In the Fall of 2015 I began testing clay body additives, looking for alternatives to glaze for finishing my ceramic sculptures. During that time, I happened to see an exhibition of Japanese ceramics at the Phoenix Art Museum. Among the collection was a traditional stoneware rock vase from the Shigaraki region of Japan. The naturally occurring feldspathic inclusions that bubble to the surface were a striking feature of the piece. This got me thinking about particle size, and what might be possible if I reverse engineered the powdered materials in my glaze lab back into chunks of rock.
Defining the Terms
Feldspar: An abundant rock-forming mineral typically occurring as colorless or pale-colored crystals and consisting of aluminosilicates of potassium, sodium, and calcium. Feldspathic glazes (glazes made from crushed feldspar rock) are some of the earliest known in existence.
Frit: A powdered glass that can be either a part or the majority of a glaze. A frit is made to a specific chemical composition, batched, melted, cooled, crushed, and ground into a powder.
Shigaraki: A pottery and stoneware made in the Shigaraki region of Japan. Wares from this region are known for containing large feldspar inclusions in the clay body that melt and swell during the firing process.
Sintering: The process caused by kiln firing which solidifies the clay but does not lead to vitrification. This occurs in bodies with a low flux content or at low temperatures, such as in earthenware.
Reverse Engineering Feldspar
The nice thing about reverse engineering feldspar from powder back into rock form is you have an opportunity to control not only its particle size, but also the melting temperature and color.
To begin, make and bisque fire several small (approximately 100g volume) clay crucibles, number them to help keep track of test batches. Dust the interior of each bisque-fired crucible lightly with alumina hydrate to prevent the feldspar from sticking.
I began by taking different types of ground feldspar and sintering them at cone 05. What I found was that although most feldspars fuse at 1900°F (1038°C), they’re chalky, and once added to clay and fired to cone 10, they didn’t bubble out of the clay body like I was hoping. To adjust the melt, I did a line-blend test (1, 2), introducing a frit in with the feldspar in increments of 5% to decrease the melting temperature and harden the fusion of feldspar when sinter firing. I then fired the fused ingots of feldspar at the vitrification temperature to achieve the desired result (3). I want the mixture to fully fuse when sintering because once a colorant is added and the ingot is crushed, the feldspar can stain the clay body, which can be problematic when using white clay. The sinter firing temperature changes in relation to the final firing temperature; you want the ingot to fuse without being chalky, but not melt onto the crucible.
I have been making work at cone 6 (2232°F), and wanted to achieve a bright-color responsive formula, so I looked at commonly available frits. Because I’m making sculptural work, toxicity was not a particular concern, so after some research I settled on Ferro frit FB-284-M, a high barium, leadless frit for bright colors and satin finishes. After I got the ratio between the feldspar and frit correct, adding in colorants with reliable results was easy (4).
After the feldspar ingot is fired, I remove it from the crucible, then smash it (5) (wearing eye protection and heavy-duty gloves!) until I reach a desired chip size (6). I sieve out all the dust, otherwise the clay just looks contaminated. Once sieved, the feldspar chips can be wedged into the clay. By weight, I have been very happy with a 1:8 ratio of chips to clay in my sculptures.
Elliott Kayser is an MFA candidate in ceramics and an instructor at Arizona State University. He worked at Motawi Tileworks and is a member of the Ann Arbor Potter’s Guild in Michigan. Learn more about his work at www.kayserceramics.com.