Sand, straw, and clay are the ingredients of cob. But put together in different proportions and different levels of fineness, these ingredients can form the basic materials for many other building projects in and around the house.
Cob, as many of you know, can be formed into strong, load bearing walls. It can also be used for beautifully flowing garden walls, fire places, and furniture (mainly benches). Of course, if you take just the straw and leave it in its original bales, you can build straw bale walls, well known for their beauty and high insulation value. Or, if you take just the clay and amend it with a little sand and some flour paste, you can create clay paints. These paints work well on cob and straw bale walls, as well as on conventional drywall. It can provide an ideal soft, earthy finish that is easy to make and doesn’t off-gas toxins.
More interesting combinations Now it becomes even more interesting. If you take loose straw, mix it with a tiny bit of soupy clay, and toss it like dressing on a salad, you get a product called “light straw-clay.” Each piece of straw is lightly coated with clay; when tamped in between two wall studs, it sticks together and can form an insulative wall. This can be done between 2×4, 2×6 and 2×8 studs. Put an earthen plaster on both sides and you are done! A variation on this is to simply grab a handful of straw and dunk it in a “clay-soup.” Now it becomes a very wet light straw-clay, and it can be stacked up without any forms. This can make a strong, organically-formed interior wall, but it probably is not strong enough to carry the load of the roof. To come full circle, add some sand to this wet light straw-clay: this makes a very soupy cob mix, extremely high in straw. Kiko Denzer calls this “super cob;” he mixes it on a tarp as with regular cob. He has successfully used this method to build load bearing walls for small structures.
If you take just sand and clay in a 70%-30% proportion, you can create what is called an earthen plaster. The dry ingredients are sifted through a window screen, and amended with flour paste or a little bit of carpenter’s glue so that the finished plaster won’t dust off. The mixture should have the consistency of a cake batter, and can be plastered on many wall surfaces. If you replace the flour paste with finely chopped straw (the size of shredded coconut), you now have the perfect material for an earthen floor. Troweled on a hard surface, allowed to dry, and finished with several coats of linseed oil, this can become a very pleasant, durable, and beautiful floor.
The “Fix-all” formula And last, but not least, there is what I call “Fix-all.” Popularized by the Steens of Arizona, it consists of just clay with a lot of chopped straw (1″ to 3″ in length). How much chopped straw? As much as you can get in there. The more you add, the stronger it becomes when it dries. It can be used as a plaster, as well as to fill irregularities in a cob or straw bale wall. If you weave a wall out of willow branches, you can smear this material on and you’ll get a very strong wall. This is a version of a technique called “waddle and daub.”
If you count foundation rock as a bigger version of sand, you can see how entire houses can be built using almost exclusively sand, straw, and clay. But you don’t need to build a whole new house to have your own earthen home: you can renovate your existing house and use sand, straw, and clay to create earthen floors, interior cob benches, clay paints, and plasters.
No doubt there are many more variations on combining these materials and new ones are “invented” all the time. To find out more about these techniques as well as many others, it’s is really worthwhile to read “The Art of Natural Building” (edited by Joe Kennedy, Michael Smith and Catherine Wanek), a collection of articles written by professional natural builders. Worldwide, an estimated two billion people still live in earthen homes, using the earth around them to create shelter. It is hopeful to see so many people in the industrialized world rediscovering these timeless building techniques.