The Uses of Mica in the Arts

Manufacturer and builder 7, 1869

Under the term mica is comprised a whole group of minerals, either occurring massive or disseminated in rocks, and all possessing a more or less foliated structure and pearly lustre. They are tough, elastic and transparent or translucent. The color may be white, gray, brown, pale green, violet, yellow, dark olive, or green. The word mica may bo derived from the Latin mica, which means a crumb or grain, or from micare, which, like the German name glimmer, signifies to shine. The term mica was formerly applied particularly to the mica in scales. The micaslate is called in German katzensilber - that is, cat-silver. The variety meet extensively ueed in the arts is the massive. This does not, however, occur in this such as are met with in the trade, but in large plates, which sometimes exceed a yard in diameter; but, being of an eminently banal cleavage, they readily afford very thin, clastic foliæ.

Mica was first need as a substitute for glass in windows; then, on account of its durability when exposed to high temperatures, it came to be extensively used for doors of stoves; and for this purpose, it was largely imported up to the year 1855, when the beds in this country were first taken advantage of. The property just spoken of has induced Mr. Warth, inspector of the royal salines in Wurtemberg to insert plates of mica in the fire-doors of steam-boilers, his aim being, tp allow a continued obeervation of the fires without opening the doors, and so permitting the entrance of cold air. The foliæ are inclosed in small sheet-iron frames, and protested by wire grates stretched over them.

Mica is also made into reflectors, sea-compasses, inlayings for wood, (instead of enamel,) and many other articles. But one of its most important uses is as a substitute for glass in spectacles, which are intended to protect the eyes of fire and metal-workers against sparks, and fragments of metel and rocks. Such spectacles are constructed by The Hermann Cohn, in Breslau, who, in examining the eyes of one thousand two hundred and eighty-three metal-workers, found that a great majority of them had been more or less injured. At first these spectacles were made without hinge, and could net be invoniently carried in the pocket; then rude hinge's were contrived, by bending a wire, and drawing it through the frame of the mica-glass; more recently, however, the inventor has done away entirely with the brass frame, and uses a ribbon to keep the spectacles in place. It may be well to mention that it simple woolen or silk ribbon serves better for this purpose than a band of India-rubber, as the latter causes pain when worn for a long time. These spectacles are, of course, easily carried in the vest-pocket. In order to protect the eyes against the action of gases, Dr. Cohn devieeel spectacles with a plain mica-plate in front, and a brass box behind, which is bent in the form of a nut-shell. They are called "nut-shell mica spectacles," and have given great satisfaction in the fire departments of Upper Silesia, as well as in a factory of artificial fertilisers in Magdeburg, where it is important that the eyes or the workmen ehould be protected from the contact of injurious vapors produced by the action of chemicals. Some firms have expressed a desire, that the brass frames, should be made large enough to protect the sides of the eyes from flying chips, and that openings should be provided in the upper and lower parts of the mounting, so that the evaporation of the moisture of the eye might take place freely. Experiments made in Verescoe and Upper Hungary have shown that mica spectacles are useful to icon peddlers; but they have to be frequently changed during such work, as the mica laminates, and becomes opaque in a very short time. Each workman, therefore, has two pairs of spectacles, and uses them alternately, his work lasting from three to five hours. Cohn at first considered it an advantage that his spectacless were very light, but an establishment in Prussia complained that their men, accustomed to dealing with hundredweights, could not habituate themselves to these light spectacles. Accordingly large mica-plates were adopted, with very massive mountings, covering the whole orbit of the eye, and provided with s second hinge, that they might be fastened to the ears; and although they were more expensive, these spectables found the readiest sale. Some difficulty wae found in meeting a demand for blue mica apectacles, which came from the directors of the royal railway of Upper Silesia. Mica, being very smooth, does not retain paint, at least without varnish, and lacque colors are easily removed by breathing upon the plate and rubbing it. In order to obviate this difficulty, an experiment was made with a solution of gum arabic or albumen, colored with blue ink, which was apread on one mica-plate and covered with another, soas to be inclosed between two floæ. Such spectacles could be cleaned well enough, but the fine air-bubbles which sometimes occurred in the gum arabic solution were not easily removed. Finally the gum Arabic was abandoned, and in its place was used the commercial blue gelatine, of a color corresponding to No. 4 of the blue cobalt glass. The melting-point of this gelatine is 158° Fahrenheit, a temperature higher than any to which the eye is exposed.

M. Puscher, the well-known chemist in Nuremberg, has lately pointed out some other uses for mica. By cleaning thin lamella with sulphuric acid and then silvering them like glass mirrors, we can obtain a very high and exceedingly beautiful lustre, admirably adapted to inlaid and other ornamental articles. Material for ornaments may also be produced by warming mica-plates, and then exposing them in a muffle to s strong red heat; in a short time, they obtain a dull, silver-white lustre when looked upon from above, and a gray, gauzy appearance if light is transmitted through them. To get this effect, the lamina, must be single, and the heat to which they are subjected must not be too strong nor too long continued, or they will turn yellow and 1ose their lustre. Mica thus made into "mica-silver" loses some of its flexibility. It is superior to the metal, inasmuch as it is not tarnished by sulphur compounds, by the action of the light and air, nor by alkalies and acid. The figures should be cut out before heating. They may be painted over with variously colored varnishes and oil paints. The uses to which they may be applied are almost innumerable. The waste of the mica-silver may also be utilized. When small particles of it are scattered over readily cast foliæ of isinglass or gelatine, and these are brushed over with glue water colored with soot, and then allowed to dry, the result is s good imitation of the appearance of granite. Splendid effects may also be produced by spreading crushed mica-silver over foliæ of colored gelatine or paper. A very fine powder rubbed together with gum arabic is used for silver ink. The finest powder of this mica-glass is sold for nine kretzers, or eight cents a pound.

F. Rotter, in Amberg, manufactures mica-brocades, which are extensively used in the production of wall and marble-paper as well as in calico-printing. These brocades contain no ingredients injurious to health; they are very light, may be produced in every shade, and comport themselves with perfect neutrality toward water, varnish, and sulphurous exhalations. Employed either between or on colored gelatine-plates, they give rise to superb crystallizations, which are used as inlayings for buttons and various other articles. Glass upon which brocade has been fixed by means of damar varnish may be employed as a background for photograph, etagères, signs, and glass closets. It is used also for lamp-stands, boxes, carvings, toys, figures of bronze and plaster of Paris, and very extensively in making artificial flowers. Decorators of theatres and churches employ it for imitating gold, rain, snow, etc. Glass pearls melted with this powder look much more like the genuine. These colors may be applied with a paste consisting of four parts of glue and one of glycerine, or of three parts of starch and one of glycerine, or damar, pale copal, and sandarac varnish. The objects are first coated with one of them pastes, and then the brocade is sifted over them. The surplus is removed shortly after by shaking, and the adhering powder is pressed in; finally, after drying, the color which does not adhere is brushed away.

With regard to the occurrence of mica iee this country, it is found in Acworth, Grafton, and Alstead, in New-Hampshire, in perfectly transparent plates, sometimes exceeding a yard in diameter, (Dana.) It is also met with in St. Lawrence County, New-York, eight miles from Potsdam, in plates seven inches across; and near Warwick, in a vein of feldspar, in plates sometimes a foot in diameter, (Dana,) not to mention a multitude of other places.

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