space not less than four inches remains between the two on every side, by which means the fire may be made wholly to envelope the muffle. This receptacle is provided with a tube proceeding from its front, and narrowing towards its extremity, without-side the furnace wall, the use of which tube is to examine the state of the glass from time to time during the process of firing. The iron plates for supporting and separating the glass, and which are fitted to the shape of the muffle, are kept at their proper distances, usually about one inch asunder, by legs of the requisite length placed at their four corners. The shape of the muffle is usually wider at the top than at the bottom, so that pieces of various dimen, sions may be contained in its different compartments. The number of these bears reference to the size of the apparatus, some small muffles having no more than five or six, while others of greater dimensions are provided with double that number of shelves.

If the plates of glass were placed in immediate contact with the iron shelves within the muffle, the metal would have an injurious effect upon some colours. The iron is, besides, liable to be warped; and, when the glass was brought into a softened state by heat, would communicate its own distorted shape. Another and a greater evil than even these would arise from the too sudden variations of temperature whereunto the glass would be subjected, owing to the strong conducting power of the iron, and which would imminently endanger the cracking of the glass.

A perfect remedy for all these evils is found in previously preparing a smooth and even bed for the glass, by sifting pounded whiting to the depth of a quarter or three eighths of an inch over the entire surface of the iron shelves. Upon this bed the glass must be deposited with every possible care, so as to avoid rubbing the colours.

If more than one piece is committed to each shelf, they must on no account be brought into contact, nor must they be allowed to touch or even to approach within half

an inch of the side of the muffle. When, after proceeding in this manner, the muffle has been filled, or all the glass that is ready for burning has been deposited, the cover must be put on to the muffle, and the fire lighted. It is usual to employ coke and charcoal as fuel for burning glass, both because they afford a steadier and more effective heat than coal, and because the sulphur which the latter so commonly contains might have an evil effect upon the colours.

The proper management of the fire in respect to the degrees of heat employed, is a thing which must be acquired through practice, it being impossible to give any written directions concerning it that will be efficacious. It may, however, be stated, generally, that caution is necessary in the first stage of heating, so as to avoid all suddenly great accessions of temperature; but that when, on inspection, the glass placed in the centre of the muffle is seen to have acquired a dull red heat, the fire may be urged with safety, so that the whole contents of the kiln may be made to acquire an uniform white heat. When this effect has once been produced, no more firing is requisite; the fuel which is already in the furnace must be allowed to burn itself out; and the kiln remaining thereafter closed, must be left to cool gradually during ten or twelve hours, before it is attempted to remove the glass at the end of this time it may be considered properly annealed.

The process of the second or third firing is conducted in precisely a similar manner in all respects.

The same powdered whiting which has already served may be used again for an indefinite number of times, upon being ground and sifted as for its first application.

Specimens of ancient stained glass have been occasionally found, on parts of which the colours retain their full brilliancy, while on other portions they appear to be wholly obliterated; a circumstance which has excited some surprise; and no little ingenuity has been shown in the formation of theories to account for this partial disappearance of colours. There is reason to believe,

however, that no decay has really ensued, but that, while some of the colours have been produced upon these specimens by processes similar to the foregoing description, other pieces of glass already stained in the manner of pot metal have been applied to uncoloured parts, and made to adhere by the interposition of some fluxing material, which, being softer than the glass, has been decomposed in the course of time, and these adjunctive pieces have fallen away.

CHAP. XV.

ON THE ART OF CUTTING, ENGRAVING, AND ETCHING ON GLASS.

ORIGIN OF THE ART OF CUTTING GLASS. -IMPLEMENTS. -- MANNER OF THEIR EMPLOYMENT. FROSTING. PATTERNS PRODUCED BY MOULDING. ENGRAVING ON GLASS. -EXECUTED WITH THE DIAMOND. -ETCHING. SCHWANHARD. DIFFERENCE OF HIS PRACTICE FROM THAT NOW USED. METHOD OF ETCHING. FLUORIC ACID. GLASS INCRUSTATIONS.-ORIGIN OF THE ART. IMPROVEMENTS THEREON.

THE art of cutting glass is a much more modern invention than that of painting and staining it, which has been described in the preceding chapter.

It is generally believed, that Caspar Lehmann, originally a cutter of iron and steel in the service of the emperor Rudolphus II., was the first person who attempted this mode of embellishing the material. It was about the year 1609, when, having procured from the emperor an exclusive patent for using the art, together with the appointment of lapidary and glass-cutter to the court, Lehmann prosecuted his invention with much success in the city of Prague.

Before that time, many artists had engraved figures upon glass, by means of the diamond; and their labours were greatly admired. Some glaziers had also discovered a mode of cutting glass by the employment of emery powder, and sharp-pointed instruments of hardened steel, as well as with heated irons; but these methods were greatly different in the manner of their performance, as well as inferior in their effect, to Lehmann's process, by which they were consequently, for the most part, superseded. It was, however, very long after the period already mentioned, that the art attained to any thing like the degree of perfection which it now exhibits.

At the end of the seventeenth century, glass-cutting

was prosecuted to a great extent, and in a very improved style, at Nuremberg; the artists of that place having much simplified the tools employed, as well as the methods used for their employment.

In the present advanced state of the art, the glass utensils and ornaments which contribute so greatly to the embellishment of our tables and saloons, owe much of their richness and brilliancy to the elaborate manner in which they are cut. This mode of ornamenting glass, although it does not indeed offer any field for exercising the higher faculties of genius and invention, yet calls for a considerable degree of taste in the arrangement of forms and figures.

The implements employed by the glass-cutter, although, owing to the great variety of the work which he has to execute, they are of necessity numerous, yet partake of the simplicity observable throughout the various processes of the manufacture.

In some principal establishments, steam power is used for giving motion to a shaft which causes the revolution of numerous large wheels or drums fixed thereon, and each of these being connected by a band with a pulley on the axle of a smaller wheel, occasions the latter to