rst
New Member
Posts: 24
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Post by rst on Sept 10, 2014 7:53:36 GMT -8
Hello, I made a cast core and attempted to follow the guidelines for mixing the material in Matthew Walker's youtube video. I used Oatey Fireplace Cement, Perlite, found clay (soil seems to be almost entirely clay under the top soil here), and shredded fiberglass insulation, attempting to follow the suggested ratios (maybe a little more fireplace cement than the minimum suggested). I attempted to pack it against my forms as I added it.
Though I fired it up and it worked fine, the interior sides had many small chunks crumbling off of it, and there were some seemingly noteworthy cracks formed on the top. I also noticed today that a big chunk of the top part of the burn tunnel was barely hanging on and about to crumble off. Basically, it's falling, or has fallen, apart. Thankfully I recognized this before I actually installed it in my house.
I guess I need to start completely over, but I don't know what I did wrong. I'm assuming there was a problem with my mix. Advice would be appreciated.
Thanks in advance.
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Post by DCish on Sept 10, 2014 9:19:34 GMT -8
How's your clay? I just did the jar test on the clay at my house and found that the sample is only about 25-30% clay despite appearing to me to be straight up red clay.
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Post by matthewwalker on Sept 11, 2014 6:11:18 GMT -8
Rst, I doubt you did anything wrong. It cracks and is soft, even with store bought fireclay. Your found clay undoubtedly makes it even less stable. However, it will probably still work great, unless it's so bad that it can't be kept together to get into position. I have tried some found clays that just didn't get firm enough, once dried they just sorta turned to powder, so it's possible your clay is not suitable at all as DC points out. The best thing to do if you are not sure is to make a test brick and see how it holds up. Keep in mind, using this technique even with store bought clay will not lead to a very hard mix.
If you can get it in place, take some handfuls of mix and slap it up there where the chunk was falling off. Smear more around the cracks and problem area. It needs to be surrounded by cob to hold it all together. Once you get it all dry and stabilized it should stop shedding pieces, with the exception of the feed, which will be worn down by the wood and need maintenance.
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hpmer
Full Member
Posts: 240
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Post by hpmer on Sept 13, 2014 14:39:40 GMT -8
Try adding a little ash to your mix. I use a 1:1 mix of clay:ash along with some chopped up ornamental grass leaves to prevent cracking and it works great. Don't know if it's the ash or if I just have really good clay, but the mix fires really strong and is relatively light.
I'm going to start increasing the amount of ash in the mix to see how far I can push it, but the mix comes out very sticky and is super easy to work with. I mix it with water to to the consistency of pourable concrete like you'd get delivered by a concrete truck, and then pack it into my mold, tamping it down with a 2x4 (I wet the end down before tamping so it doesn't stick to the mix).
As Matt suggests though, best to do a few test pucks with different mixes to see what works best for you. I use a tuna fish can as a mold to make mine so that they are all the same size.
Another suggestion regarding Matt's comment on the feed getting worn down, I mold a small cradle for a piece of rebar to sit about 1/4" away from the front of the burn tunnel for the wood to rest against. Saves ALOT of wear and tear on that part of the feed tube.
Good luck.
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Post by Robert on Sept 14, 2014 5:09:04 GMT -8
hey RST. What i would advice you is just make a core out of refractory moldable concrete... it is not so expensive although requires some attention, espacially vibrating it... if you want to place it in your house please just make it rigid enough so you can be happy with it for next 20 years.
there is also something interesting goin on with Karl`s geopolymer experiments but i could not go through it yet.
good luck.
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