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fermented cob ???  RSS feed

 
Roger Merry
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Hi
Watched the "Human Planet" series (BBC) last night - awesome photography as ever.

Part of it was the Cob Mosque in Djenne, Mali - the largest cob building in the world. They give the mosque an annual waterproofing coat of cob plaster............ so far so "I knew that already"  but they where waiting to start until "the rice husk cob had fermented" before using it

I've heard of rice husk being used in plaster but anyone heard of fermenting it ??

They didn't go into details of why or to what benefit - but it took a while to ferment and they inspected it every few days to see if it was ready before use.

If you missed the episode - http://www.bbc.co.uk/iplayer/episode/b00rrd7w/Human_Planet_Rivers_Friend_and_Foe/

 
                                  
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I have never heard of fermented rice husk cob. However, I have heard of using a litema (pronounced dee-tay-ma) dung plaster that has been fermented. Apparently this binds the dung to the clay quite well and it creates a very smooth consistency that makes it easy to apply. It also helps with its water resistance and durability.
 
Jonathan 'yukkuri' Kame
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I am pretty sure it has to do with the humic acids that are produced when organic matter is starts composting.  I think the humic acids make certain ionic bonds that make the cob stronger. 
 
Roger Merry
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It would be interesting to know if it makes cob more or less waterproof......... in the film they were applying the fresh plaster before the rains - which only really makes sense if it is water proof.

In terms of cob building Djenne architects are the worlds best, so it would be interesting for someone to try it out - maybe in a wetter region ............. I don't have a cob building to experiment with sadly 

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Roger 
 
Kathleen Sanderson
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Are the dung plasters dung mixed with earth, or with lime?  (I could try dung mixed with earth -- we are finally going to do at least a small cob building this summer.)

Kathleen
 
solomon martin
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yeah, my buddy who is a cob builder likes to chop his straw really fine and let it ferment in the clay mixture for a year or more.  He says it is stronger and finishes much nicer.  He said he learned the trick from reading about Japanese cob builders. (where they probably used rice husk)  I've seen this fermented cob, it is a little slimier and darker in color than fresh cob, but it isnt foul, and spreads nicely.
 
Roger Merry
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interesting Sol
Do you think it makes it waterproof ??
 
solomon martin
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No, cob by itself will never be water proof.  You can add cement or hydrated lime to the outer coat to make a stucco application that is more weather resistant.  Another strategy is to treat the cob with a natural oil stain (such as used by timber-framers) to make the cob moisture resistant (such as preventing drink spill damage for a kitchen floor for example) but the clay content will always want to absorb moisture.
 
Roger Merry
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Sol
sure you're right but I was interested by the Djenne practice of re "plastering" the cob buildings (4 and 5 stories high) before the rains rather than as a repair after the rains. Although Mali is a desert country, when it does rain, boy does it rain !! It seems the fermented cob provides sufficient water proofing to protect from rain damage / erosion of the buildings - they've been there a thousand years so I guess it works 
Wish I could experiment myself but still waiting to build a cob building 

Roger
 
Jonathan 'yukkuri' Kame
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youtube with Michael G. Smith on "on't cob buildings just melt away?"
http://www.youtube.com/watch?v=MCfS2-m-cpE&feature=player_embedded#at=165
still no idea how to embed video on youtube
 
solomon martin
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Hmm,  so it seems that fermented cob may provide some water resistance, yet those thousand year old buildings do need constant maintenance... 

Just thinking here, but I have read about african building techniques such as used in Mali, many of the "cob" type applications use dung and/or blood in the recipe, I wonder what this contributes to strength and weather resistance?
 
Jonathan 'yukkuri' Kame
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Sol wrote:Just thinking here, but I have read about african building techniques such as used in Mali, many of the "cob" type applications use dung and/or blood in the recipe, I wonder what this contributes to strength and weather resistance?


As far as I know, it is the humic acids in either manure or composted plant matter.
 
Kirk Mobert
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I always ferment my earthen plasters. It smoothes out the mix, makes it stickier, easier to apply and more resilient and resistant to damage (of all kinds).
When mixing up a batch, weather using horse (or cow) manure, straw, rice hulls, shredded newspaper, etc.,  I'll let it set for a few days until the mix smells like a beer wart. If it sits too long it'll smell like sewage which is unpleasant to work with. Natural enzymes will go to work on the organic material(s) in the mix, slightly breaking down the straw (or rice hulls, manure etc.), releasing lignins,  mucilage and various other gluey substances.

Sometimes, if I have some lead time, like several months, I'll whip up some clay slip, screen out the rocks  and add a handful of manure or chopped straw, manure is better. This won't be the final mix, which will have MUCH more straw/manure/whatever, it's just adding a little food and inoculating the batch with enzymes. I'll then whip the batch with a paddle mixer every so often and let turn into a thriving community of glop. When the time is near to use the stuff, I'll add the rest of the final mix in and let sit for a couple days (depending on temperature). This way of doing it typically does smell pretty rough and it's more suitable for exterior applications.. It makes for a damn fine finish!

NEVER use cementitious stucco on earthen walls! Lime is fine, (portland) cement is not.
All over the world in the last century, it was thought that one way to protect ancient earthen buildings was to stucco them.. Now, it turns out that stucco is the NUMBER ONE destroyer of those same buildings. DON'T DO IT!
Cement is hydrophilic, it wicks and sweats moisture, it also becomes a condensation point for water vapour moving through walls. Part of the problem is that since stucco is fairly resilient, you don't see the damage underneath until it's MAJOR.
 
Ardilla Esch
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I have used year-old sewage smelling, long fermented plaster Donkey warns about.  It does stink like you wouldn't believe.  But the plaster is really nice to work with (smooth & sticky).

In the end, it isn't that much easier to work than day or two old plaster.  I'll take the less stinky stuff any day.
 
Kirk Mobert
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I have noticed that the ultra nasty stuff that's been sitting around too long dries harder and stands up to more abuse.
Up to a point, the nastier and gloppier, the stronger the stuff seems to get.
I'd rather NOT work with the stuff though! And I'd worry about cuts on my hands getting infected by it.
Depending on the weather (mainly temp.) a couple days and a gentle beer wart smell is good enough.
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