Uses of Cob with Earthbags

We employ cob for a myriad of purposes. We use it as a sculptural medium for fleshing out dormered windows, gutter systems, built-in furniture, and floors, and as a base layer over a dome designed for an earthen or lime plaster finish. We refer to cob as the duct tape of natural building due to its invaluable variety of uses (Fig. 14.11).

In the Southwestern US, cob is also called sculptural, monolithic, or coursed adobe. The emphasis is on the mix being a stiff soil with a clay content of about 30 percent, with the balance made up of well-graded, coarse sand and as much long straw as it will hold and still be sticky. The reject sand we get from our local gravel yard forms the basis of our cob mix, with the addition of a few extra shovels of high-clay-content soil and gobs of long straw. A well-graded

14.9: Shooting mud onto an earthbag wall with a plaster sprayer.

coarse sand is optimum for an earthen building soil, but for a sculptural medium, we can use a wider variety of soils, as long as the clay content is high enough and of a stable quality. We like to make cob in a cement mixer, adding copious amounts of straw. The mixer will literally spit out loaves when the mix is done. The cob loaves should feel firm but not dry (Fig. 14.12).

To provide a key-in for relief work, drive nails into the wall so that they protrude at opposing angles. Installing extra-wide chicken wire cradles around the fan bags surrounding an arched opening

14.11: Sculpting the secondstory dormers around the arch forms of the Honey House.

provides built-in pockets to fill with cob for building out drip edges, or for interior relief work. Leave the surface of the cob rough to receive additional plaster layers.

We first learned about cob in a book entitled The Bread Ovens of Quebec that examines the historical construction of clay ovens in Quebec, Canada, since the mid-17th century (Fig. 14.13).

14.12: To mix cob loaves stiffer than the mixer, we throw a chunk of the mix into a wheelbarrow lined with straw and knead it like bread, jabbing the center of the loaves with our thumbs.

14.13: Making our first cob oven was the inspiration for building the Honey House earthbag dome.

14.12: To mix cob loaves stiffer than the mixer, we throw a chunk of the mix into a wheelbarrow lined with straw and knead it like bread, jabbing the center of the loaves with our thumbs.

14.13: Making our first cob oven was the inspiration for building the Honey House earthbag dome.

180 EARTHBAG BUILDING

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