Tuesday, 16 September 2008

There will now be a short intermission...

... while we raise two children.  And fix up the rest of the garden.  And the house.  And train the dog.  Back soon.

Monday, 15 September 2008

Just call me King Alfred...

So I've been told that you can cook stuff in a wood fired oven at a higher temperature than normal. But not this high. This is yeasted dough (not sourdough - more on my failures with sourdough soon...) with a high milk content. If I had thought about this a bit I would have realised the milk would make it brown easily. But I didn't. Instead I cooked it with the hearth at 280C...
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

First pizzas

Not many photos here, due to mild panic about getting all the timings right. Instead I just have a picture showing my cooking table before I started. The marble slab was £1.20 from ebay - sold as a particularly unpleasant occasional table that clearly nobody wanted. A bit of spanner work soon separated the good bit from the legs to give this marble slab. I'm using it to assemble the pizzas.

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!

Finished oven!

It's finally finished! The last week has seen a lot of activity - laying a floor in the summerhouse, adding table and chairs, and kitting the summerhouse out with the tools I made over the last few months. I've treated all the wood now, which hides the fact that much of it was recycled - you can barely tell now that this used to be a fence!

The floor is concrete paving blocks, laid on a dumpy bag of sand that was a gift from a neighbour who wanted rid of it. I was originally going to lay it in herringbone pattern, then realised that meant cutting a lot of bricks, so went for a basketweave pattern instead. A bit of judicious fiddling and I only needed to cut the blocks to fit round the oven circle.

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

A much better door

After my last dismal door attempt, I decided to make a new one. This one's a bit more complicated though. On hand, I had some aluminium scraps, some aluminium mesh, and a lot of vermiculite. And a nice wooden handle made from an oak branch, salvaged from my last two doors that didn't work.

I also bought a couple of meters of knitted stainless steel mesh tube from ebay: example link here. I cut two sheets of scrap aluminium (old land rover) to fit exactly in the door opening of my oven, and used the stainless and aluminium mesh to cover the sides. The whole lot was held together with a single thin stainless cable, passing through holes in the aluminium and through the mesh - stitching the panels together. I'd filled the mesh tubes with vermiculite before I'd assembled them into the door, and when it was all assembled I filled the door with loose vermiculite, packed it fairly tight, then screwed a plate over the hole I'd used to fill it. Finally, I bolted the oak handle back on.

As a picture tells a thousand words, these pictures are probably an easier way of describing it...

As luck would have it my oven entry is angled inwards, so this door forms a tight plug. I tested it yesterday and was pleased with it - from the graphs below it looks like it's at least as good as my previous Thermalite door and maybe even a bit better at holding the heat in.

It's about 4" thick, but is nice and light due to all the vermiculite. What would I do differently next time? Not use the knitted tubing - it looks good, but the plain vanilla aluminium mesh would have been fine.

Thursday, 29 May 2008

A tale of two bad doors

The first step of building a door was to fix together some 1" thick bits of oak to make a door. I framed it with aluminium channel to protect it from the heat. This didn't work (more on this in a bit...). The handle was made from an oak branch, planed on one side. I coated the lot with sodium silicate, which dried to a nice shine and looked pretty. This also turned out subsequently to be a stupid idea - I'd forgotten sodium silicate was intumescent (i.e. would swell up like popcorn in the heat) ...

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 thermalite block was still solid, if a bit more brittle than before, but the wood was badly burned around the edges. This wasn't a great example of how to build a door - perhaps more of a warning to others! Don't use wood for the door - I'd underestimated just how hot it would get round the edges. Now I need to make door number 3 - an all metal version.

The science bit...

The advantage in having lots of thermocouples is that you can spend inordinate amounts of time watching how your oven heats up and cools down. Here's the results of the first large fire I had - temperature scale is degrees C. The dome surface got up to about 500 C in this burn.
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

Flint wall finished

I was cursing the idea of using the flints after three days of hard work, but now I'm really pleased with it. It's a very cheap and appropriate way to finish the oven. These days it's an expensive way to build (due to the labour costs), but around here it's a method that's been used for at least the last couple of hundred years, as the building material was free.

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.

Using found flints

I needed to find a cheap way to cover my patchwork walls on the front of the oven. I had considered rendering, but the walls were too uneven. I also considered using bricks, but I was running low on them and didn't want to buy any more. Then I remembered that many of the cottages round here have flint block walls, as the chalky soil round here is full of loads of flints.

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!

Finishing the insulation

The next few posts will be work that I've done over the last week - I've been so busy building I've not had time to post anything on the blog. I've also added a box on the right hand side of the blog with all the costs in, after a request to know how much this had all cost. I've only described the cost of the oven - we were going to build the summerhouse anyway, so I only have to justify the cost of the oven to the family...

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

Insulation, insulation, insulation...

I've decided to build the rest of the oven as a cylinder - it's the easiest option for me, and means I can get lots of insulation in. I'm using more of the Thermalite blocks - many of these I've bought for cheap because they were damaged. They are fragile, but helpfully very easy to cut to shape. The top part of the oven will look a bit like a patchwork quilt though until I finish the surface off.

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.

This gives me a good 6" gap between the dome and the thermalite blocks at the bottom, and much more above, so this should keep the heat in well.

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

First fire!

I'd planned this for the weekend, but since the weather was so windy and wet I saved it until today. I started a small fire using a couple of handfulls of wood offcuts and paper, and kept it going in the dome for about an hour and a half before leaving the embers to cool. Thanks to my new thermometer I could see that the temperature got up to about 220C at the inner surface of the dome, and about 180C an inch into the dome. The hearth brick never got much above 80C - the thermocouple in the hearth insulation never changed from its starting temperature.

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

Fitting the oven thermometer

Before I start burning stuff, I want to know what temperature my oven is. A pyrometer was way too expensive for me, so my cheap fix for this was a PID temperature controller - the sort of thing you'd usually use to control the temperature of a kiln. The one I bought was from www.auberins.com and cost me less than £20. I bought my thermocouples from there too - pretty good for under £4 each! I bought the ones rated to 1000C - hopefully should be plenty. They've been buried in a variety of parts of the oven - fortunately I took extensive notes:

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

Filling the gaps

Thanks to being busy at work, and some pretty foul weather I've not done much on the oven recently. This meant that it's had even longer to dry out with the 500W halogen light - another 3 weeks almost. Two days ago I mixed up some vermiculite concrete (1 part Portland cement to 5 parts vermiculite), mixing it up well when dry and then slowly adding water until it was wet, but not runny. I packed all this around the dome and the chimney supports. I didn't fill the gaps around the entry arch - that would come later.

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

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.