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!