it's a machine to "mull" foundry sand (which is basically sand with about 8-10% clay added).
The sand is given a sort of squeezing/mixing action which seems ideal to mix cob.
I'm planning to build one - I do some aluminum casting as a paying hobby - and it would
be relatively simple to adapt it for cob mixing; perhaps a toothed roller, rather than smooth,
and a heavier squeeze?
Howdy Gregg, help me understand what you are trying to do so I can learn. It seems that these machines sort of grind or pulverize the sand /clay mixture. Is that what you are trying to do? Or do you want to mix the cob with water in one of these?
I'm thinking that I can add sand, clay, straw and water to the mixer, and save myself the labour
of mixing the cob "by hand" so to speak.
I usually work as a two-man team (with my son), and would like a machine that could keep us supplied
with cob, same as a cement mixer would keep us supplied with render on a conventional construction
Umm.. These may work.
I think that the turnover motion should be enhanced a bit..
You may find that the straw tends to get tangled into EVERYTHING. It may make the process impossible or at least difficult at best.
I've used cement mixers to mix the sand and soil together, but I ALWAYS dumped it out and mixed in the straw manually (by foot).
As you can see there are all sorts of sizes and designs but basically it is just a big mixer with two shafts with paddles that intertwine and move the material from one end to the other while it is mixed.
First off, what are you making? an oven or a structure? for small scale projects I might consider a small mixer machine, but if you are trying to make a building, forget it. I have tried: cement mixers, mortar mixers, rototillers, small tractors, and excavators. Hands down the excavator wins. in a matter of a few hours you can have a 10 or 15 cubic yard pile of cob to last you weeks of building. It is by far the most efficient mixer, and does a pretty good job of it. nothing to get straw all wound up on. and it can deliver the cob right where you want it. And, you could get a pond out of the deal if you mix in a pit near the building. And it can dig your footings, do all the site utilities, move big rocks, lift beams, and more.
The next best mixer is a small loader tractor, ideally 4 WD. You can lay out the various components using the bucket, then with a combination of driving over it to mash, and the bucket to turn can make a lot of mix in a reasonable time. It takes practice to get a consistent mix.
Mortar mixers can work, but you have to make the mix more on the wet side, and the straw can wind up on the mixer. I've had best results adding the straw and clay first, making a wet mix, then add sand to stiffen it up. Patrick Henneberry of Cobworks on Mayne Island, BC uses a mortar mixer.
Gord and Ann Baird built their home in Victoria BC with a rototiller. My experience was not as positive as theirs. It's hard work, you still have to pick up and move the material by hand, long straw can get wound up on the tines, and you are slogging in the cob all day.
Anything that you have to bucket the material in, and wheelbarrow it out means carrying your entire building multiple times. I've foot mixed entire buildings, so I know what it is like. Plus, a diesel tractor can run on SVO if you want to improve the sustainability factor.
Hope this helps,
I have just finished up a small cob cottage and I used a mortar mixer and thought it worked pretty well. In the beginning I mixed the clay and sand in the mortar mixer then dumped it out onto the tarp and used my feet to mix in the straw. I was really worried that the straw would bind up my mixer but I found that if I made my batches a little smaller the mixer did just fine. If you have access to a mixer it will save you a lot of time and energy but I must say that mixing with your feet is also really fun.
I have seen mullers used for small rammed earth mixers, but not cob. I think the difference is the moisture content. A muller would gum up as soon as the cob got wet enough to be cob. It would be fine for pre-mixing sand and clay and pulverizing the clay for faster cobbing by hand. Sand and unsifted clay through the muller, then sift to a tarp to incorporate water and straw.
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I have been reading this thread about "Mechanical Mixer for Cob" and I have done some research. I live in China and I found these 2 types of mixers. See pictures below. Can anyone tell me which one would probably work better for mixing cob?
The only problem I have with cob in a cement mixer is that the straw tends to get caught in the paddles of the mixer. I save my mixer for making lime plaster, where the length of the straw is much shorter.
For mixing cob, I form a shallow depression in the earth (our soil has a high clay content), then add straw and water. I drive the bobcat into the resulting puddle and spin around in circles until the cob is of desired consistency. I think this method would work well with any machine that is not prone to getting stuck--an ATV or pickup truck with 4-wheel-drive should be able to do the same work. It reduces the mixing time to almost nothing, so I can spend more time applying.
I've seen numerous home made foundry sand mullers, and I can see where the straw would get hung up on certain designs. One design I remember hearing about involved a cement mixer and cannon balls. Having some really large and heavy iron balls to force the material around and no plow to get straw tangled up on. I'm not certain if this would do a complete mix, but if you are limited on manpower then this could be an option to get the materials together and perhaps just need a bit of fine tuning to get it right. I'm not really sure how bad a plow, paddle, or other device would work or if any designs would be less likely to catch straw and bind up.
The good thing with this is you don't need perfect shape or diameter. You could probably find someone willing to attempt to cast them for you for a reasonable price, and it wouldn't matter how spherical they come out. Something roughly round should tumble and push material around, and do so substantially if it is heavy enough. It might be worth checking out.
Hi - Is anyone interested in continuing this conversation, or maybe it's moved to another thread?
I built a small cob studio last year (2018) using an excavator and forming system with some other folks. It sped up the process dramatically, but there is still a lot of room for improvement. I would love to talk with other folks who are interested in this topic and have any ideas about it. A friend made a short four minute video of the process as an intro...
A paddle mixer is very good for cob.
Its action is slow blades that wipe the material around. Sometimes used to mix plaster.
Thy are self emptying as well, because you simply opne the side and its swept ot though that door.
John Daley Bendigo, Australia
The Enemy of progress is the hope of a perfect plan
Hi John, thanks for this info! I've been looking at paddle (mortar) mixers and was thinking of renting one to try it out with cob, as opposed to a cement mixer (for those who need a breakdown of the difference):
My suspicion is that a large paddle mixer to make the cob, and a light utility loader to load the cob into the forms is the best way to go. As you can see in the video above that I posted, the excavator can both mix the cob and place it in the forms, but the action of dumping the cob into the forms backwards (or underhanded) rather than forwards (or overhanded as a front loader/light utility loader) is awkward and hard to do with accuracy.
John, can you recommend a specific brand of paddle mixer for cob? Also I am more interested in electric mixers, even if they are 220V, than gas as I don't like the pollution they generate on the job site.