THE NEW-YORK EXHIBITION ILLUSTRATED.

cesses. The shades so much in use for covering clocks and small articles of vertu,are made by the cylinder process of blowing.

Crown Glass. —As gravity and inflation are the simple means by which cylin-der glass is blown, so in crown glass resort is had to the effect of centrifugal forceto produce a wheel of glass out of a globe previously formed by inflation and gra-vity. The metal employed for crown glass may he the same already described, orany other hard glass material. The workman gathers the glass upon his pipewith the same precautions which are adopted when cylinder glass is to be blown,but he proceeds to blow a sphere or hollow globe, with walls as evenly thickas possible. The furnace before which he operates has a circular openingcalled the glory hole , from which a powerful radiation and flame pro-ceed. This fire is fed by powdered rosin thrown in from time to time in smalldoses by a boy stationed near by. The globe being formed is brought beforethis hole and rapidly revolved on a crotch conveniently set for the purpose.An opening is made in the apex opposite to the point of attachment, as in cylin-der blowing, but the process afterwmrd is entirely different. By the exposure tothe flame of the glory hole, the revolving and now opened sphere, becomes flat-tened at the pole by centrifugal force.; the opening, at first small, gradually en-larges ; and the whole vessel flattens under the rapid revolution of the pipe, theworkman approaches the flattened surface nearer and nearer the flame, the open-ing still widens, the original globular form is exchanged for that of a flat vesselwith contracted edges; the heat and revolution are now at a maximum, when sud-denly the whole mass flashes into a regular wheel, nearly six feet in diameter, ofwhich the pipe is the nave. Hence this process is technically termed “ flashing."The wheel still glowing with fervid redness would at once collapse and fall into auseless mass if the revolution was suddenly checked; hence the workman gradu-ally withdraws it from the fire, which is also reduced in fierceness until the glasshas become cool enough to retain its form. It is then detached from the pipeby the touch of a cold iron at the point of contact, and the wheel still very hot ispassed into an oven called an annealing kiln, where it cools very slowly, in orderto temper the glass and render it tough. The wheels thus formed are neverquite flat, but are always a little arched, or crowning from the edges to thecentre. Hence the term crown glass. If the process has been skillfully per-formed, the sheets cut from the wheel are remarkably uniform in thickness, butthere is always a bulge at the centre called the “bull’s eye,” and this limits thesize of crown glass to about 36 inches in its largest dimension.

The lustre of crown glass is always superior to that of cylinder glass, which isowing to its being exposed to the high temperature of the flame during the flashing,and also to the fact that it is completed, so to speak, at one operation, while cylinderglass is several times cooled and reheated, a process which tends to devitrifica-tion, and would if often repeated render glass nearly opaque like porcelain.

Colored sheet glass for church windows, &c., is rarely colored in the pot, buta good quantity of hard glass is selected and colored of the desired tint by oneof two processes. The first consists in dipping the pipe into a crucible of moltenglass of the desired color, and gathering a small quantity on the end; it is thendipped into the pot of hohl glass, and the requisite quantity for the cylinder oper-ation accumulated as usual. The process of blowing already described, must, aswill be easily understood, result in spreading the colored glass all over the extend-ed surface of the cylinder in a thin film, like a transparent veneer. This is theusual process on the large scale, and is the mode before alluded to as practisedin giving the Bohemian ruby to vessels of that color.

The other process employs the enamelling furnace in which the glass to becolored is heated until a fusible paste, with which its surface has been previouslycovered, is melted and flows over, adhering to the glass, and acquiring at the sametime the desired color from the mineral oxyds which had been added to the paste.That paste is always of fusible lead glass, ground to a fine powder and laid on theglass surface with a brush and water. This last process is employed to producepainted glass, on which it is designed to show more than one color or tint.

Cast Plate Glass. —Plate glass is made by a process entirely distinct from thoseemployed in producing window glass. So far as rve are informed, this divisionof the glass manufacture has not been as yet established in the United States;nor is it by any means a common branch of the business in the Old World, where;f is comparatively a modern art. Abraham Ihbvart is regarded as the origina-tor oi Up idea of casting the molten glass from the fusion pots upon a table of metal.This was at the close of the seventeenth century (1688) and the St. Gobianestablishment, still so celebrated for its plate glass, w r as founded by him. TheVenetian plates were ground down from blown glass preparatory to silvering, aprocess still in use for cheap mirrors. In England the first company, “ TheBritish Plate Glass Company,” was established in Lancashire as late as 1173 ; al-though the second Duke of Buckingham, who imported his workmen from Venice,had previously met with much success in making plate glass for mirrors and coachwindows at Lambeth.

Plate glass is a soda lime glass, soda being preferred because of the muchgreater fluidity which it gives to the molten metal. The proportion of materialsused at St. Gobian are 100 parts of pure sand, 35 pure carbonate of soda, 5 ofair-slaked lime, 100 of cullet or broken glass, and such decolorizing materials

(ox. manganese) as are needed. The furnace employed is peculiar, it being ne-cessary to ladle out the melted glass from the fusion pots into quadrangular cis-terns called cuvettes, formed of the most refractory fire-clay. The materials arefused in the circular pot in about 16 hours, and the metal is then carefullyskimmed with a copper blade, taken out in copper ladles, and turned into thecuvette. Care is taken not to disturb the un fused particles of sand and variousimpurites which have settled at the bottom of the pot. In the cuvette the glassremains 24 or even 48 hours, until it is perfectly fined ; the heat is then somewhatabated for three or four hours, that the glass may fall to the proper temperaturefor casting. Preparatory to this, one of the movable walls of the furnace is takendown, and a pair of strong quadrangular tongs are attached to the cuvette, andby their aid the glass is poured out upon the casting table. This table is of castiron 10 or 15 feet long, half that breadth, and 6 or 7 inches thick to prevent itswarping when heated on one side. The surface of the bed-plate is first heated byhot coals, so as not to chill the melted glass too suddenly, and while one set ofworkmen are again preparing this surface quite clean, another set have removedthe cuvette and hung it in a crane. Being brought into this position, its contentsare turned out in a fiery cascade, which is kept within certain bounds upon thetable by iron guides, while, at the same time, a heavy iron cylinder is drawn for-ward on its axis, and pressing upon the molten surface, produces a plate of uni-form thickness and solidity. The lower surface in contact with the bed is not sosmooth, but is more accurately level than the upper surface. In five minutesfrom the time when the cuvette left the furnace, the cast plate is slid off fromits bed by a proper tool into the annealing arch, where it rests on a bed of sandfor 12 or 14 days before it is considered safe to remove it. Next comes the la-borious process of grinding after a selection has been made of those plates whichare judged to be most perfect. This is accomplished by coarse sand and waterstrewn over the upper surface of the plate, the lower being firmly bedded inplaster of paris. A smaller plate of glass attached to a stone, and heavily weight-ed, forms the muller, and this is moved either by machinery or by hand. Afterthe coarse sapd has reduced the surface to one plane, emery in different grades isemployed to make the plane surface smooth, and finally the polish is given byred oxyd of iron (colcothar or rouge) applied on cloth backed by wood. Now,when it is remembered that 7 grades of sand, and 15 of emery are used, besides thecolcothar, and that this series of processes is to be repeated for each side ofthe plate, it will readily be understood that the preparation of large mirrorplates must be a very costly and time-consuming affair. Moreover, it is onlyafter the fine grinding that the blemishes in the substance of the glass appear(air bubbles and discolorations), requiring the plates to be cut into smaller onesto save them from total loss; and added to all other sources of cost is thedanger of fracture in such repeated handlings and so many mechanical opera-tions. There is need, too, of the most scrupulous care in the choice and compound-ing of the original materials as well as in the casting, that blemishes of color, andirregularities in the inherent structure of the glass, or striaa, may be avoided; sincethe first and last requisite of a perfect mirror is the power of rendering an exactreflection, both in color and form, of the objects before it. The same care istherefore needed in the manufacture of this description of glass as in the prepara-tion of glass for optical purposes. It is not wonderful, then, that large mirrorplates should be very costly, nor that a heavy capital should be required to con-duct the manufacture with advantage.

We trace the history of our modern silvered mirrors (i. e. tin amalgam) toVenice, where they were produced by the present process in the 16th century.The ancients employed small metallic mirrors, highly polished, the form and con-struction of which we see perfectly in the specimens from Pompeii preserved inthe Museo Borbonico, at Naples. The process of “silvering” glass mirrors isvery simple. The sheet of tin foil, somewhat larger than the mirror, is laid upona smooth table, and quicksilver poured over it until it covers the tin foil with athickness of one-tenth of an inch or more; when the mercury has been swept bythe edge of a stick to clean off the drops from its surface, the glass plate scrupulouslyclean is brought even with the edge of the table, and pushed gently forward side-ways, so as to slide over the bath of mercury, its edge just dipping beneath itssurface, so as to push before it all impurities, and to exclude all air-bubbles.Weights are then evenly applied over the hack of the mirror, and the whole tableinclined to such an angle as to favor the drawing off of the superfluo—'mrcury.This requires some days or weeks, according to the size of the plate. .

additional risk and cause of cost in large mirrors, since the time consumed is notsmall, and the danger of fracture imminent. The amalgam sometimes crystal-lizes, producing imperfections which require the renewal of the whole process,and the health of those engaged in it also suffers, and is finally destroyed by mer-curial salivation.

Silvered Globes.- —It must have attracted the attention of the most casual ob-server, that within the last few years spheres of glass, sometimes of large dimen-sions, have appeared in our shops, brilliantly lined with a silver coating. Theseglobes obviously cannot be covered on their interior by the amalgamative processjust described for mirrors, and the reflecting surface is really what it seems to be,a film of metallic silver. This is put on by an extremely simple process, known