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.
It’s great to see small scale 3D cob modeling like at Ryan’s site Colorado Cob. I really enjoyed this model and walk through. Working out the details in small scale is so smart! It allows you consider aspects of the design and to solve spatial challenges you might not recognize in paper or Sketchup. While I haven’t done much 3D modeling myself, I definitely will in the future.
I’m trying my hand at structolite ( plaster of paris and perlite ) and fine sand as a base coat for a diamond finish. Something completely new for me, as I’ve usually used a clay alis over a earthen plaster or homemade lime plaster. I am shaping with earthen plaster then adding structolite for a super smooth finish. My goal is to get a dust-free, durable finish in the kitchen and bath areas.
I was at the hardware store looking for helpful tips from the old-timers. One was a plastic spreader for applying and shaping the plaster. One can use old yogurt lids, sans the rim, for applying and burnishing – it’s the same idea.
After using the bondo spreaders for a short time I’m convinced these are worth having in the toolbox. It’s a little thicker plastic and doesn’t break as fast as yogurt lids. This pack has small, medium and large.
Great tools for getting in those strange tight spaces.