|
|
Post by martinpolley on Jun 2, 2015 1:14:24 GMT -8
So we're buiding a cob base for our rocket-fired mud oven. There's one part of the wall that is pretty thick. What I'm wondering is: can I build rocks into the middle of the wall to save myself some work (mixing cob) and sand (which I have to buy and bring in)? Have any of you done this? How much rock can I get away with without weakening the wall? Can the rocks touch, or is it better to leave space so the cob can connect between the two sides of the wall? It currently looks like this:  |
|
|
|
Post by josephcrawley on Jun 4, 2015 8:11:06 GMT -8
I have put all kinds of garbage in cob beer cans, car doors, shoes, bottles, rocks you name it.
|
|
|
|
Post by martinpolley on Jun 4, 2015 11:03:22 GMT -8
Thanks josephcrawley! I now feel pretty confident about tossing some rocks in there and seeing how it works  |
|
|
|
Post by PNW Dave on Jun 5, 2015 8:33:32 GMT -8
I'm not speaking from experience, but I imagine it would be best if the rocks did not touch. Maybe slap a big mound of cob down and press a rock into it and repeat for the rest of the "course". Then cob over the rocks and repeat with another course. Just a thought.
|
|
|
|
Post by martinpolley on Jun 5, 2015 9:35:20 GMT -8
PNW Dave That's my gut feeling too. Make sure there are lots of places where the outside cob connects with the inside cob. That's what I'm going to try, we'll see how it goes
|
|
|
|
Post by satamax on Jun 5, 2015 19:55:34 GMT -8
I'm by no means whatsoever an expert on cob. I've never done any.
But, i think there's another way. Chose your stones like you were dry stacking theses. So they would kind of hold on top of each other, may be rocking a little; this is usualy taken care of by the weight of the layers on top. And just use the cob as a mortar. Just like most of the traditional techniques of making mortared walls all around the world. You would end up with a stone heavy wall, and lots of mass to hold the heat.
|
|
|
|
Post by patamos on Jun 5, 2015 22:20:40 GMT -8
Yes to all of the above. With cobb it is best to dry stack the rock work for the first few layers out of the ground to prevent wicking of moisture up into the structure. If you have good overhead protection the need is lessened somewhat, but splash from the nearby ground can travel a few feet. For the areas that you have already cobbed close to the ground you could set a piece of slate up against them to divert splash. Or make sure and give them a good lime wash or coat of oil or sodium silicate. Have you thought about making a little sitting bench? If you exhaust out the bottom of the oven deck you could work in a little horizontal flue run. Winter bliss... Happy embedding  |
|
|
|
Post by martinpolley on Jun 7, 2015 5:27:09 GMT -8
satamax Actually, I was planning on just dry stacking the stones to form the base. But the rocks we have here aren't nice, flat, easy-to-stack stones. They're mostly rounded river rocks. They don't stack easily, and they don't look that great if you do manage to stack them (at least, they don't when I do it ). So what I'm going to do is just embed the rocks inside the wall. I've started and it seems to work pretty well so far. And storing heat is not really an issue in this particular case—this cob is around the rocket core that will be powering the oven, and insulated from it by plenty of vermiculite or perlite. Thanks for the suggestion though! patamos I've got a couple of layers of dry-stacked rock, and under that, lots of stones, gravel, urbanite, etc. which should keep the whole thing dry. The start of the cob is still pretty low though. We didn't build a roof yet. Unless we get some totally freaky weather, it won't rain again here until October, so we've got time to figure that part out. I guess we can just make the roof wide enough so that splashing won't be an issue. The bench is an excellent idea! We'll have to make sure the roof is big enough to cover that too though… Here's what my embedded rocks look like at this stage (excuse me showing off, but I'm really enjoying the process!): 
|
|
|
|
Post by shilo on Jun 7, 2015 9:59:56 GMT -8
no problem to add rocks to cob (actually it's an advantage if the cob is for thermal mass). just don't forget to wash them in water or clay sleep bucket.
|
|
|
|
Post by martinpolley on Jun 7, 2015 10:50:47 GMT -8
shilo Ah, good point. I did that with the firebricks so they wouldn't suck the water out of the mortar. But does it matter so much for rocks that don't absorb water?
|
|
|
|
Post by shilo on Jun 7, 2015 11:25:57 GMT -8
are they clean with no dust?
|
|
|
|
Post by martinpolley on Jun 7, 2015 20:21:01 GMT -8
shilo Hmm, I didn't think about dust .
|
|
|
|
Post by satamax on Jun 8, 2015 21:33:07 GMT -8
Martin, there's a way to stack pebbles with less "mortar" Tho i don't know it. You could also break the rocks with a sledge hammer. That's may be less work than making tons of cob Ahhhhh, that's more like it! |
|
|
|
Post by martinpolley on Jun 9, 2015 5:55:27 GMT -8
satamax That's a useful approach to know about—thanks. I'm trying to avoid concrete because of the embodied energy thing. Also, the plastered cob aesthetic is one that my other half an I agree on. Stacked or mortared rocks, not so much Making cob is a lot of work, but it's very satisfying. Still, if I can reduce the amount of cob I need to make and still achieve the same result, then it's even better
|
|
|
|
Post by patamos on Jun 10, 2015 7:54:03 GMT -8
Old beat up bricks and scraps of urbanite can serve as wedges between the round rocks. This further takes up space with (less) cob still as the mortar.
Most people are looking for full bricks, but the scrappy chunks are very good for this kind of shaping...
|
|
