How I Process My Wild Clay

[Tyler Miller]

There has been some interest in how to use your own dug clays, so I thought I'd share what I've been up to and maybe help a few people on their own journey to using materials claimed themselves.  Today I was processing some clay I harvested this week and last year, and I thought i'd share what I do.

 

To begin your search, I would advise you track down a publication from your State, provincial, or regional geological office.  In my case, this is "The Clay and Shale resources of Ontario"  (link here: OFR5134 - Clay and shale resources of Ontario - Geology ...  ) an excellent publication that analyzes the composition of clay and shale from around the province, fires them, and characterizes other features, such as the geology of the areas where they found the clay etc.  Your area likely has something similar.

 

When you've located a promising sample, before anything else, determine if it's worth your while even to consider it.  Test its plasticity. (All links from this point on are to my gallery)  Edit:  To actually perform this test, wedge the clay in your hands out of the ground with some water to a good working consistency.  Roll into a snake, and bend over itself and twist.

 

http://community.ceramicartsdaily.org/gallery/image/4146-img-0472/

 

If you can't get your clay to do that, move on.  It might be a good glaze, you might have a good additive for a clay body, but you're not going to be able to use it out of the ground.

 

If your plasticity test is good, awesome, dig up a bucket full.  In my case, I know this clay is good, so I picked up several hundred pounds of the stuff.    Here's a small sample.

 

http://community.ceramicartsdaily.org/gallery/image/4147-img-0473/

 

Next, cover it water. Cover it, and let it fester in a warm place for as long as you can stand.  For this particular batch, I'm going to leave it for two weeks.  It's what I can afford to leave it for.

 

http://community.ceramicartsdaily.org/gallery/image/4149-img-0475/

 

For the batch below, I left it to sit for a year.  The longer the better, I've found.  I don't have a real explanation but, I think it has to do with a change in chemistry.  Throws better.  The water is cloudy because I stirred it up to feel how it was working.

 

http://community.ceramicartsdaily.org/gallery/image/4148-img-0474/

 

Next, take a quantity and REALLY stir it, until it's like a milk shake.  Mix it as thoroughly as possible.  You'll notice that there's clumps of fibre, rocks, and other stuff in the mix.  The goal is to wash this debris of the clay its holding onto and get as much of the clay into solution as possible.

 

http://community.ceramicartsdaily.org/gallery/image/4150-img-0476/

 

Secure a window screen over a second bucket.  In my case, the trimming scrap bucket for the same clay.  You may find you need a finer mesh screen to get your clay to work for you.  Experiment.  Window screening works for me because there's literally nothing in my clay but roots and moss.  You'll find that as you pour, the screen will get clogged with debris.  Scrub this debris against the screen, squeeze it in your hands (the clay will escape, the most of the debris will stay in your hand), and return it to your first bucket. 

 

http://community.ceramicartsdaily.org/gallery/image/4152-img-0478/

 

 

Give the bucket a thorough stirring to "wash" the clay off the debris.  At the end, you'll be left with a little cow pat of a bit of clay, debris, and maybe stones.

 

http://community.ceramicartsdaily.org/gallery/image/4153-img-0480/

 

 

Once filtered through a window screen, it will pass through a 40 mesh sieve without any debris on the screen.  You may find you have a different situation.  However, you may want some debris in your clay--it adds character.  John Baymore has discussed this and the results can be pleasing.

 

After you've filtered your clay, let it settle out, siphon off the excess water and dry, pug, wedge, and otherwise process as you would reclaim.

 

It's now time to test it.  Form it into bars, measure the bars when wet, then dry, and fire them at different temperatures, if they survive drying.  How does it look at cone 04, 1, 5, and 10?  Did it spall because of lime?  Find a new source if so, but remember that some pottery styles (esp. in Mexico) embrace lime pops.

 

My particular clay melts at cone 4 and slumps at around cone 1.5.  Measure your bars after firing to each cone.  I've got a pretty heavy shrink from wet to dry and dry to mature.  This is something I've got to work toward fixing.

 

Another thing to remember is to break the bars to check for carbon coring.  This is a real pain with wild clays as there is usually MUCH more organic matter than what you'd buy ready processed.  If you've got a dark interior to your clay, adjust your firing schedule to include a good long soak (sometimes over an hour) in oxidation from when the pots begin to get colour up to about cone 04.  

 

If you decide your clay needs grog, a cost effective way of making it is to allow a portion of your clay to dry, pulverize, screen to a desirable size, and fire in a bowl to bisque temps.  I have not found that crushing failed bisqueware is a cost effective way of making grog without a ball mill.

 

I hope you guys found this useful.  I thought I'd share what I do.  Here's a pot I threw a few weeks ago with clay processed this way.  I'm drying it as slowly as possible.  I'll be firing it either this week coming or next.  Not sure it will survive, as i'm still debating if I need grog or not, but it threw nicely all the same.

 

http://community.ceramicartsdaily.org/gallery/image/4154-img-0444/

[Guest JBaymore]

I can add one thing in there in the early part of the process (unless I missed it).

 

VERY ealry on take a small sample of the clay and place an excess of vinegar onto it. If it fizzes all over, you have some calcium compounds (limestone / carbonate likely) evenly spread into the mix. That's generally OK.

 

If it fizzed from single localized little SPOTS....you have nodules of limestone in the clay. These will cause you what are known as "lime pop-outs". Almost impossibel t clean out of the body.

 

Find a new clay depsoit.

 

best,

 

....................john

[Tyler Miller]

John, excellent point.  I didn't know about that test.  I'll confess when looking for my deposit, I just steered clear of where Clay and Shale Resources of Ontario noted lime pops.  

[Benzine]

Tyler, great information! There are a lot people brining up the topic, as of late, so I'm sure they'll appreciate it.

 

I was given some "found clay" several years ago, by a Science teacher I worked with. I still have some. It is a blue green body, that turns golden yellow, when fired. It throws reasonably well, but is relatively week. I was able to snap my test tile by hand. I have never tried to fire it hotter, as I only do low fire in my classroom. However, a classmate of mine in college, brought in a similar clay. When it was taken up to our glaze temp, Cone 6???, it melted.

 

I'll have to try using it again sometime.

 

The picture you posted for your plasticity test, looks like the same consistency as mine..... It also looks like what my dog leaves behind on the lawn... hehe

[Tyler Miller]

I suspect what you might have gotten was marl, a lime-containing clay like what John was talking about.  Maybe try that vinegar test to see what happens, it will fizz if I'm right.  It's been used for millenia--the Egyptians used it and I think delftware used it to some degree, but fires a little hotter than a usual low fire.

[Benzine]

Tyler, should I water the clay down to a slip consistency, before trying the vinegar?

[Tyler Miller]

I don't know, hopefully John will chime in.  I was aware of lime containing clays, but the test is new to me.

 

I will be very interested to know how the test turns out.  Maybe it can be worked into a nice body?

[Guest JBaymore]

You can do it with a layer of dry clay powder.... or with a pinched out flat half-dollar size patty of wet clay, or whatever.

 

The key thing you are looking for is that the bubbles originate at POINTS...... spots in the clay (that might not be all that visually obvious without the budbbles). 

 

Even levels of fizzing throughout means that the limestone is evenly divided into the clay.  That isw not a problem... it will act as a source of calcium flux in the clay.

 

Limestone turns to quciklime when you heat it up past red heat.  When that quicklime in the fired body is then exposed to moisture (food or atmospheric absorbtion) it expands.  Eventually taking a piece of clay out of the wall with it.

 

best,

 

.....................john