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Old 06-05-2008, 12:03 PM
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Default Two bad ways to make a door

I've spent ages getting round to building a door for my oven, and I wanted to build it out of materials I had to hand. Unfortunately, I botched it. Twice. I thought I'd post the details, since it might help someone else. I can't be a good example on this - so instead I'm being a warning to others...

My first step of building a door was to fix together some 1" thick bits of oak (old floor boards) 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, cracked a lot 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. It works in small thin sections, but big bits crack. A lot.

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 AAC 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 I could see that this door worked well. I'll add the graph below - notice that this graph's over 28h, and the dome surface 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!

Lesson 2: Don't use wood for the door - I'd underestimated just how hot it would get round the edges. This is probably my own stupid fault for putting it on when the oven was so hot...

Now I need to make door number 3 - an all metal version, probably filled with vermiculite...
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Old 06-05-2008, 12:05 PM
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Default Re: Two bad ways to make a door

Temperature graphs as promised above...
Attached Thumbnails
Two bad ways to make a door-burn-3-28h-c.jpg   Two bad ways to make a door-burn-3-28h-f.jpg  
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Old 06-06-2008, 12:52 AM
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Default Re: Two bad ways to make a door

Thanks Carl...

Its door making for me this weekend, and a pizza party on Saturday, so I'm pleased you've stopped me from using Wood as a door !!

I think a trip to the local steel merchant and some 6mm plate springs to mind...

Cheers

Peter
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Old 06-06-2008, 05:40 AM
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Default Re: Two bad ways to make a door

I'm glad you posted this Carl... my door mk 2 was going to be aerated concrete and wood.

Back to the drawing board!
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Old 06-06-2008, 11:03 AM
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Default Re: Two bad ways to make a door

Frustrating isn't it. The AAC is great stuff to work with, but fragile, and with the wood expanding/contrcating as it heats (or chars, in my case) it seems to put too much stress on the AAC.

My door Mk3 will be aluminium, stainless and vermiculite.
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Old 06-06-2008, 11:40 AM
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Default Re: Two bad ways to make a door

Well speaking as someone who has yet to have a completed FWO (and as a consequence is of limited experience), it seems to me that perhaps two doors are in order.

One that is really an insulative door that can stand higher temps and which is put in place at the end of a pizza session (for instance) to hold the temp for cooking the next day. This door needs to withstand temps in excess of the kindling temp of wood (aprox 500F). It can be more delicate cast refractory with maybe two handles to be more easily placed. It is used to hold the heat for many hours/overnite/perhaps days.


The second door needs to be more easily moved to open and close the oven for the insertion of food items like bread, pot of beans, pork shoulder etc. but only needs to withstand temps of maybe 350 to 400 degrees max. It perhaps could made of wood so it can be dipped/quasi-soaked in water to add steam for bread making. It needs to be light and easily moved and placed. Holding heat is necessary but it's insulative value can be significantly lower than the first door.

Just some thoughts,
Wiley
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Old 06-06-2008, 12:31 PM
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Default Re: Two bad ways to make a door

In the northern hemisphere summer when our wood basks in hot long days, and bursts into flame almost spontaneously when in the oven, don't forget those cold damp days of winter when it seems impossible to get the dome to white. I don't have one yet, but I think a metal draft door which shoots combustion air in the bottom and directs the air out the chimney is a wise thing to fabricate.

Here's the picture of Jim's excellent blacksmith made example:

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Old 06-06-2008, 12:37 PM
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Location: Nottingham
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Default Re: Two bad ways to make a door

I'm going for something similar - a steel door with some sort of insulation layer. It may be a 'box' of steel mesh on the back filled with perlite, which should reduce the loss hopefully.

Out with the welder and plasma cutter tomorrow !

Peter
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Old 06-06-2008, 01:45 PM
Master Builder
 
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Default Re: Two bad ways to make a door

dmun, The metal fireplace door which you show works basically on the same idea of the newspaper trick I went on about some time back. The one you picture is quite nice in that the slide gate regulates the amount of draft. I suspect after a bit this door gets pretty hot.

One probably should mention that this door fits in front of the chimney opening so the combustion gases have someplace to go. The other doors we have been talking about are for containing the heat and closing off an oven such that the chimney is not able to take heat from the oven itself.

Wiley
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Old 06-08-2010, 11:24 AM
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Default Re: Two bad ways to make a door

Hi, all-

I made my door out of birch plywood on the exterior, followed by another layer of baltic birch , followed by a layer of the FB insulating board, followed by a layer of cement board. Then I trimmed the plug (about 4"depth) with aluminum sheet. Works great! It chars a bit on the edges, but so far has not spontaneously combusted! Keeps the high heat in all the way down to the low.

Tom
Menifee,CA
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