Straw alternatives for cob

[hpmer]

What are the main attributes one looks to the straw for in cob? I don't have easy access to straw so was wondering what I could use as an alternative.

Should the material be rigid or flexible? Should it be soaked prior to mixing? How long should the fibers be (1", 6", more?) ?

Any suggested substitutions?

[Deleted]

The reason for using straw is simple: If you are a farmer then you have stalk fiber (like straw, hay) in abundance.
planer shavings are stalk fiber too.
Another source is horse shit.
One can use any kind of natural fiber like cotton, jute, flax, ramie, sisal, hemp, coir (coconut) or dissolved paper.
Mineral fibers would do it too.

[hpmer]

Thanks, but that doesn't answer the main question. What properties and lengths are we looking for in the binding material? Also in what proportions should it be mixed in?

[Deleted]

Roughly 15% of dry weight.
0.5" -1".
Long fibers tend to build unwanted cluster.
I prefer toilet paper.
Cheap and Easy to get and use.

[Donkey]

If yer building something small like a rocket stove, the fibers don't need to be too long. Paper pulp is fine.
If yer building a home, longer fibers are preferred, around the length of your forearm is good.

The answers that you will get from ANY natural builder will be (should be, had BETTER be) something like "it all depends". Personally, I think that if you get some pat answer and it's snap... You probably should think about just walking away (though that depends too).

SO! It all depends, longer fibers are as a general rule, desirable because they provide tensile strength to the mix. Sand (or other aggregates) provide load strength.. Take a handful of sand and crush it, it resists, but pull that handful apart and it shows no resistance whatsoever.. Do the same with a handful of straw (fiber), very little resistance to crushing but difficult to pull apart. Generally speaking, the longer the fiber, the better it's woven into the wall (whatever thing you're making), the more tensile strength they provide. As I said before though, it becomes less critical if you're building small things, like benches and so on.

As to proportions, that all depends too. What do you want it to do? Where is (this particular) mix located? Questions like that.
When I'm building arches and larger overhangs, I add LOTS of straw to the mix, though when I'm building near a stove I'll use very little. Think about what I said earlier.. Is this thing (that I'm building) going to experience a lot of downward loading, or will it be pulled in multiple directions?
There IS NO one-true-mix.. You've got to fool with the stuff and see how YOUR clay does with YOUR sand(aggregate) and YOUR straw (fiber). You may find that your local soils need no added sand for a good every-day cob. You may find that the kind and amount of roots in your soils make up for the lack of local straw.. No way to know without trying.
So make tests with different mixes, let them dry out and test each one to destruction. Don't be afraid to try crazy shit, you may be surprised at the results.

[hpmer]

Thanks, that's the background I was looking for. I'm inquiring for a stove as I've had cracking issues in the past and I like to know the "why" when I am doing something.

Sounds like I need a different alternative. I guess I'll go with the traditional fire brick.

[Donkey]

Cracking often (but not always) indicates a heavy amount of rather expansive clays. The fix is most often more sand, though more fiber can do as well.
If you have cracking, but the material is hard (when dry), it's likely clay heavy. If the material is crumbly, it's likely got too much silt (or sand/topsoil/loam/etc). Clay heavy is easy to fix, silt heavy, not.

[hpmer]

May 22, 2012 21:27:29 GMT -8 Donkey said:

Cracking often (but not always) indicates a heavy amount of rather expansive clays. The fix is most often more sand, though more fiber can do as well.

And grog even better than sand, true?

[Donkey]

In high heat areas, grog will help. Out of the heat, I'd choose sand.
Depends on the sand, of course.. Silica sand in it's non-crystalline form is likely to out do the grog.

[hpmer]

Like mason's sand?

[Donkey]

Hard to say. Different aggregate companies often call different sand the same name.
I've only ever seen troubles with crystalline sands, like quartz and silica (in it's crystalline form).. Non crystalline sand seems just fine in rocket stoves.
Though in my forge, the sand is NOT up to the heat and it's melting into slaggy/glassy bits all over the place. Looks like I gotta chip it back and use some other mix around the fire.

[hpmer]

How does one tell one type of sand from another? Are there visual clues? What words am I looking for (or looking to avoid) on a bag of sand from Home Depot? Will it say quartz and/or silica? What's the good stuff called? Is beach sand generally no good?

[Deleted]

Silicon sands like silica sands (silicon dioxide, or SiO2), usually in the form of quartz are the most common sands, as the largest part of the Earth's mantle is made of silicon.
I can only wonder why Donkey has problems with silica.
Silica is crystalline by nature.
The Melting point is between 1600-1725 °C, 1873-1998 K, 2912-3137 °F.
Large amounts of flux are needed to lower the melting point.
Sands formed by water are not good for buildings because of their rounded form,
crushed sands are preferable.
Beach sand especialy in warm areas is is often made of calcium carbonate,
which has a lower Melting point 825 °C (aragonite) and 1339 °C (calcite)[2].
en.wikipedia.org/wiki/Sand

Large amounts of woodash can lower the melting point of silica significantly,
because of relatively high contents of alkali metals, but hardly enough to explain Donkey's experience.
Maybe Donkey's soil contains lots of feldspars.
en.wikipedia.org/wiki/Feldspar

[Donkey]

Crystals, including crystalline silica change shape DIFFERENTIALLY at heat. Clays. as a general rule, do not.
You have to take into consideration the properties of everything in the mix and how they will play together.
This is why I stay away from crystalline sand.. Yes, the sand will handle the heat, but it will crack apart the clays that are used as binder. It's better to have the mix melt ever so slightly than crack all to pieces.

[Deleted]

Thermal expansion generally decreases with increasing bond energy, which also has an effect on the hardness of solids, so, harder materials are more likely to have lower thermal expansion.
crystalline silica is harder than non crystalline and thus has a lower thermal expansion.
en.wikipedia.org/wiki/Thermal_expansion
Non crystalline silica can only form when silica melts.
Few things in nature can create the needed energy.
Clays formed by eroding quarz will be crystalline too.