I is for isinglass

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 may recognise 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.
Mica mining in the United States began in 1805 in New Hampshire pegmatites. Some mines produced sheets over a metre wide.
Mica can resist temperatures as high as 700?C, and can be readily split into flexible, transparent sheets thinner than a human hair. This makes mica an excellent material for use as windows in wood or coal burning stoves.  Because the US in the 19th century relied on stoves for heating and cooking, it outpaced other countries in mica production.
Mica can withstand an electrical charge of over 1500 volts per millimeter of thickness without damage.  Thus, the growth of the electrical industry in the late 1870s created a demand for mica, which depleted US mica resources. The high cost of hand-splitting and trimming mica also created a disadvantage for domestic production when competing with low cost foreign labour. By 1885, India was a major supplier of sheet mica to the US, and 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, which further depleted US reserves and increased imports from India.
The development of the vacuum tube in 1904, its use in radios during and after the First World War, and the development and use of sophisticated electronic equipment in the Second World War, maintained the demand for mica. By then the US was almost wholly dependent on sheet mica imports.
In 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 US production of sheets and large flakes of mica has been virtually non-existent since 1976.  The US still produces tiny 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 US companies fabricate built up mica sheets by mechanised 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, eg national defence and fire alarm systems.
Some built up mica sheets are gorgeous, and, 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.
Bill Langer is a geologist with the US Geological Survey. Email:

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