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Post by patamos on Apr 30, 2015 17:59:50 GMT -8
Hi Folks,
So i am getting ready to lay up the last shaping layer of cob on a brick/cob heater body... and am beginning to second guess my use of burlap in the belly of the cob. I know it offers superb strength. And when i set it 8-10mm from the surface it works fine. But when i set it closer to the surface there has been the odd instance when it delaminates from the cob underneath. This happens in fairly low temperature zones. So i am a bit puzzled.
My guess is that the burlap expands and contracts in different ways than the cob surrounding it. But this is just a guess.
Any insights?
thanks
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Post by Donkey on May 1, 2015 15:33:49 GMT -8
Did you soak the burlap in clay slip first?? Could be because the burlap is dry. ?
Also, if the burlap is between 2 layers of cob, it would tend to hold the layers apart/keep them from becoming one piece. The strength of cob is in the fact that it's one monolithic piece. Letting the stuff dry completely before adding on and/or putting something between layers prevents this.
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Post by patamos on May 1, 2015 20:42:46 GMT -8
Thanks Donkey.
Ya i soak the burlap in clay slip, wring it out, and then make sure to seat it into the inner layer of cob by smearing it with hand or wooden trowel. I usually then add the next layer of cob over top shortly after with the notion that it will bind better to the soft wet substrate than a dry one. I also only use the fine thread loose weave (usually brazilian) coffee sacks… so the cob material can mingle in the spaces between the jute fibers.
The more i reflect on it, i've never had delamination so long as there is 1/4" or more cob over top. But less than that and ya, a year or two down the road it has loosened up here and there - in both high and low heat locations.
I think i am second guessing whether to use it at all. Except maybe around the lip of the bench.
I wonder if the burlap is less resilient than cob, so it wants to move less with the thermal cycling?
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Post by Donkey on May 4, 2015 8:22:10 GMT -8
I don't really understand the need for the sack material. I'm of the opinion that the straw is already doing whatever the sacking is intended for. I would maybe just add more straw to the mix where you would be tempted to use the weave.
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Post by patamos on May 4, 2015 9:54:48 GMT -8
Ya that is exactly the conclusion i am arriving into. Thanks for the notion.
On a related note, i've taken to using your (basic) recipes for thermal cob in high heat zones. And wonder: at what temperature application would you transition from fibre to fibre less?
thanks
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Post by Donkey on May 11, 2015 9:39:11 GMT -8
I add in a little chopped fiber, pretty much always. It helps with forming and drying and just chars away when fired. So far I've seen no problems at all with including a little sacrificial fiber in even the hottest areas, I just wouldn't go big on it there.
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Post by patamos on May 12, 2015 19:09:11 GMT -8
Ya it makes for an interesting trade off. The fiber enables more clay content with out cracking, which should hold up better in the heat. I'm familiar with the various ratios mixing floors and brown coats... But the heat factor is relatively new for me.
So iIt is good to get your take on things.
Thanks
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