THE NEW-YORK EXHIBITION ILLUSTRATED.

it has always been continued on a limited scale. Indeed, the Bohemianglass, and the Venetian with its slender, graceful forms, and curved spiral stems,parti-colored, engraved, or plain, have never been surpassed either in beauty ofform, or excellence of materials, nor have they even been successfully imitatedelsewhere until a very late period.

Such is a brief historical sketch of this art. Let us now attend more parti-cularly to its details.

Glass is, essentially, a compound of silica (th & flint is nearly pure silica), ren-dered fusible by an alkali, as soda, potash, or lime. Sometimes one of these, butmore commonly two of them, and occasionally all three, enter into the constitu-tion of glass. The oxyd of lead, is also an important constituent of what is calledflint glass. This metallic oxyd has the remarkable property of dissolving large quan-tities of silex, and of giving to the colorless glass which it forms a peculiarbrilliancy, such as can in no other way be procured. Such glass is also peculiarlyheavy, and to its density owes the high refracting power which it possesses. Ina chemical sense, glass is regarded as a salt, and belongs to the large family ofsilicates , of which numerous examples are to be observed in nature. Glass

is, however, peculiar in this respect, by which it is distinguished from nearly allnatural compounds of silex, namely, that it is entirely without any crystallinestructure. Its particles on cooling assume no regular internal arrangement, theyare homogeneous, but are without form, or amorphous, as it would be expressedin mineralogical language.

Silica by itself is a very infusible substance, and by no means could it alonebe formed into vessels by aid of heat. In its most pure form it occurs in beauti-ful transparent colorless crystals, called rock crystal or quartz, exceedingly hard,and not easily reduced to powder. Silicious sand is found, however, in manyplaces remarkably pure, and some sandstones exist that are quite pure enough toanswer the purpose of the glass house. Flints found so abundantly in the chalk cliffsof England, are also nearly pure silica, and being heated and quenched in water, theycrumble easily, and form the material of a large part of the English glass. The termflint glass came thus into use. In this country fine glass sand is found in the county ofBerkshire in Massachusetts, at St. Genevieve in Missouri, and at St. Paul’s in Minneso-ta, on the Mississippi River. A specimen of the latter (which is asyet only imperfectlyknown to the manufacturer) exists in the Mineralogical Cabinet of the Associa-tion (No. 181 Mineralogical Catalogue). M. Le Due, the Minnesota Commissioner,who deposited this specimen, has placed beside it a specimen of flint glass made from

it, which is remarkable for its purity of color. The flexible sandstone from NorthCarolina (No. 163, Class. 1), would also, no doubt, prove a good glass material.The existence of a very small quantity of any of the compounds of iron, destroysthe value of the sand in which it is found, from the color which it imparts to theglass made from it.

The heat required to fuse glass depends very much on the quantity of flux(alkali or oxyd of lead), which is used in forming the compound, but the goodqualities of the glass require that no more flux should be employed than will ren-der it easy to fashion the vessels in the process of blowing. In badly compound-ed glass, so much alkali is sometimes used that the resulting glass is soluble inwater, thus destroying one of its most essential qualities, and rendering it valueless,and where a much less excess is used it still causes the glass to sweat, or attractmoisture to its surface, and finally to become rusty or opaque. In the strictest senseall glass is somewhat soluble. The very hardest chemical glass when finely pul-verised and moistened with water, yields an alkaline reaction to tests, and waterwhich has been boiled for some time in a vessel of glass is found to contain ap-preciable traces of silica. Soda when employed alone, or in connection withlime, gives to the glass made from it a greenish color, more or less decided, whilepotash salts give a yellowish tint. To coiTect this color in soda glass, and to re-move any tint of a similar color from small quantities of iron present in the sand,it the custom of the glass blower to use some metallic oxyd, which will aid indecolorizing the product. This is accomplished either by a change which theoxyd produces in the chemical compounds present (e. g. as by reducing the per-oxyd of iron to the condition of protoxyd, which forms nearly colorless compoundswhen cold), or by supplying another color complementary to the offensive one,thus rendering the product colorless. Black oxyd of manganese is such a sub-stance, and has been long used by the glass makers for this purpose, and as itseemed to the uninformed workmen to wash out the color of their material itwas familiarly called glass maker’s soap. It requires to be used, however, withgreat caution, as it possesses the power of giving a pink, amethystine, ordeep violet color to the glass, when present even in slight excess. Its power toneutralize the green color of soda glass is probably owing to the optical effect ofthe red color of the manganese compound, which, when not in excess, wouldprove exactly complementary to the green, and white glass, would result. It is,however, quite common to see in glass articles of common use, a violet tint in thethicker parts (as in the bottoms of tumblers), due to the manganese. The whiteoxyd of arsenic is another substance constantly used in the glass house to decolorizeglass, as well as to render it, when used in excess, opaline or opaque. Borax andnitre, more costly substances, are less often employed as decolorizing agents,although they possess this property in an eminent degree.

It is thus plain that a good deal of science connects itself with the glass maker’sart, and that it is indeed truly a chemical art. To its improvement the first chem-ists living have devoted much attention, and the scientific principles involved inthe selection and compounding of the materials of glass, and of the pots in whichit is fused, are perfectly understood, and the success of the art depends on theskill and good judgment with which these principles are applied in practice.

Colors are given to glass by the use of metallic oxyds, whose combinationsresult in the production of various transparent colors. Some foreign substancesalso, as carbon and oxyd of iron, produce also various shades of color, from me-chanical suspension in the fluid glass.

Yellow is produced in cheap ordinary glass by smoke soot, or any other formof finely divided carbon, which in greater quantity renders the glass dark brownor black, but of a dirty and lustreless aspect. Glass of antimony produces a fineyellow in glass, and cheaply. Oxyd of silver, applied in a peculiar way, also formsa delicate orange in glass containing alumina, and most costly of all is the beauti-ful yellow green formed by the oxyd of uranium.

Bed. —This color is produced cheaply by the addition of finely pulverized redoxyd of iron, which, being mechanically suspended in the glass, produces abrownish red color of no great beauty. The sub-oxyd of copper , (the scaleswhich are thrown off when metallic copper is quenched in water,) producesa red of great beauty and depth of color. The metallic oxyd was also employedby the ancients to produce red glass, as the analysis of some of their specimens hasshown most conclusively. It was used likewise by mediaeval artists in coloring theglass of church windows, and its employment for the same purpose in moderntimes is but the re-discovery of an old fact. Singularly enough, this metallic oxydproduces its appropriate red color in perfection only after the glass has been cooledand heated a second time. It is in the first instance, on leaving the crucible, nearlycolorless, with a slight tinge of green, and becomes deep red on reheating, a changewhich has not been well explained. Should any decolorizing material be used inconnection w Y ith sub-oxyd of copper, the glass will be colored green instead of red,the sub-oxyd (Cu 2 0) being converted into the oxyd (CuO) of copper, which producesgreen tints. Its coloring power is very intense, and any considerable mass orthickness of glass containing it appears black. Hence it is almost invariably usedonly to flash or cover one surface of vessels to be colored red.

Cold in the form of the purple of Cassius, (a compound of gold and metallictin, produced by cautiously precipitating a solution of gold by one of tin.) will pro-duce a brilliant red color in glass, which may be graduated to produce scarlet, car-mine, rose, or ruby tints. This color is very powerful as well as expensive, onepart of gold, it is asserted, producing a decided rosy tint in 30,000 parts of glass.The same peculiarity obtains in this color also that was mentioned of the copperred, namely, that glass colored with gold is nearly colorless or slightly yellow,until it is cooled and heated a second time, when it assumes its proper tint. TheBohemian ruby glass is a peculiar color prepared in special manufactories, and soldin cakes to the manufacturer ; but the essential thing is after all gold in one of itsforms of combination, (viz., fulminating gold.) The Bohemian ruby contains notin, which probably, by its tendency to form opaline or milky glass, may have anunfavorable effect on the rose color, while a small quantity of oxyd of antimonyadded in the Bohemian red glass, heightens the brilliancy of the ruby tint. Man-ganese, as already stated, produces an amethystine tint in glass, a peculiaritybelonging to the peroxyd only, as the protoxyd of manganese gives no colors.

Green. —This color is produced cheaply in common ware by the use of protoxydof iron, but this color is feeble and. of little brilliancy; but mingled with protoxydof copper, it forms a beautiful emerald color. A grass or yellow' green is producedby using the sesquioxyd of chromium, a substance abundantly obtained from thechrome iron ores of Maryland and Pennsylvania, (No. 137, class I.) In Bohemia, the“modem emerald green,” as it is called, is produced from a mixture of the oxydsof nickel and uranium. The preparations of antimony mingled with oxyd of cop-per, also produce a fine green color.

Blue is produced almost solely by the use of oxyd of cobalt, a metal associatedwith nickel, and whose oxyd possesses the power of imparting a decided bluishtinge to at least twenty thousand times its weight of glass. The exquisite bluecolor produced in glass by oxyd of cobalt was known long before the separateexistence of cobalt as a metal was suspected; and the manufacture of glass coloredwith it, under the name of smalts , and used for giving color to pottery ware orglass, has been carried on in Germany for centuries. Zaffre is another name bywhich the impure oxyd of cobalt thus prepared is known in commerce. Cobaltand nickel are found at several places in the United States, and specimens fromConnecticut and Maryland are in the present Exhibition, (Nos. 23 and 135,class I.)

The admixture of the primary colors just enumerated gives to the glass-makerthe power of producing an almost endless variety of tints. The effect of opal-escence is gained by the use of arsenic, of oxyd of tin and alumina; and bone earth(phosphate of lime) is added to produce opacity or milkiness. Black is usuallyproduced by using some coloring matter in excess; not being a color, but only itsabsence, black is inconsistent with transparency.

Enamels are formed by the use of the colors already named, with a lead glass

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