Manufacture Of Glass
( Originally Published 1851 )
In the whole circle of our manufactures there is not any thing more curious than the one that is depicted in the above engraving. Materials which appear of themselves but little fitted for any useful purpose, are blended together so as to form corn-pounds of a new and entirely distinct character. Indeed, an uninitiated person looking at the sand, lead, and pearl ashes, as they are prepared for the glass-house, would consider that nothing less than the wand of the enchanter could accomplish their change into a hard and crystalline body.
The ingredients usually employed in the manufacture of glass, with their relative proportions, may be thus briefly described:
120 parts of well-washed white sand
40 purified pearl ashes
35 „ litharge
13 „ nitre.
1 " black oxide of manganese.
When these materials are collected and properly proportioned, they receive a certain amount of calcination prior to their being placed in the melting-pot. This operation is called fritting, and is performed either in small furnaces adjoining to the proper glass furnace, and heated by the same fuel, after its principal force has been expended on the glass-pots, or else in ovens constructed for the purpose. The use of this preparatory process is to discharge all moisture from the ingredients, and to drive off the carbonic gas. This operation is performed gradually, and carried to the point of semi-vitrification. When the materials are sufficiently fritted," they are thrown with clean iron shovels, through the side opening of the furnace, Into the glass-pots the fire having been previously raised to its greatest intensity. When filled, the opening is closed with wet clay, excepting a small hole for examining the interior of the furnace. The mass soon begins to heave, and exhibit a mass of liquid grandeur like the waves of the ocean on fire. During this process, samples for examination are frequently brought out by the aid of an iron rod, and the glass becomes beautifully clear and trans-parent. The glass may now be considered as completely made, hut it requires some time to cool down to the requisite working temperature. It should be just soft enough to yield with ease to any external impression, even to the force of the breath, when impelled against the glowing mass, and in that state it may be bent into any required form. Such, in-deed, is its tenacity, that it may be rapidly drawn into a solid string, and wound on a reel, many miles in length. Having thus brought the glass to a state fit for what is technically called " blowing," we may introduce our readers into the workshop itself, which will be best done by the aid of a graphic illustration, and the engraved view at the head of this article, will admirably answer the purpose. In the present season of the year the temperature of the blowing-house would shame the hottest portions of the torrid zone, and while we now write, we are laboring under the enervating effects of a visit, many hours back, when the thermometer stood at 140°.
The workmen who are represented in the engraving, are each engaged in one of the operations essential to the manufacture of a common drinking-glass. For this purpose the operator takes a hollow tube, about four feet long, called a blowing-iron, and dipping it into the melting-pot, turns it round till a portion of the glass adheres to the surface.
He then holds it near the ground, so that the mass is extended by its own weight, and blows strongly into the tube. The breath penetrating the red-hot mass, enlarges it, and it becomes an elongated sphere of the requisite dimensions. To separate this globe from the iron tube, an assistant dips the end of a solid rod into the glass-pot, and bringing out at its extremity some of the melted glass, thrusts it immediately against the globe at the part directly opposite the neck, so that it may be firmly united The workman then wets a small piece of iron with his mouth, and lays it on the neck of the globe, and it immediately cracks off, leaving the globe open at the neck. This is again introduced into the fire by the new bar of iron, and afterwards rounded on the rails of a sort of arm-chair. In order to detach the foot from the iron, moisture is again applied, and it drops off. There is a final process called annealling, which consists in raising the temperature in a separate oven, and afterwards allowing the glass to cool gradually; it is less likely to break.
Pliny attributes the invention of glass entirely to chance, and relates, that it was first made in Syria by some mariners who were driven on shore, on the banks of the river Belus; and who having occasion to make large fires on the sands, burnt the kali which abounded on that shore; and that the alkali of the plant uniting with a portion of the sand on which the fire stood, produced the first stream of melted glass that had ever been observed.