Carved in Stone

Brooke Wisdom

September 1, 2010

I is for isinglass

Once commonly used in coal and wood-burning stoves, U.S. production of mica has all but ceased.

By Bill Langer

ˈī-zәn-glas (noun): two materials, both of which can occur in thin, relatively transparent sheets: (1) thin sheets of mica; (2) a gelatin made from fish bladders.

(Author’s note: This article is about mica, not fish guts. I am a geologist, not an ichthyologist.)


When I hear the term isinglass, I start singing ‘with isinglass curtains y’ can roll right down, in case there’s a change in the weather.’ Those of you who are Broadway stage or old movie buffs, or have kids who perform in high school plays, might recognize the lyrics from The Surrey With The Fringe On Top, a song from Rodgers and Hammerstein’s 1943 Broadway production and 1955 motion picture film, Oklahoma. The lyrics refer to oiled canvas side curtains with large isinglass (mica) inserts used on horse-drawn carriages and early automobiles. (Go ahead, hum a few bars.)

Mica mining in the United States began in 1805 in New Hampshire pegmatites. Some mines produced sheets over 3 feet wide.

Mica can resist temperatures as high as 1,300 degrees F and can readily be split into flexible, transparent sheets that are thinner than a human hair. This makes mica an excellent material for use as windows in wood or coal-burning stoves. Because the 19th century United States relied on stoves for heating and cooking, whereas the Europeans relied on open fireplaces, the United States outpaced other countries in mica production.

Mica can withstand an electrical charge of over 3,800 volts per 1/10th of an inch of thickness without being destroyed. Consequently, the growth of the electrical industry in the late 1870s created a huge demand for mica, which rapidly depleted U.S. mica resources. Furthermore, the high cost of hand-splitting and trimming mica created a great disadvantage for domestic production when competing with low-cost foreign labor. By 1885, India had become a major supplier of sheet mica to the United States, and, soon thereafter, tariffs were imposed on mica imports.

A patent was issued in 1892 for built-up mica, whereby flakes of mica were bound together in a way that maintained their dielectric properties. Mica flakes were also used as insulators in electric motors, spark plugs, and magnetos in gasoline engines, and as a sound diaphragm in phonographs. Built-up mica was even used for decorative purposes like lamp shades made by the famous Arts & Crafts coppersmith Dirk Van Erp. These uses further depleted U.S. reserves and increased imports from India.

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

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