Monday, February 1, 2010
I want to share a bit about earthen plasters. I'm still learning, so I'll do my best to get the details right.
When most Americans think about "Plaster" they envision powdery white, chalky stuff, like Plaster of Paris, or the insides of Drywall sheets. Plasters come in many varieties, and many are not white or chalky.
A broader definition of plaster would include any mineral based coating that can be applied wet, and that cures or dries to a harder, more durable surface. Thus cement stucco could be described as one kind of plaster, and drywall compound or Spackle two other sorts.
The term "Earthen Plaster" usually describes a plaster made from earth-based materials, like clay, sand, and lime, usually mixed with some sort of fibrous matter for added strength. These materials can be purchased in relatively pure mixes, or made up from local materials found on-site. In locations with good deposits of clay, its common for the clay to be dug up and slaked in water and then mixed into the plaster.
We're using clay plasters for both inside and out, with overcoats of Lime Plaster on the outside for durability, and clay plasters and clay paint on the inside for a better vapor barrier. We don't have good clay deposits on the property, so we're bringing in bagged clay, and mixing it with local sand, chopped straw, and fresh manure.
Here's a tub of the "scratch", or second coating in a big tub:
Earthen Plaster in Tubs
This is a mix of clay, sand, chopped straw, and fresh horse manure. These two tubs, about 900 pounds each when full, then age until we're ready to use them. This will be the scratch coat -- on top of the adhesion coat, and eventually under the finish or top coat. The adhesion coat was mixed with fresh cow manure, and bringing in steaming, fresh bins of sloppy cow manure did make the place smell like a dairy barn for a few weeks.
Finish Coat Aging Behind the Wood Stove
This tub is going to be the finish coat, and differs from the scratch coat in being less fibrous (less straw), and its made with fresh cow manure. It's soft and gushy -- about like thick peanut butter. Once mixed in with the clay, sand and straw, the manure smell goes away.
Adhesion Coat is Applied
Here the first, or "Adhesion" coat is applied to the walls in lower half of photo. It is squished in pretty hard, and smooshed into all the crooks, crannies and voids in the bale wall. You can also see one of the saplings used to stiffen the wall structure, the stuffing of clay and straw packed in-between the bales, and the cardboard that is stapled to the timber frame to prevent too much mess and staining of the wood.
Adhesion Coat -- West Wall
Here's the West (kitchen) wall with the first coat of plaster in process. Wooden framing supporting the window box is still evident, and the beveled window openings can be seen here, too.
I'll send in more updates about the plastering and how it dries in later posts, but wanted to show how the process looks in each stage. Come back in a few days for another update.