Building

How to Mix Plaster or Cob Efficiently Without a Tarp

by Scott Howard, Earthen Hand Natural Building

Why, you might ask, would anyone want to mix material without a strong tarp? In Mali, where I worked recently, there are actually no good tarps at all. The best ones will rip in a day, so we have to use the ‘old’ way of mixing cob and plaster. This is a step by step explanation of how to do this.

If you have a cement pad or smooth rock to mix on top of it will be easier. If you are doing a lot of mixes, it may be worth it to use a little cement to create a dish-shaped mixing area about six feet in diameter. In general, I keep my mixing piles on the small side so I can manage them and reach into the middle of the pile while standing around the perimeter. You need a really good hoe. The best ones are a little bigger and wider than your average gardening hoe. A medium stout handle is preferred. You will also need a strong shovel. I prefer the medium handled spade with a cross-bar at the end of the handle.

Now, you need to place your materials into the mixing area in layers if you want them to ever mix together. If you have sand and clay, try placing some of each in layers before you add water. And try mixing these together for a bit before adding water. Use both the hoe and the shovel, perhaps with two or more people working at a time if you have them. Many hands makes light work, and this is definately not very light work.

When you add water, be sure to make a crater in the middle of the pile so that none of it will escape your pile. Try adding a little at a time to get a feel for how much you will need. It is really easy to add too much.

While adding water you should maintain the walls of your crater with the tool. Let it soak for a few minutes if you have the time to wait. Then, take off your shoes and roll up your pant legs, because you have to get in there! Using the hoe and your feet at the same time, mix the material with the right amount of water, all the while maintaining the walls of your crater with your hoe so that the runny stuff can’t escape your mixing area. At any time from here on you should add fibers while mixing. You can use the shovel from outside the mixing pile as it becomes more mixed, and turn the stuff from the very bottom outside into the middle where it is wetter. Doing this over and over does the same thing as flipping a tarp. If you have two or more people, it works well to have one or more people mix with the feet and hoe, while others use shovels to turn the outside material into the middle.

Another important tip is that you don’t need to worry if not all the material is fully mixed together. Sometimes, if the pile is bigger in size, you may mix only the top layers well before using them up. Then you will have to make sure the Cob or plaster you are taking to use is indeed mixed well. But the unmixed stuff stays on the very bottom, so you can use all the well-mixed top layers, and then continue mixing when you get to the un-mixed bottom part.

It really isn’t that hard once you get the hang of it!

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