In this and following posts I go more into depth with some of the different steps in my cob building project. The first will be the actual making of the cob mixture.
But let me start by making two general remarks
i) There is always plenty of ways to achieve a certain goal. Building with cob is extremely forgiving. Virtually every “mistake” can be saved. So in particular cobbing can be done in very different ways. In the end the final structure is the reward. And of course all the fun you had on the way building it. Here I just want to describe what has worked for me. So far.
ii) Working with cob is ideal for working as a team and in a group of people. Cob allows for so many different designs. Obviously, inspiring each other and combining different ideas into the same building can be a very creative process. More than that, there are also a few very practical advantages of working in a group. For example, the actual mixing of the cob is usually done in groups of 2 people.
I have been working all by myself so far. And to do so, this required to reconsider some steps, which I want to detail here. So this maybe for you, who is standing alone with the task to mix a portion of cob. Or it is for you, who is ready to take the challenge – against all odds – and make a cob building all by yourself. 8)
As I mentioned in an earlier post, my garden ground is fine sand, no clay. To work in cob I had to get some clay from outside. Luckily, a garden neighbor with fine clay under the top soil duck out a hole for a septic tank, and I received about 3 cubic meters. Some initial testing gave me the impression that it contains more than 50% clay. The rest is silt and sand and top soil. But also a few flint stones that have the tendency to break into sharp pieces, when I dig with my spate in the pile. But even broken bottle fragments or rusty iron tubings can be found in my pile of clay. “Nocking on wood” I have been able to spot them all before any major injury.
The best practice of course would be to dissolve the clay in water – liquid enough – to strain the clay through a suitable mesh thereby filtering stones and other unwanted additions like roots and potentially dangerous incorporations like glass fragments. But it does not require much water to achieve the wanted consistency of the final cob. And straining the clay would easily make it too liquid for further processing.
About 6 to 8 l of water is usually enough. But it depends very much on the weather. The clay pile is covered with a tarp. So for the clay rain does not make a big difference. But sand and soil are open to rain. And therefore it matters a lot, if it has just been raining, or if there was a period of drought before making the mix. I always try to start with as little water as possible. Although it is tempting to add more water than needed, as mixing becomes much easier with more water. Obviously.
All the extra water I first add to the clay in a separate big bucket. Preferably already well before use. This way, the clay becomes sucked in water and is easier to mix with sand and soil.
I mix one (overfilled) 10 l bucket of top soil with 2 buckets of fine sand and two buckets of the clay rich fraction. Assuming that the clay rich fraction consists of at least 50% clay, my final mix contains at least 20% clay.
Now I mix the different components by stamping them with bar feet. Toes and heel become very different tools. And they are very efficient tools for mixing and forming the clay mixture.
In the beginning I often pull over the tarp to bring back sand and clay to the center of the tarp. It is this very early phase of mixing, where it is easier to be two people. Each taking two corners of the tarp to swing and role the mixture from left to right over the ground. But also with my method, eventually, the mixture has improved to an overall stickiness, that I can form a need little croissant (more precisely a chocolate croissant) by slowly pulling over two corners of the tarp at a time from all 4 sides.
This croissant has the tendency to unroll, so to stabilize the cob in the center of the tarp I give it a few careful kicks, giving it a cone-like shape. Maybe better compared to a giant macaroon.
Now I stamp the whole mixture through again starting at the edges of the macaroon,
to then work my way through its center. The idea is to keep the mixture as much as possible contained in the center of the tarp and to mix thoroughly rather than spreading it out so quickly. …because forming a 60 l soil croissant is hard work.
This process I repeat up to 10 times. 10 croissants!
Looking at this last picture you may think that it is now properly mixed and ready to add straw. But I actually gave it another turn. One more croissant. My experience is that thorough mixing is very crucial and makes every subsequent step, like adding the straw, forming the bread shaped portions, or building them onto the wall much easier. So if I have any doubt, that the mix is complete, I rather give it another turn!! And everything else becomes a piece of cake and real joy.
First then I add straw to the mix. To do so I collect everything once again in the middle of the tarp, form the cone shaped mount and form it to a thick disc.
Then I add a first portion of straw evenly on top.
And stamp the straw into the clay mixture.
… add another portion,
and stamp again. You can now see that there is less straw in the center of the mass, because here is most of the clay mixture.
Therefore my final addition is mostly in the center of the mixture.
Now I stamp it in as good as possible.
Then I collect everything including the straw once more as before
This final time of stamping I really take care to keep the straw embedded in the clay mix. To do so I try to span the edge with my toes to avoid the straw to brake out and to preserve a closed surface. That makes it easier later to form the portions.
Now it is flattened out evenly like a pancake. You can hardly see any straw, but in fact, there is about as much straw in the clay mix as you can see in the mount of loose straw behind, placed there for comparison.
Then with a plucky pull on two corners of the tarp, the whole pancake loosens from the tarp. And its is easy take some portions to form them into breads.
Finally a few numbers…
To mix 60 l clay sand and soil and finally adding the straw takes me rawly 2 h.
Another half an hour to 1h it takes to form the 50 – 60 breads.
Cobbing the breads onto the wall can take everything from 30 min to 2 hours, depending on the particular task on the structure.
Taking together, working two such portions a day can be 10 h of work.
Usually I make 4 portions over 3 days. And have plenty of fun with the progress.
You can always call it a day. Best before adding the straw, or after forming the breads. In both cases, the final product can easily be covered or wrapped in a tarp and kept over night.
In fact, after overnight the consistency of the mix usually becomes even better for cobbing. It is probably even better sucked through.