a long time, and was very fiddly. This was about three days work, using a lime and sharp sand mortar. Here I am about half a day into it. The radio was essential to keep me entertained.
I like this next picture as it shows what a con artist I am on this build. The front half is all nice rockery stone and flints, the back is waste concrete and render. No point making the back look nice as nobody will see it!
The first job was to finish the walls on the front using the thermalite blocks. The front is a real patchwork quilt, as I was using damaged and broken blocks. I tipped all my vermiculite into this hollow cylinder. I used 6 bags, so the oven has 6" of vermiculite at the bottom, and about 18" of vermiculite around it at the top. Plus the 4" of insulating blocks. The top is open at the moment, to help with drying it (it's covered by the roof anyhow).
The back of the oven won't ever be seen, so I decided to have a go at rendering it. I was really dreading this, as I've tried plastering before and made a dreadful job of it. This time I used a 4 parts grit sand, 1 part cement, 1 part hydrated lime mix (first time I've ever used lime, and got a lovely cement burn on my hands because my rubber gloves leaked). It went on like peanut butter, and stuck beautifully. The lime makes an enormous difference to the 'stickiness' of the mix - it also helped that I'd coated the blocks with a dilute PVA solution too. I was so pleased with this that I considered using it on the front of the oven too. I still can't believe I did this - it was much easier than I had had dreaded.
Thursday, 3 April 2008
This will give me a nice insulating outer surface (4" thick), and I'll fill the space with loose fill vermiculite. This is a cheap and easy insulating option - you can see the bags of vermiculite stacked up on the left hand side below, ready to go in.
I'm tipping the loose vermiculite into this gap - this will eventually fill up all this space. I've filled all the cracks in the dome with powdered fireclay, and have coated the whole dome with ~1/4" of dry fireclay. You can just see this in the picture below. The idea is that this will sift into any cracks that form in future and fill them up - a good tip from Alan Scotts book, if I remember right.
Tuesday, 25 March 2008
The unsurprising part is that the dome cracked a little (early on, after about 30 min), but only hairline cracks that run all the way around the dome, and down the back of the dome. You can't see them from the inside, and they didn't leak any smoke. I don't think they'll be a problem.
This was a big step forwards - it works! I'm particularly pleased with the chimney - no smoke out the front at all - all went straight up. One happy builder.
Saturday, 22 March 2008
Another reason to sort this out before the fires - I'll end up accidently burning the notes... I also picked up a rotary thermocouple selector off ebay for £5. I've no idea where you'd get one of these from apart from ebay, but it allows you to select 1 of 12 different thermocouples.
I mounted them both in a bit of scrap aluminium (offcuts from chequer plating that I took off a Landrover several years ago). After a bit of work with sandpaper and wire wool it looked presentable.
The rotary dial lets me select which thermocouple I look at - the power supply to the unit is fed in through a cable I buried in the masonry a long time ago. I can look at the inside of the oven (thermocouple number 1) ...
Only at 42C at the moment - the halogen light was off for most of the day when I was fitting all this. Or I can look at the outside of the oven (thermocouple number 6) ...
11C in the base of the oven walls - I guess this will be an average of the outside temperature over the last few weeks as there's a fair few tons of concrete there to heat up and cool down. This closes off the lower front brick hole, and will remove the irriation of people asking me how the pizzas fit in that tiny hole...
Friday, 21 March 2008
You can see how the bit of the dome that's had the halogen light aimed directly at it has turned almost white (the patch in the middle). This bit is too hot to touch right now. I'm glad to say that there haven't been any new cracks over the last three weeks, so I'm hoping this means it's all dry.
I left this to dry for a couple of days, then made up a mix to fill the gaps between the oven entry and the chimney support. These are uneven, anything between 1/2" and 4" depending on the area, and I wanted to fill them with an insulating mix to stop heat from leaking out of the dome. The mix I settled on was 5 parts vermiculite, 2 parts dry fireclay, 1/2 part Portland cement and 1 part sodium silicate solution (all by volume). The addition of the sodium silicate and fireclay made a mix that was very sticky and could be packed together even when fairly dry, and could be moulded very easily. The addition of the cement meant that it started to set after about an hour, and hopefully shouldn't shrink and crack. It was fairly hard after 2 hours.
I used this mix to pack into the gaps, smoothed it all off, then washed it with a wet sponge to get rid of the excess.
This worked better than I'd hoped, and ended up being quite a neat job. Notice that I've filled the gaps between the firebricks and red bricks with the same mix too.
I filled all the sharp corners, and made things as smooth as possible to help the smoke go upwards. The gap between the two black bits of angle iron is the chimney. In the chimney itself I used the insulation mix to mould a nice smooth transition between the rectangular brick chimney and the metal chimney pipe:
This was a very easy way of filling the gaps, and an easy way to get a smooth transition between the brick chimney and the inner clay dome. It really smartens up the oven entrance - just need to wait for this to dry now.
Monday, 3 March 2008
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.