To get a good cob mix you need to use sand or a aggregate of some kind for all the slippery clay plates to attach to. If sand is not available on site you may be purchasing ‘plaster sand’ for 25 dollars a ( small ) truck load. Crusher fines are a little cheaper but less fine. I’ve found a good resource for those that have recycling centers near by – glass. This glass has been purchased at a store, the contents consumed and brought to the recycling center. They pulverize the jars and bottles and produce a sand like mix – If you don’t mind handling a material that everyone in town has had their lips on. My recycling center has a blend of super fine glass, like fine sand, and a 1/4″ to 1/2” glass chunks. A truck load is 5 dollars and they load the truck.
I’ve used this for straight up cob mass building, infill plaster and sculpting. I added some sand, horse manure, straw and clay to create a infill plaster and sculpt-able mix. There are some ‘rocks’ of glass, but none too large. If you were inclined to reveal the glass look, plaster on let dry and sponge down to reveal the sparkly glass bits.
I’ve only been cut once so far by the glass, but definitely wear gloves and some safety glasses.
No collection of Making Cob articles would be complete without “BobCob”. That is: Cobcat, Bobcat cob or tractor cob. Although foot mixing cob can be a tactile treat for your feet, after the 100th ton of mud one thinks to themselves: Is there another way to do this? Well, our friends at Clay Sand Straw have demonstrated that there is another way. Here is a video of Kindra mixin-mud in Hunt, Texas – BobCob style.
I’m fascinated by new ways of making cob. Especially ones that take some of the back-breaking labor out of the equation. Years back I visited HUG, or Hunt Utilities Group and produced a short video of the experience. Ryan Hunt gave us the tour and explained their vertical auger mixer. Which can make quick work of cob creation in mass quantities. Recently I checked in with HUG to see if there were any new innovations with mechanical cob mixing. The following is a email I received from Ryan…
The magic to it is the vertical auger. Tumble mixers make balls of it. Mortar mixers and others with blades running through the mud catch the straw on the leading edges where it builds up huge sometimes. Horizontal augers can compact the material against the side. Skidsteer or tractor mixing is inconsistent. And feet just get tired by the time you get a couple tons done.
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.
Then we bought and modified a little orange mixer hopper meant for spray-able, fibrous mixes. We modified it with a vertical auger to replace the paddles and added a variable speed motor. It has been used extensively to make small batches for the cob sculpting on campus.
With those two, we decided we needed something of an in between size that could be more portable than the big one, but make enough cob to be useful on a job site. So we commissioned the construction of one about 1/4 the size of the big blue Patz. That is now powered with a small tractor.
These are all successful, but are now underutilized. We may be willing to rent some out. Last year, we made several tons of cob for a local builder.
Then we have a gravel screener to get the rocks out of the sand and the clay (somewhat). Cob with only small rocks works well for being pumped and trowel applied or spray applied on a wall. We have a nice little pump and air compressor set up that can do that job, too.
We’d love to have some help documenting “commercial” cob. It’s not the main direction we are headed at the moment, but there is a lot that we can share with people.
Mixing cob… mesa style. This is a sloppier version of cob that’s good for infill, shaping and scratch coat plasters. Raw earth, straw and water. I’ve been using this for everything lately. In the video it shows a sloppy mix as it comes out of the mixer and the same mix after it has sat out in the sun and the wind. It stiffens up quick. I’m a fan of the cement ( cob ) mixer. It allows you to do several other things while the barrel is spinning. Electricity can come from variety of sources. Of course using power from the sun to make cob would be natural building nirvana. Even better… the human power of friends and neighbors.