Processing hand-dug red clay in western North Carolina
Since moving to a more rural home in western North Carolina, I’ve started a deep dive into the styles and practices of traditional pottery in the Catawba Valley. PBS Folkways did a few episodes on Catawba valley pottery; one that stands out to me is the Pottery Revival in Catawba Valley. Much of the pottery history in this area focuses on Burlon Craig, who is well known for his functional pottery that is decorated with bizarre faces.
Though Burlon Craig never considered himself an artist, many do. Burlon Craig was an incredibly resourceful potter who worked with what he had access to. The clay he used was harvested from the banks of the Catawba and South Fork rivers. Burlon Craig fired all of his wares in a groundhog kiln on his property, which is a rather large in-ground terraced wood fired kiln. The kiln was able to hold hundreds of pots and jugs and it took months to have enough pottery ready to be fired. Because of this, the kiln was fired at most around four times each year. While modern potters opt for a bisque (low temperature) fire, followed by glazing, and then a glaze (mid-high temperature) fire, this process is impractical when firing in the groundhog kiln. Instead, Burlon’s pots were glazed at the leather hard stage and single fired. He made glazes using a high percentage of clay slip, pulverized scrap glass, and wood ash from his groundhog kiln. The clay slip is necessary so that the leather hard clay doesn’t just dissolve. The pulverized glass and wood ash provide silica and other fluxes and alkalies that give Catawba valley pottery its unique look.
Much of Burlon Craig’s knowledge was passed down to his son (Don Craig) and grandson (Dwayne Craig) and a few other local potters (like Kim Ellington) who have carried on the Catawba Valley pottery style. This knowledge roams around the minds of many potters and comes out occasionally in conversations, studios, and even at festivals like Hart Square Village’s Heritage Festival where a groundhog kiln is fired. For reference, this kiln was built by Kim Ellington and recently restored by Preston Tolbert. I’ve been spending more time talking with many of these potters, including Gary Lee down at Rising Sun Pottery in Lincolnton, NC, to learn more about the history of pottery in this area and to start developing a practice that echoes the spirit of the Catawba Valley. Gary has been kind enough to help with test firing the hand dug clay I’ve been processing and I’d like to pay it forward by sharing with you all everything I’ve learned in the past few months through conversations, reading, and experimentation.
First, some research
Catawba valley stoneware clays are often found in areas where the soil has a high concentration of feldspar, quartz, and mica, which give the stoneware its characteristic durability and ability to withstand high firing temperatures. Specific locations for clay harvesting can include riverbanks, creek beds, and areas where erosion has exposed natural clay deposits. Exact locations are often closely guarded secrets these days where folks will go to harvest usually only a few bucket fulls at a time (although I’ve heard rumors of a group getting a few truckloads full a decade or so ago).
Feldspar is a group of rock-forming minerals that make up about 60% of the Earth's crust. These minerals are primarily composed of aluminum silicates combined with potassium, sodium, calcium, and sometimes barium. Feldspar is significant in various geological processes and is a key component in many types of rock, particularly igneous rocks such as granite.
There are two main types of feldspar:
1. Plagioclase Feldspar: This type contains sodium and calcium. It is a major component of basalt and gabbro.
2. Alkali Feldspar: This variety contains potassium and sodium and is commonly found in granite and other igneous rocks.
Feldspar is crucial in the production of ceramics and glass. In pottery, feldspar acts as a flux, which helps to lower the melting point of the mixture during the firing process. This property makes it essential in the manufacture of stoneware, where it contributes to the hardness, durability, and vitrification of the clay body.
In the Catawba Valley of North Carolina, alkali feldspar is the predominant type of feldspar found, particularly in the region's granite and pegmatite formations. This type of feldspar contains potassium and sodium, which are crucial in ceramic and glass-making processes. The presence of alkali feldspar in the local clays contributes to the unique characteristics of Catawba Valley stoneware, such as its ability to vitrify at high temperatures, resulting in durable and impermeable pottery.
Let’s just dig it..
This past summer, I had wanted to put in a french drain in front of our house in Plateau and have this go to a couple of barrels that were to be placed underground. This meant we had a good deal of digging to do. Once we broke through the grass and top soil, I noticed that the “dirt” was sticking to the shovel. It was quite a deep red. I decided to get it a bit wet and sure enough it felt like clay. I set aside a few five gallon buckets full that were later slaked down with water and sifted to remove roots, grass, and large rocks. Not knowing too much about the process, but knowing what good clay felt like to throw with, I ended up sifting through a series of screens (600µm, 400µm, 200µm), leaving me with a slurry that contained particles roughly 200 µm and smaller.
Raw red clay harvested from just a few feet below the surface and a clay slurry after slaking down with plenty of water
This is what is sifted out from the clay between the 200µm and 400µm screens. This is mostly sand and larger particles. In hindsight, leaving this material in would probably be fine and would likely result in a more sturdy clay body.
Some playing and testing
After slaking and sifting, the clay was left to settle out in the bucket. I made it a daily chore to pour off the water from the top of the bucket for about a week. Once thick enough, the thick slurry was poured out onto a canvas sheet and laid on a plaster bat to dry. Once stiff, the clay was cut wedged more times than I can count. It took a lot of wedging to get all the air bubbles out and to get moisture more evenly distributed. After this first round of wedging, I separated the clay into roughly 1 lb balls and placed them in plastic to age for a couple weeks. Before throwing on the wheel, each lump was again thoroughly wedged.
With plenty of clay in hand ready to be thrown, I still needed to find some potters willing build with it and to fire it in their kiln. Debra Zimmerman from Groovy Girl Art was the first to give it a go. She made a few textured cups that were fired up to cone 6 with a white glaze. She confirmed vitrification by filling the finished cups with water and letting them sit overnight and observing no water leaking through the bottoms.
This is one of Debra Zimmerman's cups that was made with the "Plateau Red" clay that we hand dug and processed.
While Debra’s test was underway, I decided to throw some test tiles to bring to the studio (Rising Sun Pottery in Lincolnton) to see if this clay would meet their firing schedule ( Cone 04 bisque and Cone 6 glaze) without bloating or melting. Huge thanks to Gary Lee and the studio members for being supportive of this experiment!
Outside of this test, I also made a pitcher that we needed to prime the water pump for our new cistern. In the spirit of Catawba Valley pottery, the pitcher of course needed a face.. and one that is mean enough to show he’s the gaurdian of the water and the ground he came from. While digging, we found a marble and some broken plates that became his one good eye and his teeth.
But what’s really in it ?
Ok, cool, so now I know I can safely fire to cone 6. And, after doing a pitcher and some cups that certainly hold water, I believe that we have fully virtified, I still wanted to know what was really in this clay. The red color definitely suggest some iron-oxide is present. Of course, I also wanted to know if this clay could be fired any hotter. To determine what makes up this clay body, there are two kinds of analysis that can be done : 1) Powder XRD analysis, and 2) Clay fraction analysis. The first method is for the total mineralogy quantification. The second method uses the clay fraction separation and quantification of the clay minerals only.
Since I don’t have X-Ray diffraction equipment just lying around, I did some searching to find a university with a good geology department that also provides lab services for a reasonable fee. I found Dr. Martin Pentrack at University of Illinois, who runs an XRD/XRF materials characterization lab . After reaching out to make sure he could process what I dug out of my yard, I dried up some samples and sent them to his lab. The breakdown of the clay mineralogy by mass from the Semi-Quantitative Powder X-Ray Diffraction Analysis is given below. The descriptions of the minerals is what is suggested by ChatGPT; as with any new data presented to you, it must be verified.
Clay minerals (53%)
Kaolinite (47%) : Kaolinite is a primary clay mineral often used in porcelain. It has relatively low plasticity compared to other clay minerals like montmorillonite, so this might reduce the workability of your clay body on the wheel, but it will provide good strength and smoothness to the final product.
Chlorite (6%): Chlorite adds some plasticity but in small amounts, its contribution to workability is modest.
Non-clay minerals (48%)
Quartz (25%): Quartz is a non-plastic mineral that does not aid workability, but it adds strength to the fired piece. However, too much quartz can make the clay more brittle when unfired and difficult to work with on the wheel.
Hematite (2%): This iron oxide gives color but doesn’t significantly affect plasticity. It can enhance the strength and add red tones upon firing.
Gibbsite (14%): As an aluminum hydroxide mineral, Gibbsite adds refractory properties (high melting point), but it is non-plastic and doesn’t contribute to workability on the wheel. It can, however, affect the firing temperature.
Whitmoreite (6%) and Pyroaurite (1%): These are less common minerals. Whitmoreite, being a hydrated iron phosphate, might add minor amounts of color and complexity during firing. Pyroaurite is a rare magnesium-iron hydroxide, and though present in a small amount, it’s not likely to influence workability significantly.
What’s next
I’m quite happy with how this clay throws, though in some more recent batches of the clay, it has been a bit “short”, consistent with the low plasticity characterization from the mineralogy. To remedy this, I might experiment with adding some bentonite or ball clay in small quantities.
In addition to this red clay that’s just sitting in my yard, I’m quite interested in discovering some of the clays that are easily found along river banks and swampy areas around the Catawba valley. The stuff Burlon Craig used to throw with looked a lot lighter in color and fired to a high temperature in the stoneware range; where is this clay found ?
If you’ve made it this far, you’re definitely my new friend. Thanks for reading. Clay on.
