Cob Workshop - Building Cob Walls

a cobbers thumb

stuffing the walls

Recipes for Cob

-Sand 70-80%

          provides compression strength

          use coarse sand (like mason's sand) with sharp edges that will lock together

-Clay 20-30%

          sticks it all together

          test to be sure it's not silt

          it should be glossy, not slimy

-Straw lots of it

          provides tensile strength (like rebar)

          NOT HAY - it will decompose

          longer pieces are better

          break open bales to check for mold or mildew first

Mixing Cob

-brick tests are the easiest way to determine the best mix

          combine various ratios of sifted clay and sand and press into small bread or muffin pans

          try 50/50, 75/25, 25/75, etc

          let the bricks rest for a few days

          if they crack there is too much clay, if they crumble there is too much sand

-mix the dry ingredients first

-add water gradually to make bread dough consistency and 

 stomp it until the mixture is homogonous

-add as much straw as the mix will hold without falling apart

-push the limit once

*only build a maximum of two feet of wall per day and let dry to avoid splooging

*spooging is when the weight of the cob causes the wall to leak or bow out

*dry cob weighs 100 pounds per cubic foot!

*keep wall covered with a canopy while under construction and over winter to prevent frost heave

Building a wall on the foundation (stem wall)

-make cob balls, get damp and throw it to cover the top of the rocks

-integrate it into the gravel and cracks with your fingers

-next use sloppy cob, super wet on top, smeared all over the now smooshed cob balls

-continue with regular cob on top and integrate with a cobbers thumb

-watch for shouldering!

  

a brace to help keep the wall exactly the right width

a full form

the first days work when we leapfrogged the first frames up

the piece arching out the top is an arched window we put into the wall

a wall that has been finished but not plastered

the day we left - we built that whole wall!

we made this interior wall out of adobe bricks

a snazzy star wars art window using glass bottles

After breakfast on the very first day of the workshop we learned to mix cob. First we did soil tests to learn the right ration of clay and sand. The process for making cob by foot, as they say, involves laying a small tarp on the ground, combining a lightly filled five gallon bucket each clay and sand, and shaking it back and forth until mixed. This is a tougher task than you would expect. First, it's heavy! The clay forms balls and the sand seperates. It's like mixing really heavy thick cookie dough. A combination of shaking the tarp, rolling it over, and getting inside to stomp the mix around like you're making wine will eventually result in a homogeous mixture. Very little water is added to each batch, a handful or so, and a small handful of straw.

 We looked like fools, and got super filthy. It was a blast.

One of the cool things about cob building is that most of the ingredients are the same thing just different consistencies. Clay, sand, and sometimes straw create regular cob, sloppy cob, cob mortar, cob balls, cob glue, cob plaster, cob bricks, and more. There are various methods for build the walls too. We learned how to build a wall free form, which is difficult but pretty easy to do for a few inches up off the foundation. After we built up six inches we used forms, various wooded frames to create a hollow box where the wall will be, which is then stuffed with cob. When using forms, it is important to brace them to prevent splooging (hilarious I know!) from the weight of the wet cob pressing down and forcing the walls to bow out. We also learned how to build with adobe bricks. In that method the cob is pressed into a form (a frame made of old lumber in a ladder shape) then pushed out of the frame and let dry in the sun for at least 24 hours. These bricks can then be used just like normal blocks. They are particularly good for interior walls which don't need to be load bearing. 

Cob is load bearing as long as it is at least 12 inches thick.

More explanations from above:

-Frost heave occurs when water enters the cob and is not allowed to escape before it freezes. Once frozen it will expand and force a chunk of the wall to break away. We saw the result of this and learned how to repair it.

-a cobbers thumb a super fancy device used to integrate the strands of

straw between layers of cob - it's a stick. a pointy stick.

-shouldering is when the walls narrow as they go up, which is a big no no!

That's the walls, next up - the fun part!

Until next time,

Angela Kelly