Cob Testing

I decided to follow some advice out there from my cob building, and test the soil around my house to see how it stacks up to the preferred mix of ingredients.  The ingredients for cob are quite simple; sand, clay, water and straw.  I’m reading that the best mixture has between 30-50% clay.  The sand gives it form, the straw gives it tensile strength, the clay sticks it all together, and the water allows it all to mix up and be formed as you wish.  I found some great advice on testing the mix in chapter 3 of  The Cobbers Companion.

I dug some mud from the small creek behind my house and some dirt from a hillside on north side of my house.  .I was curious how the soils were different.  I tried the soil separation test but couldn’t quite figure out the results. 

Soil Separation Test: Putting the dirt in a jar of water, shaking like crazy and waiting for it settle out.  The Sand, silt, and clay are supposed to form obvious layers. 

I figured maybe real live testing was a better bet.  So I made test bricks with the two different type soils I collected, plus one mixed with both types of soil.  Additionally I was curious if any dried material similar to straw would work.  I had a good collection of last years brush that I thought might work like straw.  Apparently Hay or other very organic material isn’t a good choice as it rots away within the wall leaving weak spots.  This stuff seemed very dry and inorganic in quality, much like straw.

Results

I let the bricks dry for a few days and then decided to see how they performed.  The bricks made of river mud cracked when I applied pressure with my hands, and literally crumbled.  When I dropped one from 6 feet onto concrete it exploded.  I concluded they had to much sand.

The bricks from the north side of my house developed large cracks in them as they dried.  This is an indication of to much clay.  However I couldn’t break them with my hands, no matter how hard I tried, and they only broke off a chuck with the drop test.  The mixed soil brick performed only slightly better than the ones dirctly from the river, possibly it would work if i adjusted the mix. 

The actual straw performed slightly better than the material I had gathered at my house.  If I was building a home to live in I would definitely use straw, but since this will be a shed to store a Lawn mower, lumber etc., I figured saving the $20 on straw was worth using the material I had on hand.  We’ll see.

Location, Location, Location

All of this testing was done prior to actually deciding exactly on the site.  Once I determined where I wanted the shed I made some more test bricks from the soil right there.  To my extreme joy they performed better than either of my previous bricks!  They did not crumble, nor did they crack.  The soil directly at the site seems to have a good blend of sand and clay.  This is good because it means and soil I excavate for the foundation will get reused in the cob itself, saving me time and effort!               

Ticker

Hours invested so far: 3

Money Invested so far:$0