Mining Glaze Materials in Colorado

Carl Judson runs Judson Pottery, a commercial ceramic studio located on the Phantom Canyon Ranch 20 miles north of Ft. Collins, Colorado. Carl mines his own glaze materials from mineral deposits on ranch property. He offered to show me where he found his materials and how he prepares them for glazing.

“It’s not difficult to find glazing materials,” Carl says. “Almost anywhere you fire at stoneware temperatures you can find materials to use in within walking distance, or at most a short drive away. Readily available materials are one reason ceramics are so widely produced across the globe,” he continues. “The main difficulty is breaking down minerals that you find into small enough particles (about 200 mesh) that will properly suspend in fluid when applied as a glaze.”

Carl got interested in ceramics as a high school student. He read several influential books, including Bernard Leach’s A Potter’s Book and Michael Cardew’s Pioneer Pottery, which contained the gem of the idea of locally-sourced materials for ceramics. He also studied chemistry and has a keen interest in geology.  He told me he would sneak out of school to explore the hillsides for minerals.

In his mid-20s, Carl attended a lecture about ancient Chinese glazes, where the lecturer mentioned the use of granite in glazes. Knowing where he could find local granite deposits, he ground it up and put the granite into a glaze as an experiment. He has continued to mine materials and refine his glazes ever since.

We drove down several unpaved gravel roads near Carl’s ranch house. We first stopped near an exposed cliff revealing some purple-tinged rock under sandstone deposits just 20-30 feet of the roadside. “This is a remnant of the Fountain Formation from about 300 million years ago, the precursor to the Rocky Mountains that appeared about 60 million years ago,” Carl explained. “The earlier Fountain Formation mountains eroded away, revealing these conglomerate deposits. That purple tint indicates iron in the rock. Iron will produce a greenish hue in any glaze.”

We next stopped at a small roadside cut-away. “That yellowish material is very low-grade, impure clay, which we use in our brown slip glaze. We add some materials like schist, which I’ll show you later, to make this brown slip glaze harder and more durable. On many of our pots, we apply this brown slip glaze across the entire piece, then use wax or some type of stencil resist to create patterns, and over that we apply a clear glaze. In the kiln, both the clear glaze and the brown slip glazes melt.”

A mile or two down the road we came to another roadside cliff face. “This is granite sandstone. You can see this particular layer is yellowish and not red. Nearby iron deposits haven’t leached into this rock. It’s basically just small pieces of ground-up granite held together with a mineral cement. It’s easy to grind down and forms the basis of our clear glaze. Every 2-3 years I come up here with some 5 gallon buckets and stock up with a supply.”

The primary ingredients in granite are feldspar, quartz and mica.

Our final stop was alongside the road just above a beautiful creek. Here, Carl pointed out schist, which is metamorphic rock that intruded into the granite sandstone deposits under high pressure. “You’ll see dark stripes of this schist alongside the roads driving up into the Rockies,” Carl explained.

“Shist is extremely hard rock. We grind some of the is up and use it as a hardening agent for our brown slip glaze.”

Returning to Carl’s ranch, he showed me some of the equipment he uses to grind his materials. “We use two types of machines. One is a hammer mill, the other is a ball mill. The hammer mill will grind rock down to about 8 mesh, and the ball mill will reduce 8-mesh slurry down to about 200 mesh, which is what we use in our glazes.”

Hammer mill

Ball mill

“I’m personally fascinated by mid-19th Century technology, and have built my pottery production around technology of that era. With newer technology you can get more uniformity and higher throughput, but I like the pace and the slight imperfections that Civil War era technology produces. Both the hammer mill and the ball mill were used in Civil War times. If you had a small creek running nearby, that was sufficient to drive the action of both mills. We use small electrical motors here, but the technology is still the same.

In the ball mill, for example, we build a ceramic container, fill it with porcelain balls and some stones, and then dump the 8 mesh slurry into the mill. After several days, the slurry grinding through these balls and stones will reduce in size to about 200 mesh.”

“We also produce our own ash for glazing here. We use ash from a large hay fire up the road, not wood ash. We first soak the ash in water to remove solvable caustics. These caustics, if not removed, would soak out of the glaze and into the bisque clay body, changing firing characteristics of both glaze and clay. We don’t want that.

So we essentially leach out the caustics with several rounds of water soaking. We then put the washed slurry into the ball grinder, reducing it to a 200 mesh level. After that, we dry the ash for storage.”

“Simple wood ash plus clay is thought to have been one of the first glazes used in China, about 1500-1000 B.C. Ceramics made during the Shang dynasty frequently use ash glazes (as seen on the right). That simple glaze formula was used extensively in Asian ceramics, but less so in the West or today because it’s not particularly durable.

It needs some alumina and silica. Our home-made ash constitutes about 20% of our clear glaze. The granite sandstone constitutes about 40% of our clear glaze. Lime from local limestone quarry is about 10% of our clear glaze formula.”

Judson Pottery is open for visitors. More information can be found at their website. If you do drop by, say “hi” to Carl for me.

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