Clay Tests January 5th, 2024

I thought I was for the most part done with clay testing. Alas, here I am again. I will say It was nice to take a little break from clay testing, even if the testing shifted to glaze. Now that I’m back again I have a new interest in what I am trying to test for. This round is more for exploration than anything else. I want to play with materials and their proportions just to see what happens, what colors I can make. Of course it would be great to find either a better working clay body or simply explore more options to try out on larger levels.

This round of testing stemmed from the results of the last gas firing. I didn’t have many pots in the kiln, but for the pots that did come out I was disappointed. They were bloated and warped. We didn’t change anything about the firing temperature or atmosphere. I believe I may have mixed proportions slightly wrong or more likely the issues in the clay body had another chance to show themselves. I feared going back to the drawing board to try and force this recipe to work. A large part is that my feldspar “Custer Feldspar” was recently discontinued. I don’t want to create a clay recipe that already has a discontinued ingredient.

I will preface that my tests are a bit all over the place. There are a few different paths of reasoning that I followed. I had planned out the tests prior to mixing them. A lot of the first tests were simple changes to see how they affected the recipe. For example one was to switch the kaolin source from grolleg to EPK. I was interested to see if that would change the color as EPK fires slightly darker.

Below, the first round of tests are shown. All are fired at Cone 10 Oxidation

For reference, the base recipe for my clay is as follows:

“Base Recipe” - Newman Red Clay 35.00, C & C Ball Clay 7.00, Kyanite 13.00, Custer Feldspar 11.00, Barnard Slip 10.00, Unami Creek Clay 8.00, Grolleg Kaolin 6.00,

Some issues I had noticed about the recipe is mainly the use of Custer Feldspar. Can’t use that anymore. The other which isn’t terribly bad is the use of local clay from PA. That’s the Unami Creek Clay. Since I currently live in Vermont its very difficult for me to take a 7 hour drive down to PA to dig it up as well as process it. I would love to use it but it would be worthwhile to see if the removal of it had a huge effect on the clay.

The last gas firing showed me that the clay is still being overfired. The bloating and warping together prove that.

The first recipe “A” simply increases the kaolin. It is a highly refractory ingredient. My theory was increasing it would cause the clay to also become more refractory.

“A” - Newman Red Clay 36.00, C & C Ball Clay 18.00, Grolleg Kaolin, 13.00 Kyanite, 13.00 Barnard Slip 10.00, Custer Feldspar 10.00

Recipe “B” is the exact same as “A” except I replace Custer with Minspar 200 Feldspar. The idea is that the refractory addition of kaolin would counter Minspar’s slightly higher melt.

“B” - Newman Red Clay 36.00, C & C Ball Clay 18.00, Grolleg Kaolin 13.00, Kyanite 13.00, Barnard Slip 10.00, Minspar 200 10.00

Recipe “C” continues from recipe “B” and adds extra C + C ball clay. Ball clay is also refractory, not as much as kaolin, but adds extra plasticity to the clay.

“C” - Newman Red Clay 31.00, C & C Ball Clay 23.00, Grolleg Kaolin 13.00, Kyanite 13.00, Barnard Slip 10.00, Minspar 200 10.00

Recipe “D” goes off the trail but borrows some similar ideas from the other tests. This one increases fireclay and C + C while also replacing the kaolin from Grolleg to EPK. Notice the increase in Newmans and C + C have a slightly darker firing color.

“D” - Newman Red Clay 39.00, C & C Ball Clay 20.00, Kyanite 14.00, Barnard Slip 10.00, Custer Feldspar 10.00, EP Kaolin 7.0

Tests “A - D” were the first round of testing to start gaining some momentum. I was able to flush out some lingering thoughts on what I could change in the recipe to get some better desired results. I am understanding that with such subtle differences in color and texture, I will need to run some extra tests to see if the differences actually made larger changes than what I can see. I am referencing more extensive shrinkage testing as well as absorption tests. These can tell me much more about the maturity of the recipes. A major win for the moment os mostly that each test came out of the kiln without crazing or dunting.

A quick test I tried, seeing that I was unloading the pots around 200 degrees, was taking them directly out of the kiln and submerging them in cold water. Its. not the most extreme stress test but its helped before. If the recipe was faulty it would most likely crack. At least is has many times in the past. None of the tests in this round of oxidation firing cracked or crazed. I am excited and anxious for the reduction tests.

For the next section of tests I chose an approach on the color of the clay. Just like test “A” I played with the proportions of my fireclay and my kaolin. For each consecutive test I would increase the kaolin amount for the same amount I decreased from my fireclay. So if I decreased 5% fireclay I would increase the kaolin by 5%. Here are the results of those tests below.

The recipes for each go from theft to right named from Pt 1 to Pt 6:

Pt 1 - Grolleg Kaolin 20.00, C & C Ball Clay 22.00, Kyanite 13.00, Newman Red Clay 25.00, Barnard Slip 10.00, Minspar 200 10.00

Pt 2 - Grolleg Kaolin 25.00, C & C Ball Clay 22.00, Kyanite 13.00, Newman Red Clay 20.00, Barnard Slip 10.00, Minspar 200 10.00

Pt 3 - Grolleg Kaolin 30.00, C & C Ball Clay 22.00, Kyanite 13.00, Newman Red Clay 15.00, Barnard Slip 10.00, Minspar 200 10.00

Pt 4 - Grolleg Kaolin 35.00, C & C Ball Clay 22.00, Kyanite 13.00, Newman Red Clay 10.00, Barnard Slip 10.00, Minspar 200 10.00

Pt 5 - Grolleg Kaolin 39.00, C & C Ball Clay 22.00, Kyanite 13.00, Newman Red Clay 5.00, Barnard Slip 10.00, Minspar 200 11.00,

Pt 6 - Grolleg Kaolin 42.00, C & C Ball Clay 23.00, Kyanite 14.00, Minspar 200 11.00, Barnard Slip 10.00

The final grouping of tests were an experiment to see how my Pennsylvania clay colored the base recipe without any help from the Barnard slip. On its own the PA clay is a dark brown/ maroon color. It’s quite a beautiful material. For testing I took the base recipe without barnard and substituted PA clay in increments of 2-3 percent each up to 12 percent. The differences between 2 percent and 12 percent were very subtle. One characteristic of this grouping is the sensitivity to “fingerprinting” as you’ll see below. All of the other tests were relatively solid with their coloring while this batch has irregular mottled colors. It’s very interesting but I can’t quite figure out what caused it to be so different from all of the other tests.

Thats it for this half of the testing! The other half of these tests are currently being fired in a Gas kiln. I am very excited to see how the same clays react differently in a different atmosphere. I have been trying to figure out if I actually have a goal yet for any of these tests. I think the general excitement for finishing this round is the primary goal. I would love to see some pink or purple hues in the reduction fired tests but I won’t know if that will happen until the kiln is cooled down. I guess I am looking for something new and different. I’ve used my fair share of buff and general brown clays before. I am looking for something with a little more variety and life in it. Stay tuned for the next blog post where I’ll be posting the reduction half of the tests!