Wood ash

[hpmer]

May 22, 2010 17:21:36 GMT -8 hpmer said:

Donkey said: "I've been playing with new home made refractories lately.. My current favorite is also the simplest.

Wood-ash and clay soil, at about equal proportions. Mix it together, add water and work it till it forms a homogeneous dough, much like cob.
I've been using it in my (home made) waste veggie oil burning forge to good effect. It might work really well to make bricks out of it for building rocket stoves, I suspect it will. Heck, forget the bricks, cast it directly into the works in one piece.

This possibility is very exciting. In my research, though, I've read about perlite, vermiculite, pumice, sawdust, crop residue etc. but not wood ash. I assume this means that although it will work it is somehow not as efficient? This is perhaps suggested by the ratio you mention, 1:1 ash to clay vs. the 4:1ish for other insulants. I wonder what properties of ash make the ratio so different? It would seem to be perfect; free, easily available, non-combustible, light weight, etc.

I'm going to try it in various proportions to see how it compares to test pucks I've made from vermiculite and perlite.

[Donkey]

It does come out quite dense (so far for me) and maybe not so insulative.
I've used a little rice hull in the mix and so far so good.

Good Idea hpmer.. Give us the lowdown of yer testing... ??

[hpmer]

Seems like it should work. The stoves that I've made so far have been out of food cans and I used a 6" can with ash in the bottom 2" or so of the feed tube. After a 12 hour burn, the can was a bit worse for wear and badly discolored - except for the bottom 2" which looked near brand new. It appears the wood ash was a very good insulator.

Not sure why the thought of combining it with clay as a castable mix never occurred to me, but I'll give it a go and post my results. I have to rebuild my old stove anyway since the cans have burned out, so this option is very appealing.

[Donkey]

It works GREAT!
I'm very impressed. Its a great insulator, contrary to my earlier statement. It can take a while to dry and is deceptively heavy early on.

Make a thick clay slip in a large barrel. Put some water in the barrel first, if you add the dirt first, it'll stick. Toss in some dirt and mix with a paddle mixer, add clay rich dirt and water till you make enough. It should have the consistency of thin pudding, somewhere between pudding and a milkshake.. It'll be full of rocks, roots and other gunk. Mix it as well as you can, then let it sit overnight, or longer.

Make a screen over a frame. I used two layers, 1/2 inch aviary wire (for strength) laid underneath and metal fly screen over top.. It should be big enough to span a wheelbarrow.

Mix up the mud really well. Adjust for consistency, too thick and it'll never get through the screen, too thin and you'll make weird, watery mixes that don't work.. You'll need to bucket the slip out of the barrel onto the screen and work it through with yer hands. Sometimes bumping the screen up and down helps. When you've got one load through, you can tilt up the screen and smack the underside and the rejects will fly off. Repeat.

Ash can then be added to the screened slip. you want a firm but sticky consistency, it needs to not slump but be sticky. The rice hulls are a plus, they can be added pretty liberally. I noticed that it wasn't mixable with a hoe beyond a point, seems that foot mixing cobbers style get's it done best.. Though lye in the ash (especially hardwoods) is rough on the skin.

[hpmer]

May 22, 2010 17:21:36 GMT -8 hpmer said:

Wood-ash and clay soil, at about equal proportions. Mix it together, add water and work it till it forms a homogeneous dough, much like cob.

How did you settle on the ratio of 1:1? Any magic to it? I'm trying test pucks from 1:1 up to 10:1 ash:soil and they all seem to be holding up well. You're right in that they seem to take a long time to dry out so it may abe a bit too early to tell for sure.

I've made the pucks with a form and a similar volume of material so hopefully I'll be able to compare them and reach some reasonable conclusions.

[Donkey]

1:1 was what told to me. Though for my soil, the ash content should be higher.

The ones that don't crack when they dry will be the ones you want.
One of the things about doing little pucks is that a little clay body might not crack where a larger mass will. I've learned that lesson doing earthen floors.. Just cause the little test piece stays together, don't mean the project will!!
So, after doing the little ones, take your candidate mix and make something larger with it and see what happens.

[hpmer]

Well, NONE of them have cracked, including the pure clay one (so I suppose it isn't pure clay). I've noticed quite a difference in weight as the insulator is mixed in, but that's about it. I did the "water in a jar" test and that didn't seem to yield much as far as information goes. I dug it from a part of the yard that I get standing water in after spring rains, but we have such a high water table that perhaps mine is not as conclusive as some others.

[Donkey]

The "shake test" is almost useless for assessing clay. Sure, it'll tell you you've GOT clay, but it won't say anything about the qualities of the stuff you found.

You may have a high quantity of sand or silt in your soil. Are the pucks crumbly, do they dust off easily or crumble when you rub them? If so, it's likely silt or small rounded sand. If not, possibly you got sand.

I'd make a MUCH larger test piece. Take the one you like best (purely subjective), crumbles least, hardest, lightest that holds together well, whatever the criteria, and make a 1 foot by 1 foot by 2 inch (or bigger) tile and see what happens.

Remember, at this stage anything you make can be re-constituted, re-mixed and re-used.
After you fire it, it's not clay anymore but pottery and can be used as "grog" in place of sand. Grog won't change shape under heat, so it doesn't tend crack yer castings like sand will.

[hpmer]

Jun 3, 2010 13:42:43 GMT -8 Donkey said:

You may have a high quantity of sand or silt in your soil. Are the pucks crumbly, do they dust off easily or crumble when you rub them? If so, it's likely silt or small rounded sand. If not, possibly you got sand.

The pucks are not crumbly at all, but hold together well. I suppose that must be a good thing. And they're not dusty. I'll break them in half and see what kind of force it takes to do that. Any other suggestions for determining what to do with them? I guess the end game is to try to determine a reasonable ratio to mix the insulants in with the dirt.

By the way, I dried and then crushed and screened the dirt to filter out the small stones and organic matter before mixing in the insulants (I've tried vermiculite, perlite and ash). Not sure if that is material or not but thought I'd throw it out there in case it matters somehow.

[Donkey]

There are advantages to working both wet or dry. Whatever gets the job done.

My inclination would be to keep adding "insulants" to your test mixes till one of them is clearly wrong. It's good to have a "screwed up" mix or two around to show where the limits are.

Yer likely keeper will be a step (or two) back from the failure point.

[hpmer]

Jun 4, 2010 14:10:44 GMT -8 Donkey said:

There are advantages to working both wet or dry.

Could you elaborate?

[Donkey]

Working with dry soil, measurements are easier and working from dry up to a moist workable dough is a snap. If yer making cob or adobe, it's better to start dry(ish).

However, if you HAVE damp soil (like working in the winter) drying it out can be a real drag, especially in any quantity. Damp soil won't screen easily as it wants to clump, etc.
The solution for me is to turn already damp soil into slip.

Slip itself has a million uses. The only thing to watch out for is that it can throw measurements off. The stuff is mostly water, not soil and mixes can get too sloppy too fast and not have enough clay soil in them.
Still, I prefer slip for mixing up insulation batches like straw/clay or pearlite/clay. 'Course, it makes great plasters. I paint it onto everything that cob will touch, etc, etc, etc.

I've nearly ALWAYS got a barrel of slip around..

[hpmer]

OK, I dried the pucks made of ash/soil and all held together quite well when dried. The ones with higher ash content were, of course, lighter. I then heated them in my cook stove and had pretty disappointing results. All of them, including the pure soil version, became quite fragile and crumbly after a two hour firing. I had hoped that the ones with a higher percentage of soil would become hard and brick-like but no such luck.

I am concerned about the ability of that mixture to hold up to the stresses of the heat riser, so now I'm thinking I may have to resort to some type of cement mixture as has been mentioned elsewhere on this board. I guess it's back to square one.

Any suggestions?

[Donkey]

Look for different soil.
Try samples from someplace else, hopefully not TOO far.

[hpmer]

OK, different soil was the answer. Apparently I did not dig deep enough and was originally still using top soil. A bit deeper and I had very clayey subsoil. Mixed it in your suggested 1:1 ratio with ash and finished off the top of my heat riser with it. Now I'll give it a couple of weeks to dry and then fire it up and see how it works.