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!