Digging Up Yo' Own Clay...halp?

[Cavy Fire Studios]

So, my fella has this blue-gray clay deposit by his family's cabin. He says it's pretty silky stuff, but needs sifting to remove the impurities. I'd like to try and work with it, but here're my questions:

 

1. How do you know what temp to fire it at? Should all naturally dug deposits be treated as earthenware?

 

2. Is stoneware found naturally in the earth? How can you tell?

 

3. Any tips for harvesting?

 

Thanks, guys! <3

[oldlady]

glad to "see" you here again, guinea.  missed you.

 

someone gave me some of the blue clay they dug up.  it was just an earthenware, not blue at all, the blue color is from something i bet everyone else knows.  mine did not hold together, too much sand.

[Marcia Selsor]

In Montana there are clays of all sorts of colors. There is some pink clay that is stoneware by the Lewistown dump,etc. Trail creek clay is a ^10 slip glaze. Slake the clay down and pour through a screen. Let is dry to workable consistency. Use some clay you know to contain your tests. Make little arched shapes and fire them to various temps starting around ^04. See what you get. I did a lot of digging clays in Montana with my students back in the 70s and 80s.

Marcia

[Benzine]

I had a colleague give me some blue green clay, from her property.  It had quite a bit of debris in it.  I slaked it down, and ran it through a screen, like Marcia suggested.

 

It was easy enough to work with, and I even through some things with it.  When fired, it turned golden yellow.  It survived to Cone 04 well enough.  But it was weak at that temperature.  The test tile I made, was about a quarter inch thick.  I could snap it in half, once bisqued.

 

I never took it beyond 04.  But in college, one of my classmates brought in a similar clay.  The instructor fired it to at least Cone 6.  It melted at that temperature.  

 

I actually still have some of it left.  I may use it again for something.

[Cavy Fire Studios]

Ooooh, this could be fun! I sure wish I had an EZ-Bake Junior kiln to test out a small batch of it at varying temperatures. I only fire Fred to ^03 (he has a blank ring), so maybe I can save up for a test kiln to heat it up more, if need be.

[curt]

I assume you have access to as much of this clay as you want. If not, forget about it. Unless the material is so important to you (eg sentimental value) that you are willing to go through all the work just to be able to use it a few times.

 

If you work at stoneware temps, first step is to fire a chunk to stoneware in a small bowl. Do not bother with other temperatures since you likely do not work at those temperatures anyway. Do not sieve or clean the clay except to get rid of large rocks, roots or sticks and other obvious debris (in any case not more than 12 mesh). Just fire it mostly raw.

 

If it turns into a bubbly molten mass in the firing, put it on a back shelf (carefully labeled) and forget about it until you have NOTHiNG else to do and want to kill a lot of time for maybe something you can use.

 

If the chunk remains intact then you have something to work with. It is either just right (a 1 in 100 event) or (more likely) somewhat refractory, but either way you have a path forward to find this out.

 

Next step would be to clean the clay to a level you are likely to be able to (and want to) replicate if you start to use it in some considerable volume. No use sieving to 100 mesh to run tests if you are unlikely to do that for every bit of it going forward when you actually start to use it. The chemistry and behaviour of the clay will almost certainly be different in these two cases.

 

Then make test bars which you will want to weigh wet, bone-dry, bisqued and high-fired to get an idea of Loss on Ignition. You will also want to measure shrinkage by measuring the bar (or some markings on it) at all stages along the way. Finally, if you want the body to be usable for functional ware, you will need to boil it for a couple of hours, then let it sit in the water for a day, and then reweigh. This will give you an estimate of porosity, or how much water the fired clay absorbs.

 

As you are going through this process, gather this information for some commercial clay bodies that you actually use (from the spec sheets from the manufacturer) and compare.

 

At the end of this, you will know if your clay is

 

a) the holy grail: stoneware out of the ground. If you get this, pop the champagne corks.

 

or a refractory clay that will need some additions to be usable in the rest of your ceramic activities. But that is a whole nother chapter.

[oldlady]

 LOVE the new avatar!  a pepper??  do wish we could enlarge the avatars, some are so tiny.   want to really see the picture it is made from.

[rakukuku]

Hi guinea, love the new avatar. every guinea needs a hat.   Saw a real Cavy at the zoo recently.

 

We have a local clay here called "blue goo" . It runs off the hills and onto roadways. A few people have messed with it at earthenware temperatures. One guy I met made a flute out of it and fired it in a bonfire. It made a beautiful flute sound. As we are a strictly high fire studio we have not done much with the blue goo. I think there are books out there on how to figure out what to do with this stuff. maybe some other clays could be mixed with it. GL.  I am far to lazy to embark on this when I can go to local pottery store and buy a nice bag.    Rakuku

[High Bridge Pottery]

Good tips Curt, I just dug up some clay while helping out the dad take down his old garage. Going to use your test procedures.

[Babs]

I found a white clay when neighbour was digging s depression to catch water, a dam  is hte local name for these, and it threw beautifully and fired to stoneware temps! I also have tonnes of throwable earthen ware clay.

As Curt advises, need a  lot of time but it is exciting, I sold a line of pots made from local clay way back, with today's global tourism sickness, I would prob have a more ready market for those type of pots but the price.............. and time seems more precious now.

[rayaldridge]

When I was a young potter, I threw many tons of clay that was dug with a backhoe and and run through a tub mixer.  It came in 25 lb blocks, but before I could use it, I had to force it through a stainless steel screen to remove the tree roots and large hematite nodules.  It threw beautifully, could be fired to Cone 12, and was pretty much trouble free.

 

It's certainly worth a try.

[Marcia Selsor]

I have a friend in Canada who digs a very high temp. clay.

Residual clays are those in veins , compacted over millennia,. Sedimentary is clay formed by erosion or flooding.

it take a lot of slaking screening, drying, testing and firing. A lot.

The Lewistown dump in central Montana has a pinkish clay that fires to stone ware temps. Frances Senska, the teacher of Rudy Audio and Peter Voulkos,shared that site.

I did research on local clays around Billings area. Pryor Mountain red clay is a good earthenware. Some yellowish clay on 24th st west was a good earthenware. Gyp Springs was an ancient ocean bed and had great raw color but that all disappeared in the firings.

It takes a lot of testing. The search through beautiful Western lands is more fun.

Marcia