Mechanical Cob Mixing Will Reduce Labor
Trying to calculate cob’s speed of construction, and thus cost of labor, depends largely on two factors:
- what mixing method is being used
- what rate of drying the weather conditions permit
Cob Cottage Company states that they can build in dry weather about 30 cm, or a foot of wall height per day. But Cob Cottage also mixes all batches manually. Each foot-trodden batch on a tarp yields probably around 30 cob ‘loaves’. The advantage in manual mixing is mainly quality control. The obvious disadvantage is how time consuming it is.
Many people looking to pick up the pace turn to mechanized mixing, usually in the form of a tractor or Bobcat machine. I also found an interesting article on cob mixing that says:
“We started with the big, blue Patz vertical Auger mixer. It’s called a TMR mixer (total mixed rations) and is usually used for cattle feed. We bought the small one made to be run by a 100hp tractor. It mixes up to 7000 lbs of cob in about 30 minutes.”
This is probably an alluring option for cob contractors, who might view an expensive mixer as a vital company asset and time-slasher for all future building projects. But the machine’s cost excludes most owner/builders, who generally only need to mix such a volume of cob for one building, and can probably organize various work parties to help out for free, or even workshops that help off-set costs.
Although it’s great to mix massive batches of cob at once, I’d still be conservative about it. For one, you’re still limited by the speed of building and drying, which means if you have a big batch of ready cob, but it takes a month to use it all, you can probably expect some straw rotting if you keep the batch wet. And if its too dry? Then more energy needs to be extended to reconstitute the mix.
Every cob project I’ve worked on has been done using the tarp method, so I have never mechanically mixed cob myself. Although, I think in many future circumstances I’d lean in that direction, or do both. Anyone want to share their experiences with mechanical mixing? Tractor? Bobcat? Cement mixer? TMR mixer? The more stats shared, the better!