Using Raw Clay

[seancisse]

Hello, I found some raw clay in a field near my studio, but I don’t know its composition: can I fire it stoneware or earthware?

If you have some experience to share: do you add something to this king of clay or do you start straight testing at different temperature?

Thanks in advance.

[Marcia Selsor]

I would make trays with a known clay, and then make test coils with the found clay and fire them on the trays at various temperatures to see how high it can go. 

Often, not always, surface clays are sedimentary which usually carries a lot of impurities making it lower temperature range.

 

Marcia

[Marcia Selsor]

once you figure out the temperature range you can begin refining it. Put into a slurry and screen it, dry it to workable consistency, . I the slurry after strained out twigs and stones, you can also add things to it if needed. 

Example, fire clay and grog would raise the firing temp. Mica would increase thermal resilience.

 

Marcia

[Benzine]

I have a couple experiences with found clay. In college, a classmate brought some in to the studio. The instructor let her fire it. Then I had a fellow staff member bring me in some clay clay from her property. In both cases, it was a blue green clay, that turned golden yellow, when fired. My college classmate fired it in a Cone 6-8 glaze firing inside a larger stoneware bowl. It slumped quite a bit, near to the point that I'd consider it melting. In the latter case, I only low fired it. It didn't seem to vitrify at Cone 04. It felt very porous much like a piece of Rakuware. In fact, the sample tile I made, I could snap with my bare hands, after firing.

 

I slaked and screened mine. I don't recall if my college classmate did, but I assume so.

 

I still have a one gallon bucket of the same clay. It seems to have lost some of the blue green color over the years. More of a cool grey now.

[curt]

What you really want to know is if this clay is usable. My advice below to a very similar post recently.

 

As with all native materials, how useful this material beyond the testing phase will ultimately depend on how much processing is necessary to get into workable form.

 

 

Posted 26 June 2016 - 07:10 PM

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.