Hi Gregg,
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,
Misha Rauchwerger
www.builtinbliss.com
