All Clay Everywhere! Best affordable option to mix... straw? ratio??

Heyoooo!!! So I wen't from reading some books about permaculture to full blown everything being planned every waking moment... It's great!!

I have pretty much 90-ish% (if not more) clay in the ground around me which makes things very interesting,

searching around I see a lot of discussion about 'if you don't have clay' and other ideas, my questions are;

Do I just use clay with a handful of straw for each brick to build a kiln to start my journey? Should I add sand?

... what would be the most affordable method without buying a bunch of sand and straw, if there is one,

~and~

what kind of ratios would be advised to try if my land is clay heavy?

All thoughts and advice are welcome

Cheers!

Clays aint clays.
Each clay is different and it has variable ratios of fines,clay, sand and course particle sizes.
If you look under adobe brick making in Australia you should find papers written about it.
There are a few tests, water, sausage, feel and shrinkability that will also help you.

I don't know about South Carolina clays, but I know there are some Georgia clays that can be used for pottery straight out of the ground (a rare situation). If your clay is similarly pure, you would probably want at least 2 or 3 times as much sand as clay to make good cob. Having a truckload of sand delivered would be sensible if you intend to do large-scale work.

A good kiln can be made straight from cob, no need to make individual bricks and build a shape. I know there are Southern potters who use "groundhog kilns" tunneled into the earth which can fire pottery as high as cone 12 (higher than even much porcelain). My local clay (which is somewhere around 10-20% clay, 10-20% silt, and lots of sand and gravel) is perfect for cob, and can be used as is for earthenware kilns. The clay vitrifies around cone 6 (electric stoneware temps) and melts at cone 10 (traditional stoneware temps).

Glenn Herbert wrote:I don't know about South Carolina clays, but I know there are some Georgia clays that can be used for pottery straight out of the ground (a rare situation). If your clay is similarly pure, you would probably want at least 2 or 3 times as much sand as clay to make good cob. Having a truckload of sand delivered would be sensible if you intend to do large-scale work.

A good kiln can be made straight from cob, no need to make individual bricks and build a shape. I know there are Southern potters who use "groundhog kilns" tunneled into the earth which can fire pottery as high as cone 12 (higher than even much porcelain). My local clay (which is somewhere around 10-20% clay, 10-20% silt, and lots of sand and gravel) is perfect for cob, and can be used as is for earthenware kilns. The clay vitrifies around cone 6 (electric stoneware temps) and melts at cone 10 (traditional stoneware temps).

I agree. I have predominately clay on my site so I bring in sand. It's pretty affordable in bulk. Typically I make use 70-85% sand and 30-15% clay for my cob mixes.

Glenn Herbert wrote:I don't know about South Carolina clays, but I know there are some Georgia clays that can be used for pottery straight out of the ground (a rare situation). If your clay is similarly pure, you would probably want at least 2 or 3 times as much sand as clay to make good cob. Having a truckload of sand delivered would be sensible if you intend to do large-scale work.

A good kiln can be made straight from cob, no need to make individual bricks and build a shape. I know there are Southern potters who use "groundhog kilns" tunneled into the earth which can fire pottery as high as cone 12 (higher than even much porcelain). My local clay (which is somewhere around 10-20% clay, 10-20% silt, and lots of sand and gravel) is perfect for cob, and can be used as is for earthenware kilns. The clay vitrifies around cone 6 (electric stoneware temps) and melts at cone 10 (traditional stoneware temps).

Awesome information! And yes, after about 6 inches it's nearly pure clay and after 2 feet it is pure... not saying I wouldn't water it and send it through a screen before making pottery, but it would hold I'm certain... Looks like I'll have to start getting quotes on truckloads of sand. Thank you so much! And awesome to know about the straight cob kiln too, that'll save some time.

Aaron Yarbrough wrote:

Glenn Herbert wrote:I don't know about South Carolina clays, but I know there are some Georgia clays that can be used for pottery straight out of the ground (a rare situation). If your clay is similarly pure, you would probably want at least 2 or 3 times as much sand as clay to make good cob. Having a truckload of sand delivered would be sensible if you intend to do large-scale work.

A good kiln can be made straight from cob, no need to make individual bricks and build a shape. I know there are Southern potters who use "groundhog kilns" tunneled into the earth which can fire pottery as high as cone 12 (higher than even much porcelain). My local clay (which is somewhere around 10-20% clay, 10-20% silt, and lots of sand and gravel) is perfect for cob, and can be used as is for earthenware kilns. The clay vitrifies around cone 6 (electric stoneware temps) and melts at cone 10 (traditional stoneware temps).

I agree. I have predominately clay on my site so I bring in sand. It's pretty affordable in bulk. Typically I make use 70-85% sand and 30-15% clay for my cob mixes.

What is 'affordable' in your area?... I'm finding over $500 for a truckload out here in the middle-of-nowhere SC and just can't justify it right now... Anybody know if wood-ashes could be an option?... Just burn the stuff I can't use anywhere else and mix the ashes in with the clay?... or would that make me feel silly long-term?..

Chris Vee wrote:
What is 'affordable' in your area?... I'm finding over $500 for a truckload out here in the middle-of-nowhere SC and just can't justify it right now... Anybody know if wood-ashes could be an option?... Just burn the stuff I can't use anywhere else and mix the ashes in with the clay?... or would that make me feel silly long-term?..

About $50 a yard for masonry sand so an 8-10 yard truckload is around $500. That would go a long way. For reference, I built a 10'x16' cob shed with 1 foot thick eight foot high walls and that took 18 yards of sandy loam.

I am late to this thread, but it perked a question.

Chris wrote: Anybody know if wood-ashes could be an option?... Just burn the stuff I can't use anywhere else and mix the ashes in with the clay?... or would that make me feel silly long-term?..

I am only a very novice potter, so I’m hoping someone with more experience will correct me if I am wrong here. It is my understanding that ash tends to be a source of potassium and or calcium (and possibly other things) in ceramic mixes, which act as a flux, decreasing the melting point of the material. It’s often called for in glaze recipes to make the glaze melt at a lower temperature. Sand is a source of silica, with a very high melting point, and add structure to a clay or a glaze, raising the melting point of the mix and reducing warping or bulging in clay bodies and reducing running in glazes. So if you’re trying to make cob which is structural and needs to be stable and not warp or melt, I would think ash would not be a helpful addition?

I have never made cob, so I may be misunderstanding the nature and goal of adding sand and straw. But I do have some cob projects in mind, so I’m hoping someone will chime in and clear up this question for me.

Eloise Rock wrote:I am late to this thread, but it perked a question.

Chris wrote: Anybody know if wood-ashes could be an option?... Just burn the stuff I can't use anywhere else and mix the ashes in with the clay?... or would that make me feel silly long-term?..

I am only a very novice potter, so I’m hoping someone with more experience will correct me if I am wrong here. It is my understanding that ash tends to be a source of potassium and or calcium (and possibly other things) in ceramic mixes, which act as a flux, decreasing the melting point of the material. It’s often called for in glaze recipes to make the glaze melt at a lower temperature. Sand is a source of silica, with a very high melting point, and add structure to a clay or a glaze, raising the melting point of the mix and reducing warping or bulging in clay bodies and reducing running in glazes. So if you’re trying to make cob which is structural and needs to be stable and not warp or melt, I would think ash would not be a helpful addition?

I have never made cob, so I may be misunderstanding the nature and goal of adding sand and straw. But I do have some cob projects in mind, so I’m hoping someone will chime in and clear up this question for me.

Sand provides compressive strength, straw provides tensile strength and clay is the glue/binder that holds them together. Warping and/or melting isn't a factor with cob since it isn't subjected to heat. Maybe if you were making and firing bricks with clay wood ash might be useful but I've never made bricks so I don't know. The only other element I sometimes add to my cob mixes besides the big three(sand, clay and straw) is hydrated lime for water resistance.  

Thanks for your explanation of the cob ingredients, Aaron. It does make sense that it would not be an issue if not subjected to heat. It sounded like the original post was in regard to creating a kiln, which got my pottery brain spinning. Not sure if wood ash would be a good choice in that scenario. But understanding the purpose of each ingredient of the cob mix is helpful!

For the record, with nearly pure red clay all around a 6 to 1 sand to clay ratio has worked great; but I’ll be testing more sand in the mix very soon & will report back.

Chris, I have been involved with clay for building for years.
In Australia we are very unlucky not to find a good naturally occurring mixture of sand and clay almost anywhere.
There are limitations, but having to 'manufacture'  a mix is rare, if you did another sample hole 100ft away you may find it.
Straw is only needed if you are making very wet mixtures, generally known as 'adobe' or 'mudbricks' within Australia.