glass art


There are various species of glass, each produced by a different process of manufacture, and composed of different materials, and of different proportions of the same material; but, in a work of this nature, which is chiefly intended for the glass-cutter and glazier, it does not seem necessary to speak of any other description of it than that which their trade leads them almost exclusively to employ. We shall therefore confine ourselves to crown, or windowglass, employing as few technical phrases as possible.

Crown, or window-glass, is usually composed of only two materials,—these are kelp and fine white sand. The place of the former, however, is sometimes supplied by pearl ashes, or by some other alkalis, but of these we consider it foreign to our purpose to speak. Kelp is a substance produced by the burning of what is called, in the learned world, fuci, but which will be better and more generally understood by the familiar name of sea-weed, or wrack. Of this marine vegetable there are various kinds, all designated by different classical names, which we need not enumerate. These are cut from the rocks in the months of May, June, and July. After being so cut and brought to shore, the weed, or wrack, is spread out to dry, that it may burn the more readily, and is then thrown into a pit lined with stones, in which a large fire of peat has been previously kindled. On this fire the weed is heaped from time to time, until a large mass is accumulated, and the whole is reduced to a state of fusion. It is then well mixed and levelled, and allowed gradually to cool. When sufficiently cold, it is taken from the pit, and broken into portable masses for the convenience of transportation. With regard to the other component part of window-glass, namely, sand, the best description for the purposes of the glass-maker is procured from Lynn Regis in Norfolk. The superiority of this sand arises from the circumstance of its containing a greater quantity ol minute transparent crystals than is found in the sand of any other place in this country.

When the two materials of which we have been speaking come into the possession of the glassmaker, for the purposes of his manufacture, they are thus treated, previously to their being employed in the formation of glass : The kelp is broken into small pieces, either by the hand or by a machine called a stamper. It is then put into a mill, ground to a fine powder, and afterwards passed through a brass-wire sieve.

Having undergone this operation, it is removed into the mixing room, the apartment where the proportions of material are adjusted, and where, as the name implies, they are mingled together previous to their being fritted, or calcined. The sand, again, is usually washed in a large vat with boiling or cold water, until the latter runs off quite clear. When not washed, the effect of this operation is produced by the use of nitre during the process of calcining, which consumes any sulphureous matter that may be present, or extraneous substances of an animal or vegetable nature, and reduces them to an earth not injurious to glass. The sand, it may be observed, is sometimes put into an annealing or calcining arch, where it is subjected to a strong heat for twenty-four hours, and kept during that time in a red-hot state, and then plunged into water. This operation has the effect of dividing the particles of sand, and making it unite more readily with the alkali during the process of calcining. When this operation is completed, the sand is also removed into the mixing room. Here the materials, the sand and the kelp powder, are carefully proportioned, generally in the degree of one part of the former to two parts of the latter, and mingled together according to the judgment of the mixer, an operation which requires great care and experience. When thoroughly mixed, the compost is put into a calcining arch, or reverberatory furnace, where it is subjected to a heat so strong as to reduce it to a semi-fluid state. This substance, which is called frit, is now taken from the furnace, spread upon a piate of iron while yet hot, and afterwards, but before it becomes quite cool, divided into large cakes. The last operation consists in throwing the frit into the melting pot.

This pot is made of the finest clay. The best is got from Stourbridge, and goes through a tedious and exceedingly troublesome process of drying, annealing, or tempering, &c. before it is fit for its ultimate purpose. To the frit thrown into this pot here is added a proportion of cullet, or broken crown-glass. In about thirty to thirty-six hours, the whole is reduced, by a powerful heat, to fine liquid glass, and is then ready for the operations of the workman.

The furnace is then slackened, and the metal being now in a workable state, the first operator who approaches the furnace in which the pot of liquid glass is placed, is the " skimmer," who skims off, or removes all extraneous or crude substances from the surface of the metal. Next follows the " gatherer," who is provided with an iron pipe, or tube, about six or seven feet in length, and of this shape:

Having previously heated that end of the tube which comes in contact with the glass, he dips it into the pot of metal, and by turning it gently round, gathers about one and a half pounds of liquid glass on the end of it. Having allowed this to cool for a short time, he again dips it into the pot, and gathers an additional quantity, of from two and a half to three pounds. This is also permitted to cool as before, when the operation of dipping is again repeated, and a sufficient quantity of metal, the tube, with the elongated sphere of glass at the end of it, is then handed to the blower, who heats it once more, and again at the furnace; and alternately, or between each blowing, he presses the end against the bullion bar, so called from the part thus pressed forming the centre of the sheet, or bull's eye, thus, by the dexterous management of this operation, the glass assumes somewhat of a spherical form. The blower heats a third time at the bottoming hole, and blows the glass into a full sized globe, thus, —Again applying it to the same furnace, the globe of glass, by the agency of the fire, assumes a circular form.

When this part of the process has been completed, and the glass has been allowed to cool a little, it is rested on the casher box, and an iron rod, called a " pontil, or punty rod," on which a little hot metal has been previously gathered to make it adhere, is attached to the flattened side, exactly opposite the hollow tube which is now detached by means of a piece of iron previously dipped in cold water, leaving a circular hole in the glass of about two inches diameter. The following figure represents the operation of attaching the punty.

Taking hold of the punty rod, the workman now presents the glass to another part of the furnace called the " nose hole," where, it must be observed, the aperture made by its separation from the tube is now presented, and at which it is kept until it has become sufficiently ductile to adapt it for the operation of the flashing furnace. Being here turned dexterously round, slowly at first, and afterwards with greater rapidity, the glass yields to the centrifugal force, and thus necessarily enlarges the aperture above alluded to. The workman, taking great care to preserve, by a regular motion, the circular figure of the glass, proceeds to whirl it round with increasing velocity, until the aperture, now diminished to a ring of only a few inches diameter, suddenly flies open with a loud ruffling noise, like the rapid unfurling of a flag in a strong wind, and leaves the glass a circular plane or sheet, of from four to four and a half feet diameter, of equal thickness throughout, except at the point called the bullion, or bull's eye, where it is attached to the iron rod. The following figure will give some idea of this very beautiful part of the process of glass making.

The sheet of glass, now fully expanded, is moved round with a moderate velocity, until it is sufficiently cool to retain its form. It is carried to the mouth of the kiln, or annealing arch, where it is rested on a bed of sand, and detached from the punty rod. The sheet or table is then lifted on a wide pronged fork, called a faucet, and put into the arch, where it is tempered by being subjected to a gradually decreasing heat for about twenty-four hours. When taken from the arch at the end of this period, the glass, after an account has been taken of it by the exciseman, is ready for the glazier's use. It is first, however, removed to the manufacturer's warehouse, where the circular sheets are cut into halves, and assorted into the different qualities well known to the tradesman by the names of seconds, thirds, and fourths.