Cob: Test Bricks

[lightearth at onebox.com]

Hey Kim,

It sounds like from the tests that you undertook that you really don't know what the consistency of the soil or clay is ACTUALLY made of.... As clayey as it might seem the red clay likely has other things then clay in it, which translates to that every time you add more clay you're adding more sand/silt/small pebbles etc. AND the sandy soil is impossible to analyze exactly without a lab test.......

SO build a little bit (with lots of good fiber) and determine your best ratio based upon strength, cracking etc. If you have TOO much clay it'll crack some or alot as it dries,,,,too much sand, it'll granule off like you'd expect from a sand castle. Silt is the killer as it just falls apart (organic matter is easier to tell) but some silt is OK. The other option is get more pure clay from the road department or pond diggers that hit deep clay that is usually more pure.

Trust your instincts after building a model or something!

Best

Marlin Nissen
-- 
     Marlin Nissen
   - Outta The Box-
  lightearth at onebox.com
(608) 213-9405  Cell/voicemail




-----Original Message-----
From:     "Kim West" <kwest at arkansas.net>
Sent:     Sat, 9 Nov 2002 18:26:20 -0600
To:       <coblist at deatech.com>
Subject:  Cob: Test Bricks

I am new here [and new to cob] and do not know what all subjects have been covered, but my interest has been piqued by the recent posts concerning the mixture to use in making cob.

After doing some tests on the soil on my property I found that it was anywhere from 80-95% sand, according to where I collected the sample, with the remainder being silt. There was no clay mixed in with the soil, but if I dug down anywhere from 6 inches to 1 foot I found solid red clay.

I made some test bricks using the soil and the red clay in three different ratios and found that 25% clay and 75% soil was a bad mixture and the bricks made from it crumbled easily.

The two other ratios that I tried were 50/50 clay/soil and 75/25 clay/soil. 

The 50/50 did not crack in the drop test until 3 feet above the ground, and it broke at 5 feet above the ground. 

The 75/25 cracked at 5 feet above the ground and broke at 7 feet above the ground.

With what little I know it seems that the 75/50 ratio is the best and should be used in any cob that I make from my land, but reading here and on other sites it seems that 75% clay is too high.

Am I missing something here? Would someone please fill me in on whether or not I should use the 75/25 clay/soil mixture. It would seem to me that I should since it passed the drop test so well, but looks can sometimes be deceiving and I would like to know if I am missing something.

Thanks,

Kim