Hi Julie: Indeed, Ianto Evans and Linda Smiley stayed with Jaki and me at Earthwood for a few days several years ago, and, while they were here, we built together a cordwood masonry panel in our garage, about 4 feet by 6 feet. Instead of our regular mortar, described below, we used
cob as mortar. We did insulate the mortar joint with sawdust in the central cavity, as usual. Ianto and I had to go out and search for good clay to build with, as there is none at Earthwood. We found it where a contractor friend was doing an excavation about15 miles away. The cob mix was about 80% coarse sand, 20% good quality (quite pure) grey clay. We used chopped hay/straw as reinforcing binder. An easy way to do this is to come down on top of a flake of
hay or straw with a rotary
lawn mower. Presto, nice 2-inch pieces of chopped reinforcing binder. Linda finished the wall with a thin coat of cob without the reinforcing. This panel has held up very well, but it is well protected with a 3-foot overhand and it is well off the ground. Chapter 20 (More Cordwood and Cob) of my book Cordwood Building: The State of the
Art, goes into more detail about this project and others, and has a picture of Linda and Jaki working on the panel. You can get it at our Earthwood site, www.cordwoodmasonry.com or, maybe, win one here on Permies.
Over the years, Jaki and I have developed a very successful cordwood mortar using portland cement, lots of builder's (hydrated) lime, sand, and soaked softwood sawdust. The mix by equal volume (shovelfuls) is 9 mason's sand, 2 portland cement, 3 lime and 3 sawdust. The softwood sawdust needs to be passed through a half-inch screen and completely soaked overnight. The soaked sawdust acts as a cement retarding agent, preventing mortar shrinkage cracks. If the right sawdust is not available, use a commercial cement retarder, such as Sika Plastiment, Daratard-17 from W.R.Grace, or equal, usually three ounces per wheelbarrow load. Do not mix sawdust and commercial retarders.
Finally, I think the use of cob and cordwood together - which we call "cobwood" - is only appropriate if you have a source of good quality clay close top the building site. Once, we did a
workshop in North Carolina using North Carolina red "clay" in a cob mortar. The wall went up beautifully, but we learned from our hosts that later, when the wall had dried, the cob mortar was crumbly. Probably not the best clay. We have had no such problem with the wall we built at Earthwood.
