The development of the vacuum tube during 1904, its use in radios during and following World War I, and the development and use of sophisticated electronic equipment during World War II maintained the demand for mica. By then, the United States was almost wholly dependent on imports for sheet mica.

During the 1950s and 1960s, the demand for mica diminished as transistors replaced vacuum tubes. The advancement of solid state electronics in the 1970s further decreased demand. The U.S. production of sheets and large flakes of mica has been virtually non-existent since 1976. The United States still produces very small mica flakes that are used in joint compound, oil-well drilling additives, paint, roofing, rubber products, and so forth. But, that’s another story.

Today, a few U.S. companies fabricate built-up mica sheets by mechanized or hand-setting overlapping, large mica flakes (imported) alternately with layers of shellac or other binder. Built-up mica is used primarily as an electrical insulation material in high-temperature, fire-resistant applications including aluminum plants, blast furnaces, kilns, smelters, and for critical wiring applications such as for national defense and fire alarm systems.

Some built-up mica sheets are gorgeous, and, just like in the early 20th century, they can be put to decorative uses. My favorite…mica lamp shades. Yet, sadly, mica is no longer used for isinglass curtains. (Still humming?)

Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at blanger@usgs.gov.