Popular Science Monthly/Volume 29/July 1886/Bohemian Glass
| BOHEMIAN GLASS. |
By Professor HEINRICH SCHWARZ.
THE northern edge of Bohemia, which borders on Silesia, Saxony, and Bavaria, is at once the principal seat of the German population of the country and of its industrial activity. A person approaching this border region from the interior will be struck at once with the contrast between the stagnation of the Czech districts and the freedom and active enterprise of the Germans, under the impulse of which a not very fertile soil has been made to support a dense population. Besides the textile industries which profitably utilize the water-powers of the mountainous region, and the large mining, metallurgical, and chemical enterprises, the ceramic establishments, and the manufactories of stone-ware, porcelain, and glass, are prominent features of the district.
Notwithstanding numerous efforts, the quality of the famous Bohemian art-glass has never been quite equaled anywhere else. The principal seat of its production is in Northeastern Bohemia, where this district is separated by the Riesengebirge from Silesia; but, as the result of the active trade which has been carried on over that chain for several centuries, branches of the manufacture have also spread into the latter country. The exquisite products of the Josephinenhütte at Warmbrunn have long maintained a rivalry with the Bohemian wares, and Frau Heckert's establishment in Petersdorff can exhibit wonderful specimens of luster and color, polish and etching, that might almost make one imagine he had been transported into Aladdin's palace. The bases of the manufacture are really the same on either side of the range. The mountains furnish a pure quartz and a limestone of snowy whiteness for raw materials, and the abundant woods, with which they were clothed, formerly supplied the best of fuel to the furnaces, while the resultant ashes afforded the necessary potash. The Bohemian crystal serves in art as the type of the most perfect glass, and is unquestionably recognized as one of the superior kinds, rivaling in transparency and clear whiteness rock-crystal itself. Pure specimens of it, free from blisters, grains, and specks, have a peculiarly attractive look, even, in their simplest forms. It is, moreover, by reason of its tenacity, its constancy of luster, hardness, and difficult fusibility, eminently adapted to artistic molding and ornamentation.
The heating of the furnace with wood only, from which a comparatively small quantity of ashes was produced, and they entering into the composition of the glass, contributed no little to the attainment of the highest perfection. While the glass-houses were at first built where they might be made a means of utilizing the superfluous wood, they have now to contend against a continually rising price of wood and increasing difficulty in procuring it. t Some factories, like the Josephinenhütte of Count Schaffgotsch, and that of Count Harrach at Neuwelt, have the extensive forests of their owners to rely upon, while the much more important establishment of Joseph Riedel in Polaun is looking forward to direct railroad connection with the lower Silesian coal-mines or the Bohemian brown coal.
While formerly only the best, finely-split, well-seasoned trunk-wood could be depended upon to heat the furnaces to the needed temperature, the required degree is now obtained from limbs, knots, roots, and even green wood, by distilling the gas from them in an imperfectly ventilated regenerator, and burning it with the aid of previously heated air. By this means is obtained a clear, excessively hot flame, by which the most infusible glass is made as fluid as water, and of a very high state of purity. Many experiments will be necessary before such excellence can be obtained with coal-gas; and, in any event, a previous washing of the gas will be required to clear it from tar and ashes. The form of the furnace, the manner of introducing, purifying, and tempering the glass, the processes of bringing it into shape, and the shaping tools, do not vary essentially from those of the old ways; except that complicated figures engraved in iron and brass molds are now applied, the complete transference of which to the glass necessitates the use of air under high pressure. This is furnished by means of a hand compression-pump, so arranged in connection with the other parts of the apparatus that the manipulator can bring it to bear upon the melted glass at the precise moment when it must be brought into the closest contact with the engraved pattern. Other pieces, of a massive character, such as lenses and ring-segments for lighthouse-lanterns, which are now made on a large scale at Polaun, are formed by subjecting the material to a light pressure between an upper and a lower mold. They are then finished and polished after they have cooled.
The after-decoration of the glass is various, and subject to the frequent changes of fashion. New patterns bring a high price and find a ready sale till they are crowded out of the market by newer ones. For a long time the old German fashion ruled in glass, and manufacturers were obliged to use crude, impure colors, as if they were working in the childhood of art. Now, when we remember that glass-work has been regarded from a time long past as properly an effort to obtain the clearness of rock-crystal and other precious stones, it should appear that it was wrong deliberately to come down from that high ideal. The question is the one involved in the old contest of the artists with the artisans, which is still carried on with reference to the modern coal-tar colors. The former dislike these colors because they are too pronounced; the latter are inclined to regard them with more and more favor, on account of their brilliant luster, purity, and strength of color.
The author's studies of the Venetian mosaic glasses satisfied him that the harmony of the designs composed out of them was due to the subdued, broken coloring of the pieces that entered into the work, and that this was due again to the application of an impure, ferruginous sand in the melting. We must not, however, forget that glass is used in our houses, along with the precious metals, to bring out the highest lights, which even the most harmonious pictures can not dispense with. The purer, the more lustrous, and more brilliant the color of the glass, the better it answers this purpose.
The ornamentation of the glass is done partly in connection with the exposure in the furnace, and partly in the finishing-shops, where the work is completed by cutting, polishing, tarnishing, etching, painting, and mounting in metal. The glass-houses have at their command a very complete color-scale for transparent, opaque, and clouded glasses. But it must not be supposed that a crucible is placed in the furnace for each color, from which glass colored for each ornament is to be made. The colors are worked out by means of what are called pastes, which are kept on hand in sticks or cakes. From pieces of these pastes previously warmed till they are soft, suitable quantities are cut off, laid upon the foundation of white or colored glass, and then spread out by drawing or blowing. By this means only is an economical use of such costly materials as gold and silver compositions possible. Some of the glasses thus treated—gold, copper, and silver glasses—remain still little, or not at all, colored after the melting, shaping, and quick cooling; and do not take on their bright hues till they are reheated. This is the case with the new yellow-silver glass, which continues uncolored after the intermelting of the silver salt until it is exposed in the furnace again. Very fine effects are produced by blending or overrunning of the paste-colors provided proper attention is given to the laws of harmony. A blue-glass cup is, for example, overlaid with silver glass at its upper edge, and this is drawn down in gradually thinner tones till it fades away at the foot of the vase. Gold and copper ruby-colors are thus combined with green glasses, etc. Another brilliant effect is produced when a still hot bulb of glass is rolled in finely pulverized aventurine glass,[1] and after this is melted, and previous to the shaping of the vessel, is overlaid with a coating of either colored or colorless glass. A still finer effect is obtained with mica-brocade. The mineral mica, which has deceived so many persons by its golden or silvery glitter, besides being applied as a substitute for metallic bronze dusts, can be colored by the aniline dyes in all manner of colors and shapes. The coarse powder called brocade is used in glass-work, and the color-effect is produced by overlaying it with colored glass. A bulb is blown, for example, out of clear blue glass, is rolled in the brocade, which readily adheres to it, and is then overlaid with yellow glass. The brocade will appear, when looked at from within, of a steel color; from without, of the color of gold. Every flake will reflect the light, colored according as it is looked at.
A recent kind of decoration is shown in those glasses which appear to be held together by a network of gold-thread. This is made by preparing a skeleton of brass wire, and then introducing the glass and blowing it till its mass, having penetrated the interstices of the network, spreads over it and tightly incloses it. The full effect is then brought out by a subsequent etching away of the metal, and galvanic gilding or silvering. Other metal ornaments, insertions, buttons, drops, or figurini are often combined with this. They are cast in steel-engraved forms of type-metal, which reproduce the finest details, and are then galvanically coppered, silvered, or gilded. Another pretty effect is obtained from the clouding which glasses mixed with bone-ashes exhibit on being heated. If a bulb of this kind of glass is blown into a metallic form which is dotted with projecting points, a quick cooling ensues at these places, which leaves its mark after the reheating and finishing in the shape of a regularly distributed clouding.
Only a little need be said, and that of the most modern operations, of finishing, of the grinding, tarnishing, and polishing, ornamentation with gold-leaf and platinum-foil, luster, and enamel coloring, etc. One of the most noteworthy of these operations is that of tarnishing by the centrifugal sand-blast. The objects to be treated by this process are fastened upon revolving wooden pegs in the walls of a wooden box; the sand is introduced into the middle of the box, and is thrown by rapidly rotating fans against its sides and against the glass figures. After it has done its work upon the figures it falls upon the funnel-shaped floor, to flow away and be lifted up again.
A charming effect is produced at the Neuwelt houses by means of a guillocheeing machine in which an engraver's tool is drawn in regularly massed lines over the slowly revolving vase. The vessel has been previously covered with etcher's varnish, which is removed from the lines of the engraving, where the bare glass is afterward exposed to hydrofluoric acid. In this way are produced the wave designs resembling those which are seen on the more finely engraved banknotes.
In another very recent style of ornamentation, fine Venetian glasspearls of various colors are glued by a very fusible enamel upon the surface of the finished vessel. As the arrangement is made in the cold, the work admits of a complete artistic freedom. The enamel is then dried and the setting is fixed by heating.
Another important function of the melting-furnaces is to furnish raw material for the now considerable small-glass industries in the shape of sticks and fragments of colored glass. The favorite color for these is a dark violet or black; but colorless glass is used for the pendants of chandeliers, and they are sometimes given a reddish tint by overlaying them thinly with gold-ruby. Sticks partly overlaid with opaque glass are used in a similar manner. There are always accumulating, in the glass-houses and other shops, piles of droppings, overflows, and pieces of many colors, which can be sold for very cheap prices. All this stuff is pounded up and mixed together with the addition of manganese or other coloring oxides, and is remelted in a special furnace. The workmen take out suitable quantities of this mass, and, by a series of deft manipulations, form it into sticks about as thick as one's thumb.
Very thin globes of about the size and shape of a vitriol-flask are made from the same dark glass, to be again broken up into sherds, which can be packed away in boxes. The manufacturer cuts from these sherds slightly curved plates, such as are used, for example, as foundations for brocades.
The shops of the small-workers are of the simplest character. Wherever one of the numerous little streams makes it possible to get water-power enough to drive a grinding and polishing wheel, and in the modest houses scattered along the mountain-slopes, may be found the establishments of these industrials, in which the working force of the whole family finds active employment. The artisan buys his sticks and sherds from the glass-house. A little wood-furnace, somewhat like a tinker's furnace, gives facilities for heating four or five of the glass sticks at once, which are taken out and used alternately as the ends are softened in the fire. The softened end is fastened upon by a pair of pincers, drawn out a little, and introduced into a mold in which is carved the figure of the object into which it is designed to be formed, and which is firmly stamped upon it by closing the mold and the application of pressure. If the mold is too cold, the form will be imperfectly made and the glass will be brittle; if it is too hot, the glass is liable to stick in it. Fortunately, it can be easily worked to a suitable temperature. The molded pieces are thrown into an earthen pot, which is kept warm by a moderate flame and serves the purpose of a cooling-vessel.
The button, or whatever is the article manufactured, is still only in the rude state, with the edges yet rough and the surfaces uneven, but already provided with holes for the after-insertion of metallic eyes. The rough edges are smoothed away by grinding on the grooved periphery of a wet sandstone, being held to it by a wooden clamp which is managed by the right hand while it is turned with the left. The surfaces are ground with wet sand on horizontal, fast-turning iron plates, and afterward polished on the face of soft wooden wheels roughened with Tripoli dust. To speed the operation, the workman presses upon the piece with both hands and gives it a peculiar rotary motion that equalizes the stronger friction to which the parts nearest in contact are exposed. The proper application pf this movement is a matter of knack, and is founded on mathematical principles, which also appear when the object is rubbed on a solid base, in the epicycloid lines which it is made to describe. On account of the relatively long time required for the operation of polishing, the smaller articles are subjected to what is called a fire-polishing, in which the smoothly ground pieces, imbedded on a plate of clay in fine sand, are heated in a muffle till their surface runs. If more strongly curved plates are wanted, to form a rose, for instance, the disks, previously prepared by notching and perforation in the middle, are placed in funnel-shaped crucibles in the hot muffle. The central part of the disk sinks on being heated. The hollowed leaves are then set one in another, in the order of their diminishing size, and fastened together by a glass-headed pin.
The foundation of the design is formed of a brass plate which has been previously shaped and perforated. Additional decorations are given by means of little beads, which are melted off in the glass-blower's lamp from thin threads of glass, and find their places in minute holes in the plate. Black sealing-wax is added to heighten the gloss and the blackness, as well as to cement the parts together. In other cases lighter figures are made by partly polishing or by etching them out on the smooth background. Iridization of beads, buttons, etc., has been much in vogue for a few years past; by this process those articles are given a metallic appearance. The luster of gold or silver is imparted by covering the black glass with a silver-or gold-leaf varnish and afterward heating moderately in a muffle. Peculiar tarnish effects are given by the application of what are called luster-colors; and, lastly, these are shaded by a brief treatment with chloride-of-tin vapor. The glass articles, hung upon a wire, having been previously warmed in the muffle-furnace, are drawn through the thick white vapors which are formed when a spoonful of the tin-salt is dropped upon red-hot iron. A long experience and considerable manual dexterity are required to make sure of getting the particular iris-color that is wanted, which is dependent upon a very well-defined minimum thickness of the coating. In this is involved a question of interference-colors, the same as is involved in soap-bubbles and the temper-colors of steel, in which there must be an exact difference in the wave-lengths of the light reflected from the upper and lower surfaces of the coating. Many colors, like steel-green, require repeated trials to be brought out in their full beauty. The advance that has been made in this art has been illustrated to me in a specimen-sheet of beads which are designed to make trimmings exactly corresponding in color for silks of a very great variety of shades.
In addition to the glass industry, a very extensive interest has been developed in the manufacture of brass, bronze, pinchbeck, etc., in which use is made of various galvanic coatings of metal. These branches of the art are carefully taught in the industrial school at Gablonz.—Translated for the Popular Science Monthly from Unsere Zeit.
- ↑ A glass containing bright metallic flakes, probably copper crystals in a brown magma. It is made with rare perfection, by a secret process, in Venice.