[Cob] Re: cob as cordwood mortar

[Pack McKibben]

Thanks for every ones response to my questions.  I do plan on using a good proportion of sand to keep shrinkage to a minimum.  I agree that lime putty is the way to go over cement. I've go 200 lbs. Of hydrated lime that has been sitting in water for at least three months, and I plan on using that. I do want some input on exact proportions.  

This as an example of my cob mix from soil on my property. I built an earthen oven as soon as I moved into my new place in the woods. The cob mix I used (after shake testing) was two parts clay soil (2 five gal. buckets) to one part pond sand (one five gal. bucket), with straw and water. The clay in my soil is about 10 per cent, so the two buckets works out to the 20 per cent clay requirement. So, the total dry mix is approx. 15 gals, so should I add 192 ozs. of lime putty to my normal cob mix for my cordwood mortar? This would represent approx. ten per cent lime putty added to the normal cob mix. I came up with the above 192 oz. number by multiplying 15 gals by 128 ozs. (per gal) = 1920 ozs. This I divided by 10 to come up with the ten per cent (192 ozs.) So that would mean I need one and one half gallons of lime putty to add to the cob mix. RIGHT? This would give me 20% clay, 70% sand/other earthen materials, and 10% lime putty. Do I need to add sawdust to the mix?  

My original plan for the earthbags was to use a mix I found on a "How to build a Coracle" website. I figured if this mix would keep out water for a boat, it would surely keep out rain. It makes one gallon and ingredients follow:  43 ounces boiled linseed oil, 21 ounces paint thinner, 34 ounces porch and deck enamel, 2 ounces Japan drier, 6 1/2 pounds silica (obtained from a potter) and 2 ounces spar varnish. These ingredients can be obtained at art supply or paint stores. This doesn't seem to be too earth friendly and may be expensive.  Practical experience is worth a lot. Who wants to re-invent the wheel. That's why I wanted input from the wonderful coblist folks. I'm such a newbie that I did not know that lime putty would NOT stick to poly earthbags, and AE would stick to CD's. Thanks again. 

Pack           www.owlswamp.com



Pack McKibben
gakayaker at yahoo.com

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