Cobposite

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Cobposite

Postby matt walker » Fri Feb 24, 2012 6:48 pm

Well, night before last I was pondering my stove, and the firebox in particular. I've built it all as you know from clay and perlite where I desire insulation, and it's working great. The downside is that where the wood bangs and rubs against it, the clay gets knocked and powders, so I'm slowly eating away the edges of the burn tunnel. I was thinking, "if only the straw would hold up in there, it needs the fiber reinforcement to be a complete system with high strength." So that got me thinking about fibers that hold up to heat, and I just went "DUH!!"

So, I grabbed a bunch of chopped fiberglass from my shop, mixed it into the thermal cob, and viola! Cobposite!! It works really well, and so far feels like concrete around the burn tunnel. It's holding up great. I'm really, really excited about this. I can see being able to build highly insulated burn units that have structural integrity far in excess of just the clay and perlite. I also have carbon fiber in my shop, which has an even higher melting point, but I do think that the regular glass can handle the temps it will see in there, especially encased in clay and perlite.

After I experimented, I went online, and found an excerpt from a ceramics book where they talk about reinforcing pottery and ceramics with fiberglass. It seems it's been done for a long time, and that it works very will to increase the strength of the piece. Even when firing temps exceed the melting point of the fibers, it apparently reforms after cooling and still adds significant strength.

I'm so stoked!!
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Re: Cobposite

Postby pa_friendly_guy » Mon Feb 27, 2012 1:43 pm

Sounds like you have made an interesting discovery. Are you seeing some deterioration in the burn tube because of the intense heat? Those kind of high temps can really deteriorate alot of things. I hope the cast unit is holding up to the heat for you. If it does break down over time from the heat I think you can simply cut out the unit, remove the barrel and keep the rest of the unit intack. After replacing or rebuilding the fire box and burn tunnel and tube you can re-cobb and you will be back in business. The rest of the cobb should not have to be replaced or even repaired, it does not get the same heat that the burn tube gets. Sounds like we can all learn from your experiance. Like an old friend of mine used to say, I don't make mistakes, I have Learning Experiances. Lets all keep learning.
Never doubt that a small group of dedicated people can change the world, indeed it is the only thing that ever has.
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Re: Cobposite

Postby matt walker » Mon Feb 27, 2012 5:12 pm

It's holding up just fine to the heat, although I've always held your opinion that any problem in the burn unit will be easy enough to rectify with more clay or adding bricks there without disassembling much of the stove. Where I'm using this is in reshaping and restricting just the collar at the feed. It does take quite a bit of abuse as I try to stuff wood that is too big or has a knot in there. The clay edges do get wear. That's where I'm using this harder material.

With regards to my build with clay/perlte instead of brick for the burn unit, I do think it's going to need attention annually at least. Bricks would certainly be more permanent, but my neighbor's system is all brick, and he often mentions how he can see the difference the insulation is making in mine.
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Re: Cobposite

Postby michaelegan » Thu Feb 26, 2015 2:40 pm

Matt
I know this is from 2012 and maybe I missed something but could you update this with what you've done since then? Specifically, did you use fiberglass in the casting of your batchbox and if you did, what kind of fiberglass? I know they sell batts and rolls online that are high temp but would the standard stuff they sell at the lumber/hardware stores work in a casted mold? I did try a few pucks using a 1 to 1 clay to ash ratio and they look good so far but would probably need some kind of fibers. I'm also wondering about using steel wool type fibers. I know steel weakens with heat but in this case it might not matter if the steel fibers get real hot and then cool off with each cycle since their structural purpose is more as a tensile binder than a load bearing material.

One more question-- how do you put pictures on these postings? I have a couple I've been trying to drag on and they don't seem to want to stick! thanks
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Re: Cobposite

Postby matt walker » Thu Feb 26, 2015 4:34 pm

Glad you brought this up, it's good to revisit these and update and I don't usually do it until someone asks.

So, yeah, the fiberglass is just fiber for tensile strength. I used matte glass for body work on cars and shredded it by hand. Stainless "needles" is what is typically used in commercial refractory mixes for tensile, so I think your SS wool could work well. Don't overthink it though, it won't be terribly strong either way but a little fiber will help it stay together when under stress.

Photos need to be hosted somewhere else like Photobucket, then linked to here. If you open a Photobucket account, put up your photos, then use the {img} link they provide and put that in your post here the photo should show up.
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