«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
Sand that's made up of various sized particles makes a strong cob mix. The pieces can vary a lot, I think some pebbles and small rocks also strengthen the cob. Don't be too fussy about this. If in doubt, make test bricks to see if what you have will work.
If the soil from the site is short on sand, have a look around your property. Soil can vary a lot even within just a few feet. Often the movement of water in streams or rivers, past or present, has left sand deposits somewhere.
If you need to buy sand, go to a concrete supply place and ask for coarse concrete sand or 'reject' sand. In most places it's pretty cheap. For an extra fee they will usually deliver it to your site in a big dump truck, if you have a decent access road. Or
It can be tricky to figure out how much sand you'll need. It depends on the size of your house, the thickness of the walls, the number and size of the window and door openings, and how much sand you'll have to add to your soil to get a good mix.
Remember: if you plan to make an earthen floor, sand will also be a major ingredient of your floor. You can roughly estimate all this by volume. If you're getting the sand yourself as you need it, it will be less important to know the total amount. If you're having sand delivered, ask what the minimum delivery is. If that seems reasonable for your plans, order that amount. You can always get more later if you need it. If there's extra, you can probably use it for landscaping, gardening or future building projects.
CLAY Clay's job is to hold things together. Clay is one of Mother Earth's magic mysteries.
Legends of many cultures tell how the first people were made out of clay.
Clay is made of very small platelet-shaped particles held together by the friction of their surfaces. The particles are small enough to remain in suspension in water for a little while. Clay expands when wet and shrinks when dry. Some clays change size more dramatically when wet or dry than others do. Clays also differ in stickiness and ability to glue things together. Most soils that I've come across, in eastern Australia, northern New Zealand, and the northwestern United States, have had plenty of clay in them so I haven't had to import clay.
If you have very sandy soil and need to add clay, you'll have to find some. Gathering and adding clay can be a lot of work so if you have to do this, figure out a cob recipe with as little clay as you can get away with. You don't need much, especially if you can find a good sticky clay and add a minimum of straw.
Where to look for clay Often the deeper you dig down, the more clay you'll find in your soil. The sides of streams or rivers are likely places to find clay deposits because the water has worn into the lower layers of soil. Look along the sides of roads where the earth has been cut. Hopefully you can find some in a location where you can load it into your truck.
Clay is used by tile and brick manufacturers and can be purchased cheaply from them.
If you need to add clay to your soil, either mix the clay in water first, or dry it and crush it to a powder before adding it to your cob mix.
Adding the clay wet Soak clay-rich soil overnight (at least). This will help it to mix well with the water and make it easier to break up the clay. If your soil has almost enough clay, you can separate clay from some of the soil that you won't be using for the cob, and add it to
One easy way to make a pit is to build the sides of the pit with straw bales and line it with a tarp. The clay will become suspended in the water and then settle out on top of the dirt. You can scoop off this purer clay and add it to your mix. Straining through a mesh sieve helps break up those stubborn lumps of clay. When you add wet clay, you can thin it down with water until you can use the clay "soup" for the liquid and clay proportions in your recipe. You may need to add a little more water directly to your mix if it is too dry.
Powdering dry clay
If you add dry clay to the sand, you'll need to powder it first and mix it with the sand while dry, then add the water. This is a dusty job and bad for your lungs, so wear a mask. The dry clods of clay can be crushed with a tamper or stomped on with boots. It can be grated through a wire-mesh screen. Driving your car over the clay clods with the windows rolled up is a good way to avoid the dust. If you plan way ahead, the clay can be left for a year or two exposed to the weather so that the sun, rain, and freezing will help it to break down naturally.
Some silt in your cob mix is OK. Cob made with a large proportion of silt is weaker but if that's all you have, try it. Make a test wall first and see how it holds up. Your test wall could be a garden wall, a bench, or any other small structure you can think of. Silt has larger, more gritty particles than clay does. One way to help you establish whether the material is clay or silt is to make it into a dough ball and cut it with a knife. If it's clay, the cut surface will be shiny - if it's dull, it is silt. Organic matter thrives in silt. If there's lush plant growth on the soil, it probably has a lot of silt in it and would be wasted in cob. It's much better used in the garden. You may find a layer of clay under the silt.
Straw adds tensile strength to the cob.
If you have a clay-rich soil and limited sand, you can use a heavier clay proportion by adding extra straw. If you have a very sandy mix, it won't be able to hold as much straw. That's OK.
Straw is the stalks of grasses or grains left after the nutritious seed heads and leaves have been removed. It's cheap stuff, often baled for easy handling and transportation.
The fresher it is, the stronger it is, so avoid old straw that's started to decompose.
Make sure to keep the straw dry until you're ready to use it. Do not use hay. It decomposes.
Some people think certain lengths are better than others but I just throw it in as it comes out of the bale and it seems to work great. The straw will soak up some of the water in your cob mix, drying it slightly.
Get at least 10 bales for making a small cottage or for each large room. More is good.
Extra straw bales are wonderful to have at the building site. They serve as scaffolding supports, ladders, walls for clay-making pits, chairs, backrests, tables, even beds.
Eventually they'll become great garden mulch or erosion control.
If you're an ambitious purist, you can gather your own grass stalks or experiment with other plant fibers.
Water I usually use plain fresh water in the cob mixes. I have tried all of the following ideas and they all seem to work fine.
Some folks say rainwater is best.
• You can soak manure in the water and use that water for making cob.
• Some olden-day recipes say that to strengthen the cob you should add a little • bit of lime putty to the water (3%). (See page 162 for how to make lime putty.) Tarps When I first started making cob, I used to turn it with a garden fork. Mixing the cob on a tarp is a much easier way to stir the ingredients and save your back, so get a good supply of tarps. Any big tarp of sturdy material will do (7x9 foot or bigger).
Those awful woven blue plastic ones work well. They're light and the dirt doesn't stick to them much, but they do disintegrate after a while. The fibers from the shredded tarps can be added to the cob mix for reinforcing and to keep them out of the
70 Here's how to make cob Cob is mixed with a combination of stirring and compression. In my experience, the best way to achieve this combination is with feet and tarps.
Clear a few places at the site for mixing areas. The closer they are to the foundation, the less energy it'll take to get the cob to its destination. Inside the building is a good place for at least one mixing area. Leave enough room to walk around the mixing sites easily. Use your brain to minimize work. If you've imported ingredients, pile them in sensible places to streamline the process. For example, if the sand pile is two steps closer to the building (but not crowding walkways) you'll save miles of steps during the building process.
If you need to add sand or clay to your soil, you can measure out proportions by the shovel full, or by the bucket full until you can do it by eye and by feel.
Throw some soil on a tarp Break up the clods with your tamper or your shoes Adding sand If you are adding sand, shovel the sand onto the tarp and stir it into the crushed soil until the two are well-mixed. You can do this by pulling up the edges of the tarp and rolling the ingredients around on the tarp.
Adding clay If the soil you are working with is so sandy that you have to add clay, you can either add clay soup or add dry powdered clay to dry sand. Wear a mask when adding and stirring dry clay to your mix.
Stirring and treading the mix Stirring and treading well is a big part of the magic that turns your ingredients into cob.
To stir, stand on one edge of the tarp and lift up the tarp on the opposite side of the mix. Pull it towards you, turning the mix over onto itself. Don't lift the edge of the tarp at your feet or push the mix to turn it. That's too hard on your back. The more you roll the tarp, the faster the cob mixes.
Stir the mix often by pulling up the edges of the tarp, while you're treading. It's impossible to tread the mix too much, so go for it. You'll notice how it changes plasticity as you tread on it.
If your soil has a lot of sharp rocks in it, you can gradually toughen up your feet or give in and wear shoes. Smooth soled ones work best because the cob doesn't stick to them so much. Another option is to sift the dirt through a 1/2 inch wire screen. This is a lot of work and puts dust into your lungs. Wear a mask if you do this! Breathing dust is bad for you! Be aware that the clods that won't go through the sieve easily are the hardest and best pieces of high clay-content soil. These can be forced through the sieve, or crushed and then sieved. Or these precious little lumps of clay can be soaked overnight, then put through the sieve wet or mushed up by foot. You can save the gravel that you end up with after sieving for your drains, or put it back into the cob after you've finished mixing it.
When your mix begins holding together in a giant loaf when you turn it with the tarp, it's time to add straw.
When you add straw to the mix, it will dry it out quite a bit. If the mix is too dry to add lots of straw, you can either add less straw or add water. You will soon get a feel for the consistency you're after. Remember this ancient knowledge is imprinted in your cells. Trust yourself. Experiment.
Flatten the cob loaf down by treading on it. Grab an armful of straw and sprinkle some onto the mix. Step on it so it sticks into the cob a little and pull the tarp up, making a cinnamon roll of dirt-dough and straw-sugar. Tread on it until it's flattened out into a pancake again, and add more straw. Make another cinnamon roll. Do this three or four times until you have enough straw in the mix. Too much straw will cause the mix to fall apart. Try adding a lot to a small mix so you can see what too much straw feels like.
Cob too dry? A cob mix that's too dry will be crumbly, won't want to stick together very well, and won't take straw well. Add water and stir until the mix is the consistency of cookie dough. Too much sand or too much straw can also cause a crumbly mix.
Cob too wet? When you build with a mix that is too wet, you'll see that it won't keep its shape because it can't hold up its own weight. It will start to bulge out on the sides of the wall. The technical term for this is "ooging". (All cob oogs. The wetter the cob, the sooner it oogs.) If it's too wet, add a little dry sand, dirt, or straw and tread it in. If you add a large amount of dry stuff, use roughly the same ratio of dirt and sand as your original recipe. You can also dry a mix by treading it into a pancake so the air can get to it, and letting it sit for a few hours or overnight. You'll quickly get a feel for it.
Letting the mix cure Because it is so much easier to mix the cob if it's on the wet side, you may want to do it that way and let the mix sit for a day or two to dry out before you put it on the
Curing is not necessary. It's fine to make a mix and put it directly onto the wall. I suggest you try letting a batch of cob sit, though, to see if you can feel an improvement.
Avoid leaving the mix covered for too long because the straw will start to rot and stink, and lose some of its strength. If you do this accidentally, it's no big deal, put it on the wall anyway. It will stop stinking as soon as it dries.
Other ways to mix cob In the old days in Europe, farm animals were often used to mix the cob. The ingredients were shoveled into a circle around a post. The horses or oxen were tied to the post and walked round and round treading the cob mixture.
In recent times people have used various machines to make big batches of cob, and to speed up the process. Caution: if you let a batch of cob sit wet for too long, the straw will start to rot. It's a good idea to only mix big batches when you know you'll have time to get the cob onto the wall fairly soon. One strategy is to mix a big batch of the clayey soil and sand and add the straw to only the portion of the mixture that you know you can use within a week or so.
Cement mortar mixers will mix cob if you can handle the fumes and noise. As with manual mixing, the clayey soil and sand is mixed first, then the straw added. The cob has to be mixed wetter than with other types of mixing to allow the cement mixer to turn without too much stress. This means the mix may have to sit a day or two to dry to the right consistency. Straw can be added manually to the mix.
Tractors or four wheel drive vehicles can make quick work of the cob mixing job.
I've seen the process happen unintentionally in peoples muddy driveways. Simply pile the soil and sand and drive back and forth over them. Add the straw after the other ingredients have been mixed well, then drive over the mixture again until it's well stirred.