What does this clay need?

[Bing]

I have a dark (iron filled, I’ve been told) natural clay I dug from a foundation job nearby. Our neighborhood has a spring fed underground stream system running through it. 
I’m able to throw using it but it has some noticeable properties: while plastic, it has a tendency to sheer off if not super well-wetted, it has detectable fine sand in it but is quite sticky, it also does not adhere to itself. I attempted to put a handle on a mug I’d thrown, but there was no affinity when I tried to get it to attach. 

I’ve done a test bisque. The deep coffee-bean colored clay bisques to a brilliant brick red. About 14% shrinkage and it held its shape well.

When I ran it through a cone six firing it returned to its deep brown color and acquired a pretty sheen that my studio teacher said was probably the beginnings of vitrification. He suggested I try it at cone two. 

Does any of this information give anyone an idea of how I might make the clay a bit more workable?

[Pres]

Bing I am going to move this thread to the Clay and Glaze Chemistry section as it is more appropriate.

 

best,

Pres

[Kelly in AK]

Beware. It’s a wild world you’re stepping into.  Maybe the clay has a high percentage of non plastic material, even though it’s throwable and feels plastic. If it’s actually sticky, then you ought to be able to “stick” a handle to it. Check out Tony Hansen on “splitting:”

https://www.digitalfire.com/glossary/splitting/

Sedimentary clays are often found in “lenses,” or lens shaped layers with sand, silt, and and silty or sandy clay. You might go back to your spot and see if there’s a more pure clay layer. Silty sandy clay can seem plastic when wet enough, yet the clay an inch below or above what you dug may be the sweet stuff. One way to differentiate that is by trying to break or crumble a totally dry piece in your fingers. The more difficult it is, the less non plastic material it has.

Sieving out the non plastics gets... complicated. The finer they are, the trickier it gets. I’ve found it much easier to collect clay that’s mostly clay.  

Another thought is to add a more plastic ingredient, like bentonite, Veegum, or ball clay. That will increase drying shrinkage, and the effort it takes to dry pots without cracks. 

14% shrinkage at bisque, cone 06-04, seems like a lot, and if it hasn’t melted by cone 6 (sounds like it’s on the cusp though) that's a wide firing range. 

Tony Hansen has done so much work on prospecting, testing, and evaluating wild clays, posting it free of charge on Digitalfire. The guy deserves a medal. It’s worth diving into what he’s written if you stay on this path.

I would say, what your clay needs is for you to test it more. Not melting by cone 6 means it’s different than 90% of clay on earth. Getting that sheen at cone 6, it’s what your teacher said, vitrification, maturation. There are just a lot of possibilities. Do some absorption testing. Too cool!!!

Edited October 10, 2022 by Kelly in AK

[Min]

I agree with Kelly, more testing is needed before you start making mugs etc.

I'ld get some more samples, make a coil about the 1cm thick from the moist clay and wrap it around your finger and see if it cracks. If it does then the clay is "short", not plastic enough to work with and will need amending with a plasticizer or try and find another location and test the clay from there.  Ball clay and/or bentonite would be the least expensive plasticizer to add if you want to try and amend the clay. If you use ball clay it will likely increase both the maturation temp and the absorption of the fired clay. 

Do you have your own kiln to run the tests with or do you need to fire them elsewhere? I would suggest firing the clay at increasing cones up to cone 6 and see what happens with it. Measure how it slumps and the absorption. Link here on how to do that if you need it. 

[Bing]

10 hours ago, Kelly in AK said:

Beware. It’s a wild world you’re stepping into.  Maybe the clay has a high percentage of non plastic material, even though it’s throwable and feels plastic. If it’s actually sticky, then you ought to be able to “stick” a handle to it. Check out Tony Hansen on “splitting:”

https://www.digitalfire.com/glossary/splitting/

Sedimentary clays are often found in “lenses,” or lens shaped layers with sand, silt, and and silty or sandy clay. You might go back to your spot and see if there’s a more pure clay layer. Silty sandy clay can seem plastic when wet enough, yet the clay an inch below or above what you dug may be the sweet stuff. One way to differentiate that is by trying to break or crumble a totally dry piece in your fingers. The more difficult it is, the less non plastic material it has.

Sieving out the non plastics gets... complicated. The finer they are, the trickier it gets. I’ve found it much easier to collect clay that’s mostly clay.  

Another thought is to add a more plastic ingredient, like bentonite, Veegum, or ball clay. That will increase drying shrinkage, and the effort it takes to dry pots without cracks. 

14% shrinkage at bisque, cone 06-04, seems like a lot, and if it hasn’t melted by cone 6 (sounds like it’s on the cusp though) that's a wide firing range. 

Tony Hansen has done so much work on prospecting, testing, and evaluating wild clays, posting it free of charge on Digitalfire. The guy deserves a medal. It’s worth diving into what he’s written if you stay on this path.

I would say, what your clay needs is for you to test it more. Not melting by cone 6 means it’s different than 90% of clay on earth. Getting that sheen at cone 6, it’s what your teacher said, vitrification, maturation. There are just a lot of possibilities. Do some absorption testing. Too cool!!!

 

[Bing]

New to this all, and thank you so much for these thoughtful responses. I will continue since the idea of the wild really intrigues me. Your comments will lead me on. Unfortunately, no kiln of my own yet but I will experiment in the ways that I can. 

[Callie Beller Diesel]

One random thought on the clay sticking to itself: maybe experiment with your attaching method. There’s a number of different ways to attach handles because sometimes it’s not the user, it’s the clay body. I’ve noticed that some clays need to be scored with a lot of slip used, and some only need to be scratched with a wet tool. The technique that worked best on the first didn’t work on the second, and vice versa.

 

 

[neilestrick]

On 10/9/2022 at 4:39 PM, Bing said:

About 14% shrinkage

That's really high shrinkage for bisque. A typical terra cotta body is going to be about half that.

On 10/9/2022 at 4:39 PM, Bing said:

acquired a pretty sheen

If it's starting to gloss up, then it's probably past vitrification and starting to melt. I'd test it again at cone 4, which is typically the max for low fire red bodies.

A common terra cotta body is 80-90% red clay and 10-20% ball clay, so I'd start with adding some ball clay and see what happens.

[Bing]

Tried the coil around the finger test; absolutely no cracking. It’s very plastic. 
Another characteristic I forgot to mention is that it seems to dry extremely slowly. Much slower than the array of porcelains and stoneware clays I work with. 

[Kelly in AK]

Plastic, but shrinks a lot. Takes forever to dry. Sticky. 

Sounds like...High in montmorillonite, bentonite, or related minerals. A clay additive that is stable over a wide range, smooth and plastic, and without major shrinkage issues could be Lincoln 60.

There are three big classifications for clay (and a lot more little ones): kaolinite, illitic and chloritic, and Montmorillonite. The last one is an oddball. Super plastic and super swelling with water, super shrinks on drying. .  

Edited November 2, 2022 by Kelly in AK
Spelling

[glazenerd]

"Sticky" is an indication that organics are present. A particular kind of humus called Ord is commonly found in natural clay. Ord is many times more plastic than bentonite: which makes clay "sticky." Have you dissolved any in water? If so, any dark particles float to the top? Without pics, I have to assume your "dark" description means dark gray? If so, most likely you have magnetite (iron) in your clay. Magnetite will fire a terra cotta color at cone 06, and turn deep brown at cone 6. Iron disulfide (pyrite) also follows that color transformation. Hematite (iron) will keep more of its terra cotta color at cone 6. Sheering in this case is most likely a lack of fine particle clay: which you indicated it has a sandy feeling. Add 20% OM4 or equal ball clay, and see if the sheering improves. Remember to let is sit 5-7 days after you mix it in; takes a bit for plasticity to develop.

Yes, a sheen can be an indication of vitrification: but it can also occur on the surface from out gassing of inorganic materials. I am making the assumption that is magnetite from your color description. The most common iron source in the USA is iron disulfide (pyrite); which on occasion can be a dark grwy color pending organic levels or other minerals in the deposit. If you fire iron disulfide quickly, you will get a sheen from sulfur off gassing. Without pics, sort of a guessing game at this point.

Tom