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cob: cane straw  RSS feed

 
Fianou Oanyi
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Location: Australia
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Can anyone tell me if cane straw (or any other straw) can be used in cob mixes? It's more readily available and cheaper than straw.
 
Thekla McDaniels
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Location: Grand Valley of Colorado's Western Slope
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Hi Fianou, I don't know what cane is, but there are many different plants that can be used in cob.

I've just checked in my copy of the Hand Sculpted House and I'll summarize what Evans Smith and Smiley say aobut straw

The functions of straw in cob are to strengthen it, and to provide insulation, )the air spaces trapped inside the straw.

You want tensile and shear strength in the cob, and the straw is what provides these qualities, but to do so, it must not be rotten, it needs to be strong, and in long pieces.

There is a difference between straw with hay. Straw is cut after the grain has ripened and is harvested, straw is sort of biologically inert. Hay is cut while green and nutritions, making it of interest to insects and easier to decompose.

To check the strength of what ever straw you are considering, take a single piece and try bending and pulling it. You do not want it to be brittle. Then take 5 good pieces and put them all together and try to break them by pulling, good straw in group of five will be impossible to break in a tug of war.

The best straw will be fresh and dry, or have been stored in absolutely dry conditions.

Other plants can be used, if you can't get straw from grain plants. Oat straw and rye straws are all good in cob, barley straw can be brittle. Rye grass and canary grass are strong and flexible but are hard to work with.

Kinds of straw to avoid : alfalfa hemp millet milo buckwheat soybean sorgum flax and corn.

They also say dry hay is better than wet or rotted straw, and that traditional cobbers use what ever is available: rushes, and tussock for example.

So, about the cane: is it long and strong? will the clay stick to it, so that it will mix in and hold things together? What are your other options? The plant fiber of what ever kind you use has very important functions, and it is not temporary, rotting out over time. Old cob buildings have had sound fresh appearing straw after 150 years. You can use what ever will provide those functions.

Make some test batches to check what happens when you use cane.

I hope this helps.

Thekla
 
Fianou Oanyi
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Location: Australia
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By Cane I mean sugar cane straw. Its the most abundant local crop but I wondered if the sugar content would be an issue?
 
Thekla McDaniels
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Location: Grand Valley of Colorado's Western Slope
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The sugar content will likely be attractive food to bacteria yeasts insects and rodents, where ever they can get at it.

I am just guessing on this and with luck someone with more experience will speak up, but when they extract sugar, don't they crush the sugar cane into fiber? Do the toughness tests on that fiber, and see if it will withstand the breaking and brittleness tests. If it will, and you can find a way to get the residual sugar out of it, it might work.

Sometimes, a combination of fibers is used, to get all the qualities needed from the fiber and no one perfect fiber is available.

Again, test batches are very much called for, and it wold be good if you could get participation from an experienced cob builder.

Thekla
 
Thekla McDaniels
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could you post a photo of "cane straw" for us?
 
Fianou Oanyi
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sorry, but no. I don't have any. But I think what you are saying about the sugar attracting bacteria and bugs and rodents is probably spot on.
 
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