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.