Woody
valid member
Posts: 49
|
Post by Woody on Jun 6, 2013 20:52:34 GMT
I was originally planning a 80cm internal diameter for the oven, however due to the rear wall it has reduced the space I have to work with. The following images shows a template with two circles. The outer circle is a diameter of 80cm and gives a 18cm gap to the rear wall. The inner circle is a 70cm diameter that gives a 23cm gap to the rear wall. The measurement to the rear wall is essentially going to dictate how much clay insulation I am able to use. I am guessing that more insulation is better, therefore I should go for the 70cm diameter oven. Does that sound right? Thanks, Woody
|
|
|
Post by cannyfradock on Jun 8, 2013 12:29:40 GMT
Woody
The minimal diameter of a wood fired oven .....for it to be perfectly functional (in my view) is 60cm so you have lots of scope. There doesn't seem to be an exacting figure on the thickness of the clay walls. I think that between 4 and 5" is enough. There will be a lot of water naturaly in the clay and you have to add water to be able to "tread in" enough sand at a ratio of 2 parts sand to one part clay so the clay doesn't crack when drying. If you add enough sand and get the ratio right this will minimalise the cracking.
I recently tried to build a clay oven with a 3" layer of clay and sand and then when still wet I laid a further layer of 2" cob.....the same mix but with hay added for the thermal layer..... that was simply too much depth of clay in one hit and because I couldn't put heat into it for 6 weeks.... (it was inside a building and had to wait 6 weeks for it to be lifted with a fork lift to put it outdoors) ...the natural drying process within a shaded room, meant cracking appeared on a large scale....it could have been parged up with more clay but I decided to take it down.
Next time I will lay a 2 1/2" to 3 " clay mix and build it outdoors. After a few days I will scoop the sand form out and start augmenting the drying process with curing fires. After a few days I will put on the cob layer....perhaps 2".....and continue with the curing fires.
My biggest mistake on my last Clay/Cob oven build was that the clay was very pure......and the weather was very cold so it became difficult to tread in the correct amount/ratio of sand needed to avoid cracking.
I've posted ALL the good, bad and ugly from my last Clay build so as you may get some tips from it (I like posting "glory pics" but sometimes we need to share the problems we have had).
So..finally, in answer to your question....you can go for larger diameter......but you need to create a way of stopping any rainwater gathering behind your oven.
Terry
|
|
|
Post by cannyfradock on Jun 8, 2013 13:55:52 GMT
Woody
The minimum diameter of a wood fired oven .....for it to be perfectly functional (in my view) is 60cm so you have lots of scope. There doesn't seem to be an exacting figure on the thickness of the clay walls. I think that between 4 and 5" is enough. There will be a lot of water naturaly in the clay and you have to add water to be able to "tread in" enough sand at a ratio of 2 parts sand to one part clay so the clay doesn't crack when drying. If you add enough sand and get the ratio right this will minimalise the cracking.
I recently tried to build a clay oven with a 3" layer of clay and sand and then when still wet I laid a further layer of 2" cob.....the same mix but with hay added for the thermal layer..... that was simply too much depth of clay in one hit and because I couldn't put heat into it for 6 weeks.... (it was inside a building and had to wait 6 weeks for it to be lifted with a fork lift to put it outdoors) ...the natural drying process within a shaded room, meant cracking appeared on a large scale....it could have been parged up with more clay but I decided to take it down.
Next time I will lay a 2 1/2" to 3 " clay mix and build it outdoors. After a few days I will scoop the sand form out and start augmenting the drying process with curing fires. After a few days I will put on the cob layer....perhaps 2".....and continue with the curing fires.
My biggest mistake on my last Clay/Cob oven build was that the clay was very pure......and the weather was very cold so it became difficult to tread in the correct amount/ratio of sand needed to avoid cracking.
I've posted ALL the good, bad and ugly from my last Clay build so as you may get some tips from it (I like posting "glory pics" but sometimes we need to share the problems we have had).
So..finally, in answer to your question....you can go for larger diameter......but you need to create a way of stopping any rainwater gathering behind your oven.
Terry [/quote]
|
|
Woody
valid member
Posts: 49
|
Post by Woody on Jun 9, 2013 19:52:16 GMT
Thanks Terry, sound advice as usual. It's nice to know I have plenty of scope whilst considering the size.
You've answered some other questions I had been mulling over as well. I think I'll follow your 'next time' example when it comes to building the clay dome.
I am hoping to build a roof once I get towards the end of the build. Nothing to fancy, maybe some fence posts, a bit of postcrete, cladding for the sides and perhaps some roof felt etc...
But before any of that good stuff, I need to lay the firebricks and order some clay.
I've read the advice on a hearing bone layout, so if I feel brave I might give that a go.
Thanks again, Woody.
|
|
Woody
valid member
Posts: 49
|
Post by Woody on Jul 8, 2013 8:42:22 GMT
After much delay I managed to find a couple of hours this weekend. Here's a couple of pics: Oven floor: Wooden former: Starting the arch way: Bit of cracking on the top bricks, should I be worried and redo this section?
|
|
|
Post by swatson on Jul 8, 2013 8:59:48 GMT
Woody you've stretched the page try deleting the large picture using the 'modify' button.
|
|
Woody
valid member
Posts: 49
|
Post by Woody on Jul 8, 2013 9:18:22 GMT
try now ....
|
|
|
Post by swatson on Jul 8, 2013 12:37:40 GMT
Much better, looks excellent!
|
|
Woody
valid member
Posts: 49
|
Post by Woody on Jul 8, 2013 16:23:16 GMT
I may have to redo this top section as it doesn't seemed to have dried well. It does look like it will just tap out.
|
|
|
Post by cannyfradock on Jul 8, 2013 19:24:06 GMT
Woody....nice hearth!! Not sure on those top voussoirs (cut arched bricks).......the only reason they would have cracks is if they had been disturbed before they had set........if they are loose then change them. If they are solid and it's just the drying out on the face of the bricks then leave them.......whoever built the arch ...will know if they need replacing or not. It looks to me as if the "shims" you put underneath your arch former were too thin.....they should have been "folding wedges" so as when dropping the arch former it should have "released" from the arch.....I don't know if this is the case, but I had a similar situation in my "Corris build".......I only put flat shims underneath my arch former.....then built a corbell effect chimney and laid an old land drain pipe on top of this... As soon as the clay pipe was fitted I tried removing the former....everything was still green. If I had used folding wedges it would have been perfect.......I didn't....I used flat shims, so the arch former caught on the the arch during removal.......and the bloody lot fell down.......no big deal.....I just built it up again. ....back to your arch. It doesn't look so bad. The perps are full and it looks like a solid arch. ?? Terry
|
|
Woody
valid member
Posts: 49
|
Post by Woody on Jul 10, 2013 20:11:53 GMT
Terry, I think you're right regarding the shims. I definitely should have gone for some thin strips of wood. I'll have to be careful removing the former. I might try unscrewing the former face from the middle batons and see if that helps. At least that will prevent dragging the whole former through the arch. I decided I wasn't happy with the voussoirs in the end and replaced them tonight. They tapped out quite easily. I also fitted the chimney! After scouring Wickes, Homebase, B&Q and ScrewFix on the weekend for a duct brick similar to Nixie's , I drew a blank. Plan B was to use an old clay pipe that I found in the garden some time back. Here's the pics:
|
|
|
Post by cannyfradock on Jul 11, 2013 8:35:16 GMT
Woody
A wood-fired oven needs something like 5" internal diameter pipe about 1 meter long......but.....I've also used ceramic flower pots with the bottom cut out....and clay pipe like yours. They all work. Without the long length of the pipe it means you will have less draw and you will get a bit more smoke coming out of the arch until the dome warms up (when fully fired)......
If your arch former is still too tight you could always screw some large screws or eyelets onto the face of your former for ease of removal.
Terry
|
|
nixie
WFO Team Player
Posts: 144
|
Post by nixie on Jul 11, 2013 8:36:00 GMT
Mine was called a cavity duct I think. Was out side with the bricks at our local B&Q extra.
Your plan B looks great, probably close to the right size too (think my duct is perhaps a little large)
|
|
|
Post by swatson on Jul 11, 2013 10:23:10 GMT
My chimney is about 17" high and 4 inches at the inner diameter £12 from keyline. Attachments:
|
|
Woody
valid member
Posts: 49
|
Post by Woody on Jul 11, 2013 19:39:29 GMT
Thanks all for the chimney suggestions. I guess time will tell now and hopefully very soon A friend has a spare bit of metal flue that I could have from when he installed a wood burning stove in his house. As an experiment I might try fitting it to my clay pipe and see if it makes a difference. I took the wooden former out with no damage to the arch. I opted to unscrew the former face straight away and not mess about. The shins were no use! ... Lesson learned for next time! Next job the sand former and clay - yay I'm not sure I'll manage it this weekend as we have various commitments. Does anyone have any advise on how much time to allow for (A) the sand former build (B) applying the first clay layer? Cheers, Woody
|
|