The Sound of Mica

It is the year 1634, and the Voice of America is on the air. Since the VOA is an AM (amplitude modulation) radio station, speech and music are encoded as fluctuations in the amplitude (intensity) of a radio-frequency carrier wave. The radio waves, emanating from the Great Stone Radio Tower, spread across the German countryside, and enter the long receiving antennae of hundreds, perhaps thousands, of makeshift crystal radio sets.

There, they set the free electrons of the antenna into motion, dancing back and forth in response to the reversals of the electromagnetic field. That is an alternating current, and it is “rectified” into a direct current (flowing in one direction only) by a device called a diode. It is the crystal, probably a galena (lead sulphide) crystal, which serves as the diode.

A capacitor (a device stores and discharges electrical energy) filters out the carrier signal, passing only the electrically encoded audio signal. Finally, an earphone transduces that signal into sound.

The Voice of America is not the only broadcaster, and if you want to hear it, and not the Voice of Luther, you need a tuning circuit. The tuning circuit has both an inductor (a coil) and a second capacitor. (At least one of these must be variable for tuning to be possible.)

Capacitors (also called “condensers”) are one of the most basic of electronic components. Their most fundamental electrical characteristic is their capacitance (ability to store electrical energy).

So how do you make a capacitor? The simplest one consists of two parallel conductive plates, and an intervening “dielectric.” You can actually use a stack of plates, not just two, but the conductive and dielectric layers will alternate. One wire will be connected to the “odd-numbered” plates, and a second wire to the “even-numbered” ones.

It turns out that mica is probably the best dielectric material which is likely to be available in the years immediately following the Ring of Fire (RoF).


The Great Stone Radio Tower was built to trick the other European powers into thinking that long-distance radio requires massive antennas. (“Radio in the 1632 Universe, Grantville Gazette, Volume One). This bit of maskirovka was successful for only a limited time. By March 1634, the Cavrianis had figured out that the Venetian Embassy was in radio contact with Grantville. (1634: The Galileo Affair, Chap. 27). It is only a matter of time before the French and other governments realize the American capabilities, if only by inference from the celerity with which the USE acts.

Those powers will quickly appreciate the advantages which would accrue to them if they, too, had radio communications for diplomatic and military purposes. We can expect that collecting information on electronics in general, and radios in particular, is going to be a fairly high priority for the multitude of spies in Grantville and Magdeburg.

The knowledge of how to build crystal radio receivers is being widely disseminated (“Waves of Change,” Granville Gazette, Volume Nine), so the focus will be on finding out how to build suitable transmitters. Details appear in another article in this issue (“Radio 3: Grantville Gazette, Volume Nine), but the simplest transmitter would be of the “spark gap” type. This can send Morse code, but not music or speech.

Now it turns out that any spark gap transmitter will need at least one capacitor that can handle high voltages. To survive the high voltages, the capacitors must use mica as the dielectric.

Mica is less critical insofar as receiver capacitors are concerned, but a receiver employing mica will have greater sensitivity than one using an alternative dielectric. That is important if you are on the fringe of the broadcast area.


While spark gap transmitters will initially be used by foreign governments, it is only a matter of time before the knowledge of how to make them is passed on to others, such as merchants. The Cavrianis used the American radio to advantage in the futures market, and their counterparts will be quick to perceive the benefits of acquiring their own radio capabilities.


As the ability to receive a radio broadcast spreads, other political groups—some hostile to the USE—will want to make sure that they can speak to the radio audience. And there will be other broadcasters, whose interests are economic rather than political.

To actually “speak,” you need a radio transmitter which can simulate a continuous wave. The Voice of America’s transmitter is a rebuilt, high-powered, “ham” radio outfit, while the Voice of Luther will broadcast from a “Fessenden Alternator” constructed with down-time materials (“Radio in the 1632 Universe,” Grantville Gazette, Volume One). I assume that the Fessenden Alternator will initially be beyond the capacity of down-timers lacking direct USE technical assistance.

The most likely alternative would be a variation on the Poulsen arc transmitter, which combined an arc lamp, a coil, and a capacitor. Like the capacitor of the spark gap transmitter, this one needs to endure high voltages.


Of course, some folks won’t want to broadcast themselves, but will be keen on jamming the transmissions of others. Capacitors can be used in radio jammers, too. (“Little Jammer Boys,” Grantville Gazette, Volume Nine).


It is time now to take a closer look at why mica is so desirable for capacitor construction. Mica, to begin with, is an insulator. All insulators can be used as dielectrics, but they differ in terms of their ability, per unit thickness, to separate charges (and thereby store energy). The measure of that ability is the dielectric constant. The dielectric constant of mica is about 4–9; of common materials, only glass is superior (about 5–10).

Another important characteristic is dielectric strength. If too great a voltage is applied across the plates of a capacitor, the current will force its way through (this is called “breakdown”) and may arc-weld the plates together. Higher voltages can be tolerated if the dielectric layer is made thicker, but that reduces capacitance. Hence, high voltage capacitors are usually made of materials with a high dielectric strength (a measure of the ability of a material, per unit thickness, to resist arcing).

Mica has superior dielectric strength (5,000 kV/inch, versus 2,000–3,000 for glass). So a thin mica capacitor can resist a high voltage. Having a high dielectric strength is particularly important if you are constructing a transmitter capacitor.

When a voltage is applied to a capacitor, charges build up on the plates, but some of the electrical energy is lost as heat. Ideally, the capacitor has a low dissipation factor. The dissipation factor for mica is.0003–.0004 for mica, versus.01–.05 for soda lime glass. (Eccosorb).

Thermal stability (how much does the capacitance change if the temperature changes?) is also of interest if the capacitor is being used outdoors or in unusual environments. For mica, “Capacitance will change only -2% at -54°C, and to +3% at +125°C. ” (McCloskey).

Mica has other great properties, too. It splits readily into very thin, flat sheets, which are flexible, heat-resistant, chemically inert, and, in some cases, transparent. The latter property led to its use in house windows in Russia (“muscovy glass”) and in oven windows in the United States (“isinglass”).

Jason Cole of the University of Waterloo writes, “When it comes to modern technology, sheet muscovite is an indispensable resource. It is used in almost every electronic device sold today as an insulator. Its high resistance to the passage of electricity and heat are so great that no substitute, artificial or natural, have proved to be economically suitable to replace it. No other mineral has better cleavage, flexibility or elasticity. It is possible to roll a sheet of muscovite less than 0.1mm thick into a cylinder 6mm thick and its elasticity would enable the sheet to flatten out again quite easily. Sheet mica is just as important to the electrical and electronic industries as copper wire and now ranks as one of the essential minerals of modern life.”


So what is mica, exactly? It is a group of aluminum silicate minerals. The two most important species for the electronics market are muscovite mica and phlogopite mica.

Muscovite micas are divided into the ruby and green varieties, based on color. The term “ruby muscovite” includes the clear forms.

Phlogopite micas are “rarely found as colorless transparent sheets”; they are sometimes called “amber” micas. Rouse (352) says that they can’t be used for capacitors because their power loss is usually over 1%, whereas the maximum permissible losss is 0.04%. They tended to be used in OTL mostly for high-temperature applications. Cole says that “Muscovite mica cannot be used in temperatures that exceed 550 degrees Celsius, whereas, phlogopite can be utilized at temperatures up to 1000 degrees Celsius.”


Commercially, mica is classified according to its thickness, size and appearance. (Paramount; Rouse, 340–1).

The term “block mica” tends to refer to large, thick pieces (at least 7 mils, one mil is one-thousand of an inch) which can be split and trimmed into usable sheets.

Sheet mica is at least 1 1/2 by 2 inches in size, and thinner (say, 1–7 mils) than a block. The thinner sheets, if of high quality, are sometimes referred to as “film.”

The term “splittings” refers to pieces which are smaller than the smallest standard sheet, but at least 0.75 square inches in area. They are usually thin, too, perhaps 1–2 mils. Below that size we have waste or scrap mica, sometimes divided into flake mica and mica powder.

It is possible to assemble splittings into what is called “built-up” mica. Also, mica flakes and powder can be be used to make “reconstituted” mica (“micanite”).

Micanite was invented in 1892 (Rajgartha, 4). Micanite is described in EB1911 as consisting of “small sheets of mica cemented with shellac or other insulating cement on cloth or paper.” That is probably a sufficient description for the up-timers to duplicate it, if need be.

Paramount says that “the dielectric material used in the production of mica paper capacitors is reconstituted mica paper that is impregnated with a polymer resin.” Nonetheless, the preferred material for capacitor dielectrics is certainly sheet mica. Chowdhury (257) says that “the highest quality mica, absolutely flawless, is required for radio and wireless purposes.” (See also Rouse, 343). And Rouse (376) says that “the power loss shown by the bentonite films [for built-up mica] . . . is too high for them to be used in condensers . . . ”


The quality rating of mica seems to have gone through many changes. Depending on which references you consult, there are anywhere from twelve to twenty possible rankings. The ASTM presently uses twelve (V-1 to V-7, V-7A, then V-8 to V-10A), which range from “clear” to “densely black and red stained. ” I would expect, based on another source, that only V-1, V-2 (“clear and slightly stained”), V-3 (“fair stained”) and V-4 (“good stained”) are considered “capacitor grade. ” (Misc. Diel.)

We may be less picky in 1632, of course. Especially for receiver capacitors (USGS). The lesser grades of mica can, of course, be used as insulators, and that is a major use of micanite.

Mica Sources Reported in USE Reference Materials

As is too often the case for raw materials needed by the USE, the best known, most prolific sources of sheet mica are far away. We may have to pass through enemy spheres of influence to reach them; they may even be under enemy control.

So where do the encyclopedias direct us? EB1911 “Mica” says that muscovite sheets are found in India, the United States (South Dakota, Colorado and Alabama), and Brazil (Goyas, Bahia and Minas Gerais), and phlogopite in Canada and Ceylon.

The other Grantville encyclopedias don’t distinguish between muscovite and phlogopite. The sources they list are:

Encyclopedia Americana: India, Madagascar, Brazil, USA (New Hampshire, North Carolina, South Dakota).

World Book Encyclopedia: India, Brazil, Madagascar, USA (North Carolina, New Mexico, South Dakota).

Modern Encyclopedia Britannica: India, South Africa, Soviet Union, Brazil, Argentina, Canada, Madagascar, USA (North Carolina, Idaho, South Dakota).

Collier’s Encyclopedia notes that “the United States is the largest producer of bulk mica, but the greater part of the capacitor grade mica is mined in India.” It adds that two-thirds of the American production was mined in North Carolina.

The encyclopedias provide no quantitative information. However, for the sake of the story writers, I will give a few numbers. The leaders in sheet mica production in the period 1913–1937, ignoring the secretive Soviet Union, were India (236,916,000 pounds, 74.6% of world production), United States (10.1%), Canada (4.8%), Madagascar (3.6%), Argentina (3.5%), Brazil (1.9%), and Rhodesia (1.4%). (Rouse, 349). In 2000, the largest producers were India (3,500 metric tons) and Russia (1,500)(USGS).


Prospectors are not, of course, limited to the known mica locations. However, without some kind of lead, the discovery of mica will be quite chancy.

Russian Mica

Russia, of course, has mica: the term “muscovite” is something of a giveaway. According to the Oxford English Dictionary, the term “Muscovy glass” was used in English to denote sheet mica at least as early as 1573. In 1604, J. Marston made reference to the ease with which thin sheets could be split off a mica “book” (“She were an excellent Lady, but that her face peeleth like Muscovy glass.”) Hooke’s Micrographica (1665) refers to the unusual optical properties of thin sheets of “Muscovy glass” (sheet mica from Russia).

What practical use was made of it? Mica sheets could serve as the clear but heat resistant panes of a lantern. T. Dekker, in 1606, referred to “a candle in a Muscovy lanthorn.” They could also be used in more conventional windows. K. Digby noted in 1644 that the windows of his cabin were made of Muscovy glass.

It is clear from the foregoing that, by 1632, mica had been discovered in Russia, and exported to other countries. I don’t have any economic data for the early seventeenth century, but in 1681, Russia exported 92,882 pounds of mica to Holland, 86,400 to England, and 18,000 to North America. (Chowdhury, 178). I have no idea what percentage of this was sheet mica, but the most likely use of the mica was in windows, and that would have required transparent (or translucent) sheets.

The Russians used it mostly as an “upscale” window material. In the late 1660s, Tsar Alexei Mikhailovitch had a summer palace built at Kolomenskoe, near Moscow. It had 3,000 mica windows.

Where does the Russian mica come from? The Hammond Citation World Atlas, which most likely made it through the RoF, contains an economic map of Russia and, lo and behold, identifies mica localities. I pity whoever goes to the Atlas’ European Russian site; it is a little west of the far northern town of Kandalaksha, in Karelia. There are four mica localities shown to exist in Asiatic Russia, two of which are in the general vicinity of Irkutsk.

Fortunately, we don’t have to find the mica ourselves, we just have to trade for it.

Indian (and Ceylonese) Mica

The encyclopedias consider India to be the premiere world source of muscovite sheet mica, so it is reasonable to consider it more closely.

According to Brown (541), “the date of the commencement of mica mining in India is lost in antiquity.” However, it is unclear which of the current mica fields were known in the early seventeenth century.

Of course, we can give our Indian trading contacts some hint as to where to look, if need be. EB1911 says that mica is mined in Haziribagh (Bengal) and Nellore (Madras), and a prominent Nellore mine is “Inikurti.”

I have also studied the economic map of India in the Hammond Citation World Atlas. To avoid inadvertent bias, I compiled my list of mica localities from the Atlas before examining even the encyclopedias, let alone any of the professional geological texts. I would estimate that the Atlas can be used to localize mica sites with an accuracy of perhaps 25–50 miles. The greatest is in Bihar, close to Hazaribagh. A second is also in Bihar, at the same longitude as Asansol, but north of the Ganges River. Monghyr, on the south bank, is nearby. The third is in Andhra Pradesh, near Nellore. That is all that the up-timers will know.

The USGS says that in India, “mica mines are operated in the States of Bihar, Andhra Pradesh, and Rajasthan. In Rajasthan, the principal mines are at Banjari, Barla, Bhojpura, Chapi, Galwa, Ganeshpur, Ghegas, Laxmi, and Sidiras. ” An Indian government report says, “the main mica-sites in Andhra Pradesh are found at Atmakur, Ravuru and Gudur of Nellore district. Large deposits of Mica are also found at Tiruvuru in Krishna District, Madhira [Khammam District], and Ankannagudem of West Godavari, all in AP. ” (MMPIndia).


The importation of mica sheets from India will be much easier if, by the early seventeenth century, it was already an article of commerce, something our traders could just ask for. Gathering information on this issue has been something of an exercise in frustration.

Mica (Hindustani abrak, from abr cloud or abru the heavens) was used in ancient Indian medicine. The medical use of mica was probably of mica powder. Those who sell the medicines, or even the powders, may not know how to obtain the mica sheets. So it would be better to identify trade goods in which the mica sheets are used in intact form.

The mineral supposedly has been employed “from time immemorial, for ornaments, decorations and glazing, as well as by artists for their transparent paintings. It finds a place in the tinsel decorations of banners, taziahs and umbrellas at festivals and weddings. Its powder is sprinkled on clothes, fans and toys, as well as being incorporated in the glazes of some forms of pottery . . . .” (Brown, 541). Chowdhury (6) says that the “early” use of mica was in medicines, ornaments and vestries for idols, decorating, glazing or transparent mediums, and as a painting base (ground).

Like Brown and Chowdhury, EB1911 notes that sheets of mica have been used in India as a surface for painting. However, I believe that this began only after India came under British rule. (Swaveda, Chennai). A computer search in the Minassian Collection of Persian, Mughal and Indian Miniature Paintings turned up six mica paintings, all of which were dated as “Company School.” Likewise, Swallow, Arts of India: 1550–1900 refers only to “Company” paintings on mica, not to Mughal works of this type.

Still, Swallow offers some hope. He claims that mica was “originally used for festive lamps and the illuminated windows of the taziyas, elaborate model tombs carried by Muslims in the Muharram festival processions . . . .” One of his sources (Skelton) says that according to a contemporary account, “artists were employed to paint sheets of mica for the festival lamps at Murshidabad in the reign of Murshid Quli Khan.” Murshid was the nawab of Bengal 1706–1725.

I think it is reasonably likely that mica was known to Indian merchants of our period, especially those familiar with the Bengalese towns of Trichinopoly, Patna, Murshidabad and Benares.


While India is known for muscovite mica, Ceylon is a source of phlogopite. “In Ceylon, the mineral forms irregular veins, rarely exceeding one or two feet in width, traversing granulite, especially near the contact of this rock with crystalline limestone.” (EB1911, “Phlogopite”). Unfortunately, the Hammond Atlas doesn’t reveal the location of the Ceylonese deposits.

New World Mica

In the United States, we can find mica in several states. Mica resources are identified on the Hammond Atlas economic maps for Alabama, Colorado, Connecticut, Georgia, Maine, New Hampshire, North Carolina, South Carolina, and South Dakota. There was low-level collection and trading of mica by the American Indians in pre-colonial times. For example, North Carolina sheet mica, cut into the shape of a hand, was found at Hopewell site in Ohio. The artifact is from AD 1–400.

However, to acquire significant supplies of mica, we would have to rediscover the OTL sites and mine the mica ourselves.


The EB1911 article on “Phlogopite” adds that “in Canada it occurs with apatite in pyroxene rocks which are intrusive in Laurentian gneisses and crystalline limestones, the principal mining district being in Ottawa county in Quebec and near Burgess in Lanark county, Ontario.” The Hammond Atlas map for Quebec shows mica west of Montreal and north of Ottawa.


In South America, Brazil and Argentina are both mentioned by the encyclopedias. According to the Hammond Atlas, in Brazil it is found northeast of Rio de Janeiro and Belo Horizonte. It doesn’t map out the Argentine sites.

African Mica

Madagascar is famous nowadays as a source of phlogopite mica. Despite Rouse’s criticism of phlogopite as susceptible to high power loss, if we can’t obtain muscovite mica in quantity, we might need to make do with phlogopite.

According to the Atlas, the Madagascaran mica is near Faradofay (Fort Dauphin), in the southeast. I suspect, from the lack of any reference to it in EB1911, that it was not discovered until later in the twentieth century. If so, then the only way to obtain Madagascar mica is probably to send prospectors and, once they are successful, set up a mining operation.

As of 1632, there was no major European presence in Madagascar; there may be scattered missionaries. Control of the island is divided among several small kingdoms.


There are three mainland African mica sites: near Louis Trichardt in the Northern Transvaal (South Africa), in Tanzania, south of Lake Rukwa, and in Zambia, south of Chipata. The first of these is probably the one contemplated by the modern EB.

Australian Mica

Close study of the Hammond Atlas would reveal that there is mica in the area north of Alice Springs, Australia. Since that is in the hostile Australian Outback, it is unlikely to be investigated (except perhaps as a sideline to gold prospecting somewhat further north).

Mica in Friendly Territory

So is there mica in friendly territory, that is, in USE-controlled Germany, Sweden, Finland, and the Swedish-controlled Baltic coast? Not to the knowledge of the people in Grantville. Sadly, the atlas offers no clues as to the whereabouts of mica occurrences in Europe, other than in Russia.

But yes, there are modest mica deposits in those areas. Rockhound databases (Mindat) can provide a long list of German, Swedish and Finnish localities where muscovite mica can be found. That, of course, doesn’t guarantee that sheet mica can be extracted in a commercially feasible manner.

Just so the storytellers of the 1632 universe know where mica could in fact be found, I have consulted some specialist up-time texts. The results appear below.


Germany. Chowdhury (183) asserts that Germany produces lithia mica (lepidolite) but no sheet mica. And Rajgartha (104) confirms that there is no “primary production” in West Germany. A 1945 U.S. government report agrees that there is no German mica production. (“Non-Metallic Mineral Resources of Germany,” p. 27).

That said, you might find a small “book” of mica here and there. Hochleitner, Minerals: Identifying, Classifying, and Collecting Them has a photo (150) of a nice little muscovite specimen from Bavaria. Its sheets look like they would work fine for a receiver capacitor.

However, even if sheet mica can’t be mined in Germany, there is some evidence of early modern use of it there. Rajgartha (3) says that “very small quantities [of mica] were employed by the toymakers of Nuremberg to serve instead of glass, and the waste flakes of mica were sprinkled over carpets and draperies, which were regarded as fashionable when thus decorated. Mica strips were used for mounting microscopic objects and also for sealing zoological objects preserved in spirits.” Any early seventeenth-century German use of mica was probably of Russian material, but it is within the realm of possibility that local specimens played a role.


Finland. The USGS reports that Finland produced 5,591 metric tons of mica in 1994. Rajgartha (93) says that muscovite mica is produced from Kemjawi as a byproduct of feldspar mining, and phlogopite at Siilinjarvi from an apatite mine.


Sweden. The USGS adds that Sweden is “known to produce mica.” Chowdhury says that Sweden produces mica at Gotenborg, and from the Bohuslan district, and that the United States imported 924 pounds from Sweden in 1928. Rouse (357) says that Swedish annual production is in the 50,000 to 250,000 pound range.

Skow (56) claims that Sweden produces virtually no high quality sheet mica. I think that Skow is sadly mistaken. According to Bowie, while the mica in the pegmatite dikes of central Sweden is “generally of a poor quality” (135), the same cannot be said of all of the Swedish deposits. At Essljung in southern Sweden, “high quality transparent muscovite occurs in large flakes and has been extracted.” (142) At Brattas, over the period 1885–1994, 132 tons of muscovite was mined (together with 35,000 tons of feldspar and 19,500 tons of quartz). Munkeby also has “abundant muscovite of high quality,” and 215 tons of good quality mica was mined there 1942–44.

Perhaps the most spectacular Swedish mica find was in Northern Jamtland, where a “book” of mica was uncovered which was 55 by 85 centimeters (21.7 by 33.5 inches) in surface area, and 18 centimeters (7 inches) thick. (Bowie, 195).

Until USE-trained geologists map the pegmatite fields of Sweden, the USE and Sweden will presumably import muscovite mica from Russia, or possibly, India.

Mica for Our Enemies

As of 1633, the USE was at war with France, Denmark, Spain, the Holy Roman Empire, and England.


France. Several up-time sources, not available in Grantville, reveal that modern France is a producer of mica. Newman says that France produced 10,000 metric tons of mica annually in 1998–2000, as a byproduct of kaolin mining in Brittany (at Lanvrian and Kerbrient). Even higher numbers are quoted by Euromines for 2000–2002, the figure including “mica recovered from mica schists and/or kaolin beneficiation.” Madhukar says that the French kaolin ore is 10% muscovite mica. What isn’t clear is what percentage of the muscovite qualifies as sheet mica. It doesn’t much matter, since any French knowledge of domestic mica locations would be purely fortuitous.

But that isn’t going to stop Richelieu, if he really wants mica. Under the Treaty of Ostend, France was given England’s North American colonies, from “Plymouth Rock” to “Jamestown” (1633, Chap. 23). And it sent over a thousand soldiers to the New World, no doubt to make sure that they take control of New Netherlands (New York), too. So, while mica is not one of their principal economic concerns, they certainly can send expeditions to look for it, most likely in Alabama, Connecticut, Georgia, Maine, New Hampshire, North Carolina, or South Carolina.

Actually, there is mica in Virginia (in the pegmatites of Amelia County); this is referred to in the Audubon Society field guide.


Spain. Bowie says that Spain produced 3,000–5,000 tons of mica annually in 1977–81. There is no reference to this production in the documented sources.

Hence, the Spanish are most likely to rely on New World sources. Because of the union of Portugal and Spain, the Spanish can develop the Brazilian deposits . . . once they find them. Moreover, the Spanish have certainly seen mica mirrors, and other mica artifacts, in the Americas.

The Olmecs used mica mirrors as early as 1600 B.C.E. (Hoopes). A Mayan king, buried about 350 C.E., was found with an apron of mica mirrors (Hofstetter). At one site in Aztec Teotihuacan, believed have belonged to a jeweler, over half the soil samples contained imported mica (Storey). The trade routes which brought mica to Mexico should still exist, and the Spanish can exploit them.


Austria. Chowdhury (183) states that there are “extensive mica deposits . . . in Styria and Corinthia in Austria.” Madhukar (90) acknowledges the point, but warns that the muscovite is of “very poor quality.” It is thus most likely that the Austrians will obtain sheet mica by trade with the Russians.


Denmark. Eventually, the Danes may obtain mica from Norway. Chowdhury (183) says that there is a pegmatite dike, one hundred feet thick, near Skatterlund, which produces sheet mica. Skow (55) agrees that Norway has small quantities of good quality sheet mica. Of course, the Norwegian sources have yet to be discovered by the down-timers, and the up-timers have no inkling that they exist. In the near term, the Danes are likely to go buy what they need from Moscow.


England. The English clearly have imported “Muscovy glass” in the past, and are likely to continue to do so in the new future.

Prospecting for Mica

All that the up-timers (or down-timers) know about where to find mica is what is in the encyclopedias or the Hammond Atlas. Nonetheless, they are not without resources.

First of all, there are definitely specimens of mica in rock and mineral collections. The schools will have them to support earth science instruction, and there are certainly a few rockhounds in Grantville, too. That means that we can show down-time merchants and miners exactly what we are looking for. There are also several field guides documented as existing in Grantville (Mannington), with nice color photographs and additional description of both muscovite and phlogopite.

Secondly, the Grantville encyclopedias clearly indicate that muscovite micas occur in pegmatites. EB1911, for example, says that “large sheets of muscovite . . . are found only in the very coarsely crystallized pegmatite veins traversing granite, gneiss or mica-schist.”

Popular rockhounding field guides, several of which are available in Grantville, agree. According to the Audubon Society Field Guide to North American Rocks and Minerals, “good muscovite mica specimens are restricted to granitic pegmatites.” Pegmatites are light-colored, coarse-grained igneous rocks, and they, too, are going to be found in those rock collections and field guides.

Now, pegmatites are famous as hunting grounds for large crystals. If those crystals have certain other properties, like hardness, then we have another name for them: gems. So we may be able to find a likely mica locality on the basis of mica’s association with something the down-timers are more interested in, that is, a gemstone. EB1911 says that the pegmatite “veins consist of felspar, quartz and mica, often with smaller amounts of other crystallized minerals, such as tourmaline, beryl and garnet.” The Audubon Society guide mentions associations with quartz and tourmaline. The entry on “granitic pegmatites” has a long list of minerals, which includes beryl, opal, topaz and zircon.

Brown’s specialist book on Indian minerals says that “the beryls of the mica-bearing pegmatites of India, in which they often attain huge dimensions, are too fissured and flawed and of too washed-out a colour to be of any value in the gem trade.” (598) Still, I would expect that jewel merchants would know something of these giant beryls, assuming that they had in fact been mined at this time.

Beryl is of interest in its own right as a source of beryllium, and Brown indicates that the productive beryl deposits in India are the pegmatites of Rajsthan, Bihar and Andhra, and that it is “recoverable in small amounts as a by-product of mica mining, particularly from the Koderma Forest area of Hazaribagh and the mica mines around Gudur in Nellore.” (268–9)

Chrysoberyl is also of interest; Brown says that “transparent yellow stones of good quality occur with beryl in mica-bearing pegmatites at Govindsagar, Kisangarh, Rajasthan.” (603).

Another association is with aquamarine (a gem variety of beryl); “some beautiful aquamarines have come from the mica mines, 1 1/2 miles west of Saidapuram, Nellore.” (Brown, 598)

Phlogopite micas are found in marbles, and hence it may be productive to question sculptors, and check out the marble localities in the Hammond Atlas. In OTL, the main commercial sources of phlogopite were Madagascar and Canada. (Brown)

Mica Mining and Processing

Mica is split into sheets, trimmed, and sorted, almost entirely manually. Splitting is done with a knife. Twentieth century attempts to make machines which could split mica “books” into sheets of specified thickness were unsuccessful. Trimming and sorting are even less suitable for automation, as they require judgment. It is a good thing that the up-timers are now in a world in which labor is inexpensive! Trimming can be done by finger pressure, or with a knife. (Rouse 244–5).

Mica Mine Productivity

The larger the sheet, and the higher the quality, the rarer it is. At one Indian mine, the workers extracted 123,200 pounds of rock daily, of which 7,500 pounds (6%) was “rough mica.” Preliminary sorting reduced this to 5,000 pounds. Splitting and trimming yielded just 1,000 pounds of unsorted sheet. During sorting, some additional trimming had to be done to take out flaws, leaving 932 pounds. (Rouse, 354)

The grade distribution of these sheets was 38.5 pounds clear, 90.95 pounds slightly stained, 26.85 pounds fair stained, and 775 pounds stained. The size distribution was 0.2 pounds of Special (36–48 square inches), 1.3 of No. 1(24–36), 3.9 of No. 2(15–24), 15 of No. 3 (10–15), 30.4 of No. 4 (6–10), 100 of No. 5 (3–6), 61.6 of No. 5 1/2 (2.5–3), and 720 of No. 6 (1–2.5).

Suppose that for a transmitter capacitor, you want sheets 24 square inches or larger, and of quality better than stained. The daily output of such material from that Indian mine was just 0.9 pounds: less than one part in one hundred thousand of the total rock excavated.

USE Mica Demand

In general, for a transmitter capacitor, you will need higher capacitances than for a receiver capacitor. To achieve a high capacitance, you want the dielectric to be as thin a sheet as its dielectric strength permits, and you want to maximize the effective cross section (the surface area of one flat dielectric sheet, times the number of those sheets). A typical thickness for a mica sheet is two mils (i.e., two-thousandth of an inch).

For a mica transmitter capacitor, we want a stack, one to three inches thick, in which a conductive (c) material (gold or silver foil) is interleaved with mica (m) sheets, like so: cmcmc . . . mcmc.

A transmitter capacitor will use perhaps forty square feet of mica, and have a capacitance of around four microfarads (4,000,000 picofarads). This could take the form of a one inch stack of alternating metal and mica sheets, in which there are 250 two mil thick mica sheets, each four by six inches. (Boatright, personal communication). The total mica content is 11.52 cubic inches. The density of mica is about 300 kilograms per cubic meter (0.0108382 pound/cubic inch), so one transmitter capacitor needs one-eighth of a pound of high quality sheet mica.

If there was demand in Europe for 1,000 transmitter capacitors, you would need 125 pounds of suitable sheet mica. That doesn’t sound like much, until you realize that you would probably have to process 12,500,000 pounds of ore in order to recover the sheets you wanted.


A receiver capacitor for a crystal radio only needs 10-20 picofarads of capacitance (Boatright). For that, we don’t need mica, just an air gap will do. However, a mica capacitor would increase the sensitivity of a crystal radio.

For the sake of argument, let us say that the receiver capacitor uses a single two mil thick sheet of mica, just a quarter inch on each side. That would have a capacitance of about 40 picofarads.

There are perhaps eleven million people in the USE and Sweden. Let us say 10% eventually obtain crystal radios, and that 10% of those sets are equipped with mica capacitors. If each set has two capacitors, then we need 22,000 of them. The individual receiver capacitors only use 1/100,000th as much mica as the transmitter capacitors. It is clear that the scrap from mining for transmitter grade capacitors will supply our needs for receiver capacitors.

Mica Economics

If the only mica which has economic value is that which was in large sheets of transmitter capacitor quality, then mica would be extremely expensive to mine. If you take only one pound out of every hundred thousand pounds of rock mined, and just toss away the rest, then you are talking about a very labor-intensive operation. Labor is cheap in the early seventeenth-century world, but not cheap enough.

Pliny, in his Natural History, refers to what is translated as “specular stone” or “mirror stone.” Translator John Bostock assumes that this is “transparent selenite or gypsum.” However, since Pliny says that it “can be split into leaves as thin as may be desired,” it seems more likely that it was mica. EB1911 agrees.

The principal use of the Roman “specular stone” was in buildings. This may well have been in windows, in which case it antedated, by many centuries, the Russian use of “Muscovy glass.”

Mica, besides transmitting light, is also heat resistant. For this reason, it has been used in more specialized windows; for example, as a stove window (“isinglass”), or in the panels of a lantern.

In pre-electric nineteenth-century America, mica was used mainly for oven windows and gas lamps, and that meant that the demand was limited to sheet mica. The requirements weren’t as stringent as for capacitors, but some deposits couldn’t be mined because they didn’t produce sheet mica in quantity, and what mines were worked produced plenty of waste.

That changed in 1878, when Edison invented the electric motor. Not only did the motor require electrical insulation, that insulation had to tolerate heat. Mica was ideal, and scrap mica did the job. This could be assembled into built-up mica, or ground mica could be used to make micanite. What had once been waste mica had become a commercial product, and that changed the economics of the industry for the better. (Anglin)

The sparkle of mica (which is called “glimmer” in German) also gives it a decorative function. Pliny says that, at the “celebration” of the game, the sands of the Circus Maximus were strewn with the “shavings” and “scales” of the mirror stone, “with the object of producing an agreeable whiteness.” The modern equivalent is the incorporation of mica powder into the sidewalks of Hollywood, so they sparkle.

Mica powder also can be mixed into paints and cosmetics. In Federico and Ginger (Grantville Gazette Volume Six), Adriane’s skin has been “liberally sprinkled with twentieth century ‘moon glitter’ to give her a more celestial appearance.” Her “glitter” was nothing more than wet ground mica.

While this variety of uses makes mica mining more practical, large sheets are likely to be disproportionately more expensive than small ones. In 1911, an “average” sheet price was four shillings a pound, while large sheets could cost as much as fifty-four shillings a pound (EB1911, “Mica”).

Mining for Associated Minerals

The economics can be improved further if some of the associated rock isn’t really waste, but rather can be used for something else.

It is not unusual for mica to be extracted as a byproduct of feldspar mining, or vice versa. Feldspars are aluminum silicates, and the “alkali feldspars” also contain sodium or potassium.

Feldspars are not a “strategic material” like mica, but they have their uses. Alkali feldspars are used as fluxes in the manufacture of glasses and ceramics. The alkalis (sodium and potassium) act to reduce the melting point of the composition. The aluminum makes the glass harder, stronger, and more resistant to chemicals.

Feldspars are also used as fillers in rubbers and plastics. Since the USE supply of rubber and plastic is limited, that may come in handy.


Micas may be associated with beryls. Beryls are beryllium compounds, with a beryllium content of about 4%. Gem varieties include emerald and aquamarine. Material which falls shy of faceting quality is mined nowadays as a source of beryllium.

Like aluminum, beryllium is a light metal. Beryllium can be alloyed with copper or nickel.

Synthetic Mica

The Encyclopedia Americana mentions that synthetic mica had been developed, but provides no clues as to how it was produced. Hopefully, one of the chemists in the USE will know that the synthetic mica was not considered a success, so the USE will not squander resources on an attempt to duplicate it.


There are two reasons why it is worth taking a look at how mica might be exploited in the 1632 universe.

First, there is a window of opportunity in which mica will play an important role in radio development. This will be the period in which Europeans rely on spark gap and arc transmitters. Bear in mind that even after the USE shifts to more advanced, vacuum tube- or transistor-based equipment, some of our rivals will still be using the older technology.

Secondly, this study provides a sampling of the problems which the up-timers face whenever they try to get their hands on a familiar raw material. You can’t just order sheet mica (or rubber, or borosilicate glass, or gasoline, for that matter) over the phone or internet, and have it on your doorstep a few days later.


While some might prefer the glitter of gold to that of mica, the latter is precious in its own way.


Mica, Generally

Rouse, Chapter XII, “Mica,” in Strategic Mineral Supplies

Chowdhury, Handbook of Mica (1941)

Spence, Mica, its Occurrence, Exploitation and Uses (1912)

Skow, Mica: A Materials Survey (USDI 1962)

Rajgarhia, Mining, Processing and Uses of Indian Mica, with Special Reference to the Bihar Mica Fields (1951)

“Mica Introduction,”

Cole, “Mica,”

Anglin, Women, Power and Dissent in the Hills of North Carolina; Chapter 3, Mica Mining, is available online at

Mineral Information Institute, “Mica,”

USGS, “Mica Statistics and Information,”

(Mineral Commodities Summaries for Sheet Mica 1996-2005, Mineral Yearbook for Mica 1994-2004, Historical Statistics for Sheet Mica) and

Glover, “The Spruce Pine Mining District [NC],”

Answers.Com, “Mica,”

Works on Mineral Resources

Mindat, “Muscovite” entry at

(detailed locality info available)

Bowie, Mineral Deposits of Europe, Vol. 1, Northwestern Europe (1978)

Newman, The Mineral Industry of France (USGS 2000),

Euromines, “France: Production of Mineral Commodities,”

Non-Metallic Mineral Resources of Germany (1945)

Mica Properties and Classification

Hyperphysics, “Dielectric constants,”

“Mica,”, Kirk-Othmer Encyclopedia of Chemical Technology

(dielectric constant)

McCloskey, “Reliability Design Guide for High-Voltage Capacitors,”

(temperature drift)

Inderchand Rajgarhia & Sons, “Physical Properties of Mica,”

“Miscellaneous Dielectrics,”

(dissipation factor, temperature drift)

Eccosorb, “Dielectric Chart,” constant vs. loss tangent)

ASTM, “ASTM Standard Qualities of Mica, Quality Classification of Mica Based on Visual Properties,”

Johnson, “Lossy Capacitors,”

Kuphaldt, Chapter 3, Lessons in Electric Circuits (2004)

(dielectric strength)

“Dielectric Constant Reference Guide,”

“Dielectric Constant, Strength, & Loss Tangent,”

Mica History

Arthur, “History of Western North Carolina – Chapter XXV Mines and Mining” (1918), online at


[Chennai], Government Museum Chennai, “Tanjore paintings,”

MMPIndia, “Brief Outline of Mica mining in Andhra Pradesh,”

Hoopes, Ancient American Civilizations: Mesoamerica (1998)

Hofstetter, “Acropolis Plaza: Structure 6F-3”,

Storey, Life and Death in the Ancient City of Teotihaucan: A Modern Paleodemographic Synthesis 33, 82-3, 97 (2001)

The Natural History of Pliny, Book XXXVI, Chap. 45, p. 369.

(Bostock transl.)


Pegmatite Interest Group,


Kilowatt Classroom, “Capacitors,”

Appendix 1: Tuning the Radio Receiver

The Voice of America is not the only broadcaster, and if you want to hear it, and not the Voice of Luther, you need to make some circuit modifications. In order to be able to receive broadcasts on different frequencies, you need a tuning circuit whose resonant frequency can be adjusted.

Does the term “resonant frequency” make you gasp for air? If you pumped a playground swing as a kid, you were matching the rate of the swing to the resonant frequency of the swing (which is a pendulum), so that each energy input increased the amplitude of the motion.

The crystal radio set’s tuning circuit is composed of an inductor and a capacitor. You can make an inductor just by wrapping wire around a dowel to make a coil. You can make a capacitor by taking two plates of a conductive material (a metal) and putting an insulator in-between. The USE’s preferred insulator is likely to be a sheet of mica; more on that shortly.

Appendix 2: What Does a Capacitor Do?

Hook a battery to a capacitor, and electrons (particles of negative electric charge) build up on one plate, but can’t cross to the other because of the intervening insulator. In the meantime, some electrons are pushed out of the other plate, so it becomes positively charged by reason of their absence. Thus, a capacitor is a device for separating charges.

By virtue of this charge separation, a capacitor stores electrical energy. Connect the two plates by a wire, and the electrons move out of the negative plate and into the positive plate, eventually restoring both to electrical neutrality. You can think of the capacitor as being a bit like a dam, which stores the energy which the water obtains by being lifted (this is an imperfect metaphor, so don’t belabor it).

But doesn’t a battery store electricity? No. A battery converts chemical energy to electrical energy when you hook it into a circuit. The flow of electrons from a battery is relatively slow. If you discharge a capacitor by connecting the plates with a wire, the discharge is fast. Like a dam breaking.

Appendix 3: How Does a Capacitor Help Tune a Receiver?

Put a capacitor together with an inductor the right way, and interesting things happen. Let us say that the top plate of the capacitor is negatively charged (electron-rich). When the plates are connected by a wire, the capacitor discharges a current of electrons. If the wire is uncoiled, that would be the end of the matter. But if it is wrapped into a coil, forming an inductor, that current has side effects. The electrons traveling through a coil create a magnetic field, and, when the current stops, that magnetic field collapses.

This “induces” a new current in the wire, which deposits electrons into the bottom plate of the capacitor, thus recharging it, but reversing its polarity (i.e., which plate is negatively charged).

Eventually the effect of the changing magnetic field is spent, and, the capacitor is free to discharge. Like receding flood waters, the current reverses. Electrons move out of the bottom plate, and pass once again through the coil. A new magnetic field is created and destroyed, this time generating a current which throws electrons onto the top plate. We have now completed a cycle, which can repeat itself as long as there is electrical energy to maintain it. The electrical energy of the capacitor is swapped back and forth with the inductor each cycle.

If all we had were an inductor and a capacitor, this swapping would eventually come to a halt. The circuit components have an inherent resistance to electrical current; as a result of this resistance, a bit of electrical energy is dissipated as heat energy with each cycle. In the absence of some additional energy input, the current would eventually dwindle away—like a swing which stops when no one is swinging or pushing it.

But that’s where the antenna comes into the picture. Those radio waves themselves carry energy. If that energy is fed in at the right intervals, then the oscillations of the inductor-capacitor system is reinforced. That “right interval” is the resonant frequency of the system. The circuit is “tuned” to Voice of America if its resonant frequency matches the carrier frequency of that radio station.

Of course, if you want to be able to select Voice of America today, and Voice of Luther tomorrow, you need to be able to change the resonant frequency of the circuit. That means that either the inductor or the capacitor must be variable.

Appendix 4: Transmitter Capacitors

Capacitors aren’t just used in receivers. They are also used in even the simplest radio transmitters, the so-called spark gap transmitter. Those are illegal nowadays, because they radiate at many frequencies, but in the 1630’s, there are only a few radio stations, and they are relatively low-powered, too.

The most basic spark gap transmitter used a circuit in which there was, in series, a capacitor, an inductor, a resistor, and the “spark gap” (a short break in the circuit). Once the voltage across the capacitor reached a high enough level, a spark crossed the gap. That then completed (temporarily) the circuit, and the capacitor discharged. Thanks to its interaction with the inductor, an electrical oscillation occurred, which produced the radio wave in an antenna.

Appendix 5: Capacitance

The capacitance of a capacitor is a measure of its ability to separate charges and store energy. If the charge on one plate is +Q, the charge on the other plate will be -Q, and the voltage across the capacitor will equal the capacitance times Q. The basic formula for the capacitance of a parallel plate capacitor is

C = 0.224 K S (N 1)/d


C = Capacitance in picofarads

K = Dielectric Constant

S = Area of one plate in square inches

N = Number of plates

d = distance between plates in inches (in effect, the thickness of the dielectric layer)

Note that N-1 is the number of dielectric layers, and (N-1) times S is really just the combined surface area (on one side) of all the dielectric layers. So one big two-plate capacitor can have the same capacitance as a stack of many small plates. Also note that it is advantageous to have a dielectric which can readily be obtained in flat, thin sheets.

Appendix 6: Dielectric Constants

(the higher the dielectric constant, the better the material is for use in a capacitor)

Air (at atmospheric Pressure) 1.0

Rubber 3.0

Paraffin Coated Paper 3.5

Cambric 4.0

Pyrex 4.5

Wood 5.0

Quartz 5.0

Fiber 5.0

Bakelite 5.0

Porcelain 6.0

Mica 6.0

Glass 8.0

(Source: Kilowatt Classroom)

Appendix 7: Dielectric Strengths

Here are the dielectric strengths, in kilovolts per inch (kV/in), for some common materials:

Vacuum ——————— 20

Air ———————— 20 to 75

Porcelain —————— 40 to 200

Paraffin Wax ————— 200 to 300

Transformer Oil ———— 400

Bakelite ——————- 300 to 550

Rubber ——————— 450 to 700

Shellac ——————– 900

Paper ———————- 1250

Teflon ——————— 1500

Glass ———————- 2000 to 3000

Mica ———————– 5000

(See )

[another source says Mica has excellent characteristics, with a dielectric strength of 3800-5600 V/mil (1 mil = 0.001″ = 0.0254 mm). Teflon is also very good, with 1000-2000 V/mil. Polystyrene has a strength of 500-700 V/mil. Pyrex and soft glass can resist 335 V/mil and 200-250 V/mil, respectively.]

Dissipation Factors

Teflon (.0001-.0002) is superior to mica, but won’t be available to the USE anytime soon. (Eccosorb)


About Iver P. Cooper

Iver P. Cooper, an intellectual property law attorney, lives in Arlington, Virginia with his wife and two children. Two cats and a chinchilla rule the household with iron paws. Iver has received legal writing awards from the American Patent Law Association, the U.S. Trademark Association, and the American Society of Composers, Authors and Publishers, and is the sole author of Biotechnology and the Law, now in its twenty-something edition. He has frequently contributed both fiction and nonfiction to The Grantville Gazette.


When not writing (or trying to get an “orange blob” off his chair so he can start writing), he has been known to teach swing dancing and folk dancing, or to compete in local photo club competitions. Iver adds, “I can’t get my wife to read my fiction, but she has no trouble cashing the checks.”

Iver’s story “The Chase” is in Ring of Fire II