USES OF MICA
Two varieties of mica are of particular economic importance: muscovite or white mica, and phlogopite or amber mica. Three other varieties, lepidolite, zinnwaldite and biotite, find occasional commercial use.
Mica is marketed as sheet or block mica, mica splittings, thin sheets split chiefly from smaller sizes of block mica, and scrap or ground mica. The uses to which sheet or block mica may be put depend upon the size, thickness and shape of the piece which can be cut from it and the quality of the material itself. Factors entering into the quality of mica are: presence or absence of stains, spots, inclusions, cracks or pin holes; flexibility and elasticity; hardness; degree of distortion of the sheets; transparency; and dielectric strength.
An essential use of sheet or block muscovite is in electrical work; from the mica are made condensers for radio equipment, magnetos and certain telephone equipment; also to a less extent for resonators in sounding boxes. This mica is also used in making spark plugs, particularly plugs in high-compression engines, for winding cores and as washers in place of porcelain. These uses were widely extended by the war to meet requirements for motor transport, airplanes and radio equipment.
A great deal of this variety of mica is used for other insulating purposes. There are a vast variety of uses, such as for sheets, washers and disks in dynamos, electric-light sockets, guards in rheostats, fuse boxes, telephones, etc.
Sheet or block phlogopite is used for general electrical insulation—particularly where mica softer than muscovite is required.
Mica splittings of both muscovite and phlogopite are employed for the manufacture of “built-up mica,” which is used widely for electric insulation in many different forms, such as sheets, tubes, cups, etc. Mica board built up from phlogopite splittings is used extensively for insulation between the copper segments of commutators.
Among the less essential uses of sheet or block mica (mainly muscovite) are in windows for stove fronts and ovens; chimneys and shades for lamps and lanterns; and for many other purposes where a transparent non-inflammable, non-shattering material is required. It is also used for heat insulation, in various electric heating devices.
Ground mica is also used for heat insulation, as in pipe and boiler covering, etc.; and as a patent roofing, both as a coating to prevent sticking when rolled, and as a filler in the roofing itself. It is also used in annealing steel, and as a lubricant for wooden bearings.
Among the non-essential uses of mica, those for which a satisfactory substitute is known, are the uses of sheet or block mica for phonograph diaphragms, and for decorative purposes, chiefly in India. On a similar basis are the uses of ground mica (mixed with oil) as a lubricant for metal bearings; as a filler for rubber goods, etc.; and for decorative purposes—in wall paper, decorative paints, ornamental stone, etc.
Substitutes.
—No other substance possesses the combination of elasticity, toughness, flexibility, transparency, ability to withstand excessive heat and sudden changes in temperature, high dielectric strength, flatness and amenability to splitting into thin films, which belongs to mica.
For the vast variety of electrical equipment in which mica is used, no satisfactory substitute has been found. In the manufacture of certain low-tension condensers sheets of oiled paper have been used instead of mica films, but attempts to substitute this material more widely have met with little success. According to one report a compressed paper product called “Pertinax” was developed in Germany during the war which is claimed to be “most satisfactory” for all electrical purposes, even for the manufacture of high-tension condensers. The fact, however, that Germany was paying $75 a pound for mica from Norway, and continued to use mica in the manufacture of condensers for airplane magnetos, indicates that complete substitution was not possible.
For a great many glazing purposes it is possible to substitute heat-withstanding or non-shattering varieties of glass.