In natural building, we make an effort to build with materials that are found locally. Unless they are recycled materials, we also prefer to harvest these materials ourselves, with the least amount of “industrialization”, so that we can be gentle on the earth.
Clay is a pivotal material in this whole picture. It is available just about world wide and is used for a great variety of techniques. Clay has the unique quality that it can absorb water and consequently becomes soft and sticky. It also expands a bit. Once wet it is usually mixed with other materials (sand, straw, wood chips, paper, grasses, etc.). In it’s soft stage and mixed with other materials, it is then formed into walls, plasters, floors, paints, furniture, etc. It takes a while to dry out and harden, giving the builder the chance to go slowly and make corrections on the way. Even when the building material is hard and dry, the process can usually be reversed or undone by wetting down whatever you want to change. This is different from lime and concrete-cement products, which set and become hard through an irreversible chemical process. Clay products set through a drying-out process.
In my workshops and building projects I use clay mostly for:
Cob. I mix it by foot with sand and straw to get a stiff material and use it to build and sculpt walls (load baring), ovens, fireplaces, built-in furniture and sculptural details.
Straw-clay plaster. A clay-soil slurry mixed with chopped straw. I use this as a rough plaster on straw bales, to patch holes and irregularities and for waddle and daub.
Light straw-clay. Here we lightly coat the straw with a little clay and tamp in into forms, usually between studs or poles to form interior or exterior walls (non-load baring).
Earthen plasters. Mixed with fine sand, we use it as a finish plaster on cob, straw-bale or light straw-clay walls. It can also be used on drywall and concrete block.
Earthen floors. A wonderful finished floor system made out of finely screened clay, sand and chopped straw.
clay paints. Soupy clay mixed with flour paste becomes a great, non-toxic and easy to work with paint. Especially if you buy or find light colored clays, added pigments can create a wide variety of beautiful soft colors.
Clay-slip. This used as a “glue” to adhere already dried cob to wet cob and to prepare straw-bales for it’s rough (straw-clay) plaster.
The qualities of clay are so unique and useful, that even these days more than half the worlds population uses clay as the central ingredient of the wall structure in their houses. Besides clay’s usefulness it is fortunately also incredibly abundant: You can find it almost anywhere and therefor can harvest it locally. From “mud huts” in Africa, to adobe brick buildings in New Mexico, clay is what holds it all together.
How to find and recognize clay
One seldom need to use clay in a pure form, like potters often do, but rather dig it up nearby the building site where it is often found with other materials mixed in: Small rocks, sand, silt and organic matter. This is totally fine for building purposes (and can even be advantages in some cases), as long as your “clay-soil” has enough clay to provide the stickiness than you need for whatever your building.
Clay can come in magical colors such as yellow, gray and blue, but is often found being anywhere from dark red to light orange. It is not unusual to find clay right below the topsoil. In our valley, I usually don’t have to dig more than between 3 and 10 inches before I hit a vein of clay, which is sometimes as thick as 5 feet before I would hit rock. You also can often see a vein of clay where they have cut into a hill for the purpose of road- or housing construction. Because clay becomes quite waterproof when wet (it expands or “swells”), one can also find it often around ponds, puddles and springs (as a vein of clay pushes the water up). Whenever you see cracks in the earth it is often caused by the clay in the soil; as it dried out it shrunk so much that it caused the cracking. (you see this often on old dirt roads). When hunting for clay, hang in there. If you don’t find clay where you are digging, there might be beautiful clay just 10 feet away from you. It can be somewhat random: There may be pockets of clay rather than a solid horizontal vein.
Where ever you find and mine your clay, Always consider what impact you will have on the local eco-system. Will your hole turn into a pond and do I want that? can I dig so that I create a useful flat area for something else?
So how can I be sure that I have clay in my soil? Well, grab a handful, mix it with a little water and kneed it until it has the consistency of modeling clay. If the “stuff” tends to stick to your hands it has probably clay in it. Roll it out to the thickness of a pencil and wrap it around your finger. If it can do that without too much cracking, you have clay! If you still not sure, roll it into a little ball and let it dry. (you can use your oven if you are in a hurry). If you squeeze this dried ball between thumb and index finger, it should be hard and stay together; if it crumbles there is probably not enough (if any) clay in it.
You can quickly become a “clay finder expert” by carrying a bottle of water with you and every now and then stop while walking or driving and do the above tests with the soils on the side of the road/path. Your hands will soon learn what clay feels like!
When digging up clay-soil for your building project, you are usually working with a material that can be anywhere from hard and rocky to soft and moist. If it seems too hard to dig up, soaking it a bit before you dig can be very helpful.
Sometimes we find that the clay-soil has a lot of rocks in it that can be bothersome in the building process. We then usually screen it with a 1/2 inch or 3/4 inch screen, built on a 3’x4’ wooden frame.
Preparing clay for you building project
Once you have your clay soil dug up, it is time to determine how moist you want the clay-soil to be for your particular project. There are no hard and fast rules for deciding this, and usually it differs a bit from person to person.
Here are the 3 basic consistencies that we work with:
“Soft-clay”: Soaking clay in a tub, bucket or troth overnight (or even just for a few hours) will generally make it soft and hydrated. That is your starting point. From there on you can use it for cobbing or turn it into softer, wetter mixes:
“clay-slurry”: Mixing the clay with water and a ho in a wheel barrow will create a cake batter type consistency. This can be used for cobbing as well as for straw-clay plaster and earthen floors. For earthen floors you want to make sure that you don’t have any rocks in it. You can screen the dry clay before you add more water, or send the “cake batter” through a screen (usually a little harder).
“Clay-slip”: We make it even wetter and mix it in a bucket or larger container with a drill and a paint peddle (used to mix paints; available at hardware stores) until it gets a paint like consistency. We use that for finish plasters, clay paints, slip coats on straw-bales, light straw clay and as an adhesion coat. Unless the slip is used for a finish plaster or paint, it doesn’t have to be very smooth. In absence of a drill you can even just mix it with your hands or a small hoe in a bucket.
One may choose to make these “clay-soil and water mixtures” on the wet or on the dry side. There are no hard and fast rules for this, and a lot of it depends on a variety of circumstances:
The amount of clay in your clay-soil. Different composition of your Clay-soil will react differently to the same amount of water. Not all clays are the same either!
What are you drying conditions. In hot dry weather I like to work with more moist materials then when it is humid and cold. Also, if you have a lot of help and you are building fast, You usually want your material to be on the dryer side. That way things will set sooner so you can continue on working.
How you like to work! Each person has different preferences and uses different tools for any of the natural building techniques.
Whatever your conditions and preferences are, remember that these materials are very forgiving and they are easy to experiment with. And to be sure, you can always do a test before you start working on the real thing.