by Rose Schreiber

A truck emerges from a processing plant along route 385, just 4 miles south of the town of Custer, South Dakota. This is the Black Hills region, close to the border with Wyoming. The truck is stacked high with heavy paper bags filled with a fine white powder: a mineral, mined from a nearby open pit, known by the eponymous name of Custer feldspar. Turning onto the highway, the truck begins its wending, often thousand-mile journey out of the Great Plains and on to distribution centers across the rest of the United States. From there, these hefty bags may be sold and shipped one more time, before they are at last sold and shipped to you. 

Feldspars are found all over the world, making up half of the earth’s crust. They are formed in all the ways rocks are formed: through cooling magma, both above and underground; through the intense heat and pressure of metamorphism deep within the crust; through erosion and the accumulation of sediment. Here, in the Black Hills, Custer feldspar derives from volcanism. Just 250 miles away, as the crow flies, sits a bentonite mine in northern Wyoming—itself a testament to one hundred million years of volcanism in this region.

Of the various feldspars we use in ceramics, Custer is rich in potassium. This potassium serves as a flux, making Custer a key ingredient in mid-range and high-fire clay bodies and glazes. Compared to sodium-heavy feldspars, Custer produces melts that are more opaque and viscous. While sodium fluxes are often used in glossy, transparent glazes, potassium fluxes like Custer are more frequently used to produce mattes. Outside of the ceramic studio, Custer feldspar is used in container glass, fiberglass, abrasives, tiles, and electrical porcelain. 

Like other ceramic materials mined in this area—such as trona, from which we derive soda ash, as well as bentonite—Custer feldspar is entangled in this region’s political fabric, both past and present. Its very name, Custer, connects this material to a romanticized vision of American military history and westward expansion. George Armstrong Custer, for which both town and material are named, was a key figure in the lead-up to, and the events of, the Black Hills War—a resource war in which the United States, in blatant disregard for its own recently signed treaty, expropriated Indigenous land, forcing the Lakota Sioux and Northern Cheyenne onto ever-shrinking territory. Although the Black Hills War was driven by white settlers’ desire not for clay minerals, but for gold, it nonetheless forms part of a broader history connecting United States imperialism with the search for mineralogical wealth and other resource extraction. To this day, this land continues to sit at the center of a land dispute between the US government and the Sioux Nation. 

Bibliography/Sources: 

Custer City. “History of Custer.” Accessed February 7, 2022. https://custer.govoffice.com/history#:~:text=When%20it%20came%20to%20naming,Civil%20War%20hero%2C%20Stonewall%20Jackson.

“feldspar, n.”. OED Online. December 2021. Oxford University Press. Accessed February 8, 2022. https://www-oed-com.ezproxy2.library.colostate.edu/view/Entry/69037?redirectedFrom=feldspar.

Hamilton, Calvin and Rosanna. “Feldspar.” Science Views. Accessed February 8 2022. https://scienceviews.com/geology/feldspar.html

Hansen, Tony. “Custer Feldspar.” Digitalfire Reference Library. Accessed February 8 2022. https://digitalfire.com/material/253

Hansen, Tony. “Feldspar.” Digitalfire Reference Library. Accessed February 8 2022. https://digitalfire.com/material/310. 

Nichols, Jeremy and Biodiversity Conservation Alliance. “Petition for Objection to Issuance of Operating Permit for Pacer Mica Processing Plant.” US Environmental Protection Agency. May 18, 2006. https://www.epa.gov/title-v-operating-permits/pacer-corporation-white-bear-mica-plant-near-custer-south-dakota-petition. 

Pacer. “Our History.” Accessed February 8, 2022. https://www.pacerminerals.com/our-history/. 

“spar, n.2”. OED Online. December 2021. Oxford University Press. Accessed February 8, 2022.https://www-oed-com.ezproxy2.library.colostate.edu/view/Entry/185620?rskey=9qw9Rf&result=2&isAdvanced=false.

The US Department of Agriculture. “Brite X Mica Mine – Steffie Expansion Project Hell Canyon Ranger District Black Hills National Forest.” 

Troll, V. R. “Magma Mixing and Crustal Recycling Recorded in Ternary Feldspar from Compositionally Zoned Peralkaline Ignimbrite A’, Gran Canaria, Canary Islands”. Journal of Petrology. 43 (2): 243–270. (2002). doi:10.1093/petrology/43.2.243.

US Environmental Protection Agency. “Emission Factor Documentation For AP-42 Section 8.27: Feldspar Processing.” Accesed February 9, 2022. https://www.epa.gov/sites/default/files/2020-10/documents/b11s27.pdf.