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In Earthworks (July/August 1999) I described the new Papercrete material made of newspaper and cement, and mentioned a more earth-friendly recipe I prefer called "Cobwood,"which is made without cement. Cobwood is basically sawdust added to clay, lime and sand, with dry grass clippings or straw for tensile strength. I use it as a soft, sculptable building material, much like Cob- the ancient Welsh earthen recipe. Cob is a mix of sand, clay and chopped straw hand applied in loaves– the "cobs"– to build a wall. Cob is also similar to Adobe mixes in which a ratio of 30% clay:70% sand is considered ideal. These natural materials have been used, in various proportions, for many enduring structures the world over.
My Cobwood bench.
Many materials can be used.
People who are looking to use unprocessed and indigenous materials, are
experimenting with harvested clay, sawdust, woodchips, hemp, weeds,
straw, dry grass, and lime, forgoing the high embodied energy Portland
cement altogether.
The mixes I experiment with have ranged widely, with satisfactory results. I am not attempting to use Cobwood for load bearing walls, or for critical structural areas, and I am not a construction engineer or architect. However, these natural materials have been used since the beginning of man's (and woman's) shelter building attempts, and are easy to play with for anyone interested in exploring alternative ideas.
And that is the beauty of using nature's gifts...free and readily
available, unwanted and uncommercialized, with no profit in restricting
your access to grass, weeds, river or beach sand, clay or rubble. In
fact, clearing away scrap shrub and weeds is seen as an improvement to
most property!
Most of what you need can be found in your own back yard, down the road,
or in a local field or stream. The only item which must be purchased is
hydrated lime, sold in 50 pound sacks, available at most builder's supply
and home improvement retailers, or feed & grain stores, and costing
about $6-$12 .
Compost bin walls are made of a three layer pour of Agstone, using board formworks.
A professional wood chipper is used to chop and shred
materials.
What is amazing is how simple it is to use these dry unwanted materials
and create something practically for free. Clearing a weedy, overgrown,
and hilly property could provide the filler material, and back-hoeing to
make a level building spot can provide the clay solving two problems, and
recycling two ingredients at one time.
The three primary ingredients of Cobwood are mixed while wet, and all are measured by volume. I use a basic recipe of 9-3-2-1 and work the mix into a 5 gallon pail. Nine wetted, aged sawdust (soaked overnight, then drained for a few hours), three clay slurry, (thick as sour cream), two lime putty (also like sour cream), and one sand, usually dry. I1ve made mixes with sand and without, and prefer the smoother texture when sand is used, as it fills in the voids between sawdust grains; more sand can also be used. Dry chopped grasses are added last before building. This pliant material can sit overnight, or for a day or so; is like a wet cob, and can be poured, hand applied, or thrown, "harled" as the Scots do with exterior renders, (wear vinyl gloves).
My Cobwood mix is high on sawdust because it is both freely available,
and comes ready-to-use, with no chopping necessary. Redwood sawdust mixed
with gray clay produces a soft gold or honey color. When completely dry
the surface is dun colored, very hard, and only slightly friable. A nail
or stick can scrape marks in the surface only with strong pressure.
Mixing is done by hand with a garden hoe, and clay is often slurried
first with a paint stem mixer attached to a 1/4 HP hand drill. For
larger construction projects a cement or mortar mixer can be used.
The outdoor garden bench I built is made entirely of various experimental Cobwood mixes poured over a rubblestone base of broken concrete chunks. Mixes were added one pail at a time, and left to dry. Hand shaping of the back, arms and seat was not difficult, and a form could have been used to build the base quickly. The design for a bench or seat can vary widely, and could take any shape. Curved driftwood can be used for the arms, river stones can stud the base and seashells or other elements can be embedded into the bench for more personal, creative expression.
Endurance
A brick, or wall, or garden bench left completely exposed to driving
rains will begin to return to the earth. If lime was used in the mix any
rainwater will cause less damage, but building under a tree for shelter
(as with my bench), or plastering a wall for protection is best. I have
no experience with freeze/thaw cycles on my Cobwood, however cob houses
in England and Wales have endured for hundreds of years through extremely
harsh winters. These houses were rendered with pure lime plasters and
have stood the test of time. John Stahl does get below freezing
temperatures in the mountains of Leggett CA, and both of us endure long,
very wet winters, and these natural materials have stood up well.
Traditional cob will also perform well, and saves adding lime or sawdust to the mix. If you can locate good soil, or mix sand and clay together for a respectable cob mix you will have a totally free building material. Lime, cement, or purchased sand does add to the overall cost, but compared to other building methods (and the tools and equipment needed) Cobwood and Agstone are an inexpensive and easy to use alternative material.