The operation of calcining, or burning frit, requires much care. After the materials arc put into the furnace, they are stirred frequently, until all the earthy matter in the kelp is burnt away, which generally requires about three hours; the heat is then raised to such a degree, as to bring the materials almost into a state of fusion, which must be prevented by constant stirring, otherwise it would get into what is termed catches, or small knots that con tain more sand than the rest of the batch, occasioned by the alkali being dissipated from an excess of heat. If it is continued at that high temperature for about two hours, it will be sufficiently calcined. It is then taken out of the furnace, and spread, while warm, upon a plate, and di vided into large cakes ; this must be done before it cools, otherwise it would he one hard lump. It is the opinion of many that frit cannot be too old. All the opulent manu facturers, therefore, lay up great quantities, and seldom use any till it is six months old.
Crown glass is made by filling the pots on the melting furnace with frit, adding about one-eighth part of broken glass. The furnace is then raised to as high a degree of heat as possible, and in about ten or twelve hours the felt is melted. The pots are again filled up in the same manner. The. heat is then continued, and if possible in creased, till the metal, as it is now called, is completely fine, that is, pure liquid glass fit to be made into window glass, altogether requiring from 30 to 36 hours of intense heat.
After the metal is completely fine, the founder, that is the workman who manages the metal making, allows the fire to slacken for about two hours, until it arrives at what is termed a working heat ; this is called settling the fur nace, and on the proper settling of the furnace the working of the metal in a great measure depends.
For whirling or flashing crown glass few tools are re quired. The principal instrument is an iron tube, the end of which being heated, is dipped into the pot of melted glass, and turned round to collect some glass upon it. This portion of glass is distributed equally upon all sides of the end of the tube, by rolling it upon a small flat table of iron. It is then heated, and again dipped into the melt ed glass, to take up an additional quantitiy. In this state, by blowing slightly through the tube, the glass is made hollow within; and the workman judges, from the manner in which it enlarges when he blows, if the glass is so ar ranged round the end cf the tube, that it may be afterwards blown out and extend itself equally on all sides, to form a large globe, or hollow vessel of any other circular figure, of a regular thickness. If he perceives any side to be too thin, he dips that side first into the melted glass, to take up a greater quantity, and reinforce it ; and he further cor rects this, by rolling it on the marble. This operation
is repeated three or four times. The last time he heats it well in the fire, and rolls the glass with great care, to form it circularly upon the end of the tube, and lengthen it out in the manner represented by Fig. I. Plate CCLXXVI. Then, by blowing through the tube, he distends the glass into the lot in of a long hollow pear, resting it all the while upon the iron table, and rolling it round, that it may pre serve a correct circular figure, and distend itself equally. To lengthen out the neck of the pear, he rolls it, as it may require, over a smooth iron rod, fixed up horizon tally, as in Fig. 2. and the pressure thus caused will lengthen the neck. lie then blows again through the tube, supporting the glass, by resting the extreme end of the pear upon the iron rod, and rolling it round at the same time ; The pressure upon the iron a od raises a small point or eminence upon the globe, opposite the end of the blowing iron. The blowing being dexterously managed, and assisted by the pressure of the rolling, the glass is en larged to the form of a sphere, Fig. 3, which remains at tached to the tube by a neck.
By the time the workman has proceeded so far, and pro duced a globe, Fig. 3. of a very regular thickness, the glass is generally so far cooled as to grow firm ; and though it will readily yield to a pressure, it will not alter its figure by its own weight. To proceed in the operation, it must be heated again. For this purpose, it is carried to a particular mouth of the furnace, to be exposed to the heat of the flames. A slight wall is erected before one half of this mouth, to screen the workman from the heat; and the screen is at a sufficient distance from the wall of the furnace, to leave room for the globe to pass between them. A hook is fixed in this wall for the support of the blowing iron, which is rested in the hook, at a few inches from the neck of the globe. This method admits of turn ing the tube rapidly round as an axis, to which the globe is affixed, and therefore revolves with it. The side of the globe opposite to the end of the blowing iron being ex posed to the heat of the fire, is gradually softened ; but the neck of the globe, which joins the rod, and all that part of the glass which is more remote from the fire, is not softened in an equal degree. In this situation, the cen trifugal force produced by the whirling, causes tile equa torial parts of the globe to fly out from the centre, by stretching or enlarging itself; but as this• enlargement will be most powerful upon the softest part of the glass, which is the end opposite to the fire, the centrifugal force will cause that end to become flattened almost to a flat plate, or at least to a slightly convex plate, as shewn in Fig. 4.