« ZurückWeiter »
made to serve for both furnaces, by which means the expense attending the alteration has been much diminished.
It is said, that the weekly expenditure of coals has been lessened, by means of this alteration, to the extent of ten chaldrons.
The dimensions of the crucibles and furnace should bear some relation to each other. If the former, from their diminutive size, are much below the efficiency of the furnace, a very needless expenditure of fuel will be the result; and should the crucibles be too large for the power of the kiln, the heat will not prove sufcient. In this case, an excess of fluxing materials must be used in order to form glass, which will be imperfect, according as it is fused below the most beneficial degree of temperature. The relative proportions of pots and furnace may be somewhat varied according to the power of promoting combustion, which partly depends upon construction and situation. Loysel recommends, that in general the aggregate area of the crucibles should be very little beyond one fourth of the area of the furnace. The experience whereon this recommendation rested was drawn from the use of wood as a combustible, which requires a greater space than coal for the developement of a given effect. The difference thus arising would, on the other hand, be lessened by the necessity which we are under for covering the pots ; whereas, if the kiln is heated by means of wood, these may remain uncovered, and their contents will, in consequence, be more efficiently acted on by the heat. Loysel states *, as the result of experiments continued during a whole year, that the weight of dry beech wood consumed in a glasshouse furnace exceeded the weight of coal burnt in the same furnace, and during an equal period, in the proportion of 45 to 28. According to Lavoisier, the heating effects of wood and coal are in the proportion of 1089 to 600.
The result of Loysel's experiment should consequently have shown similar effects from the combustion of 45 parts by weight of wood, and 24.8 parts of coal; the difference between this last quantity and 28 being, probably, the loss consequent upon covering the crucibles.
The openings already mentioned, as serving for the introduction of the materials into the crucibles, and for the removal of the melted glass, are called boccas. These may be closed either wholly or partially, according to the need of the workman, by means of movable collars, or, to speak more correctly, by temporary screens made of fire clay. On either side of each bocca is a smaller circular opening, sometimes called a boccarella, but more generally by the familiar name of nose-hole, the particular use of which will be explained hereafter.
The annealing oven, or lier, is a long, low, rectangular chamber, heated at one end, and furnished with numerous shallow iron trays, which can be passed easily along the level bottom of the chamber. These trays are called lier-pans, or fraiches; which námes, together with those of several implements used in glasshouses, are evidently adopted from the French.
As regards the structure of the annealing chamber, there is nothing that particularly requires notice, or that will not be rendered sufficiently clear by the explanation
* L'Art de la Verrerie, p. 72. .
of its use that will be given in the following chapter, while describing, in their regular course, the consecutive processes of the manufacture.
Every glasshouse should possess within itself the means of making all the crucibles and other earthen utensils that may be required for its operations. The conveyance of these from any considerable distance would add materially to their cost, not only by the mere expense of carriage, but through the greater liability to fracture, whereto such unwieldy vessels would in that case be subject.
These crucibles, or glass-pots, should be made with five parts of the best Stourbridge fire clay and one part of old broken crucibles ground to powder for that purpose. Great nicety is required in mixing these ingredients and in working them together, in order to drive out from their substance every particle of air, the presence of which would, by its expansion in the furnace, occasion the immediate breaking of the pots. The method invariably pursued for kneading the clay Fig. 3. is, that the workman treads on it for
a considerable time with his naked feet, turning it over from time to time, so that every part may, in its turn, be subjected to the required pressure, An attempt was recently made by a glass manufacturer to employ machi
nery for the purpose; but after having incurred considerable expense, and exerted much ingenuity in perfecting his apparatus, this gentleman has reverted to the old inartificial method, as being decidedly the best calculated to insure the goodness of his crucibles. This attempt at improvement was not relinquished, until the losses which were sustained in its prosecution became of serious moment.
After the crucibles are formed they are suffered to remain for a considerable time in the apartment wherein they were made, in order that they may dry equally throughout their substance. It is not, indeed, considered pur
dent to remove them from this situation until they shall have been formed for at least a twelvemonth.
For some little time before a pot is put to use, it must be placed in an apartment wherein heat can be artificially admitted, and either raised or lowered at pleasure. It is then removed for about three days to a furnace particularly appropriated to the baking of pots; and here the temperature, which at first is moderate, is gradually increased, until at last it is made nearly as intense as that of the working furnace, for insertion in which the cru. cible is then sufficiently fitted.
With all the skill and care that can be employed in their manufacture, these crucibles will frequently break during a very early period of their use, and the loss occasioned by that means to the manufacturer is in many ways one of very serious importance.
ON THE MANUFACTURE OF FLINT GLASS.
THE MOST BEAUTIFUL AND COSTLY KIND OF GLASS. -IMPORTANCE
OF ITS QUALITY FOR OPTICAL PURPOSES. - EXPERIMENTS FOR ITS IMPROVEMENT — UNDERTAKEN BY THE ROYAL SOCIETY — PROMOTED BY GOVERNMENT. – DISTINGUISHING PROPERTIES OF FLINT GLASS. TO WHAT OWING, -DIFFERENT COMPOSITIONS. PROCESS OF MELTING. — GLASS-GALL. — ITS USE. — CURIOUS PHENOMENON. — IMPLEMENTS. — COLLECTING GLASS ON ROD. - MARVER. PARAISON. BLOWING. - RE-HEATING. ELONGATING. — PONTIL. — FASHIONING. – DETACHING. — REMOVAL TO ANNEALING OVEN. -MOULDING, ANNEALING. WHY INDISPENSABLE. BOLOGNA PHIALS. — RUPERT'S DROPS.
FLINT glass — known in foreign countries under the name of crystal --retains among us the title originally imparted by its principal ingredient, although the use of flint in its composition has long since been discontinued.
This glass is the beautiful and peculiarly refulgent compound whereof the finest articles designed for domestic use or for ornament are made. Of all the vitrified compounds that are manufactured, it is the heaviest and the most brilliant; the one most easily fashioned by the hand of the workman, and that which has the greatest refractive power. It is also, from the nature of its constituent materials, the most costly.
Vessels of flint glass cannot, however, be properly applied to all purposes. If, for instance, they are used to contain carbonate of ammonia, they will very soon become so exceedingly brittle, that the very slightest apparent cause will occasion pieces to fall out. Common green bottle glass is not liable to this objection.
Were we to judge from the practice of different manufacturers, in bringing together the ingredients that form this compound, we inust believe, that to adhere to any exact proportions is by no means indispensable. In almost every different glass-house a peculiar recipe is