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By the gradual manner in which they have parted with their heat, time has been allowed for the regular contraction of the whole into an uniform and consistent substance.
In glasshouses where objects of various magnitudes and descriptions are made, two or more of these annealing ovens are usually attached to each working furnace. Pieces which are large, and of considerable substance, require that the oven in which they are annealed should be made much hotter than is necessary for thinner and smaller pieces. Glass which is afterwards to pass through the hands of the cutter is always made of considerable thickness, and requires not only that the heat of the oven should be very considerable when it is first inserted, but that it should be withdrawn from this heat very gradually; while, on the contrary, such articles as are very thin may be placed at first in a much more moderate temperature, and may be removed altogether at the expiration of a few hours.
It is impossible by written explanations to impart beyond a very faint idea of the truly curious and interesting operations of the glass-blower. This difficulty
does not arise from any complexity in the manipulations. Although, in common with nearly all the manual arts, these call for long practice to insure proficiency, they are yet exceedingly simple in their nature. But there is something more than ordinarily striking—perhaps, even, it may be said fascinating-in watching the progress through which a substance, in its usual state rigid and brittle to a proverb, passes, by rapid conversion, from a glowing and shapeless lump to a perfect article of most elegant manufacture. The absolute control which the workman exercises over its form and substance; the perfect ease and security wherewith he pulls, and twirls, and divides, and joins, a matter which we are accustomed to handle with only gentleness and care ; never fail to excite a high degree of admiration even in those who have had frequent opportunities for witnessing the processes.
The amusement to be derived from watching the operations employed in many branches of manufacture is probably much greater than would be imagined by persons who have not so indulged their curiosity; and among these manufactures, although there are doubtless many that call in a higher degree for our admiration as proofs of the genius and perseverance of man, there is not one calculated to afford, for the time, more gratifi. cation than the operations of a glasshouse.
ON THE MANUFACTURE OF CROWN GLASS, BROAD GLASS,
AND BOTTLE GLASS.
DESCRIPTION OF CROWN GLASS.-HARDER THAN FLI
- MORE DIFFICULT TO FASHION. — ITS COMPOSITION. — IN FRANCE. - IN ENGLAND - FRITTING. – CULLET. — REFIN. ING. —SULPHATE OF SODA. VEGETABLE CHARCOAL. - GATHERING. - BLOWING. - RE-HEATING, FLATTENING. TRANSFERRING TO PONTIL. — TWIRLING. — EXPANDING. OPENING. - ANNEALING, - NICE REGULATION OF TEMPERATURE REQUIRED IN THIS PROCESS. - QUALITIES OF CROWN CLASS. GERMAN GLASS. -BROAD GLASS. INFERIOR TO CROWN GLASS. — ITS COMPOSITION. PREPARATION. --WORKING. - BURSTING. - OPENING. — ANNEALING. — BOTTLE GLASS. - MANUFACTURE CHECKED BY INCREASE OF DUTY, COMPOSITION. — RESTRICTIONS AS TO MATERIALS. — THEIR BAD TENDENCY. – SUPERIORITY OF BOTTLE GLASS FOR CERTAIN PURPOSES. - MATERIALS EMPLOYED IN FRANCE. — AT NEWCASTLE. — FASHIONING. MOULDING. EXPERIMENTS SUGGESTED BY COUNT CHAPTAL. -KLINGSTEIN, VOLCANIC GRANITE
The name of crown glass is given to the best kind of glass commonly used in making windows, and for like purposes. In the composition of this material, no lead or metallic oxide enters as a fluxing agent. A small quantity of manganese is frequently used, and sometimes also a minute portion of oxide of cobalt; but the object of these additions is the correction of a faulty colour in the glass, arising from impurities in the sand and alkali. This kind is, therefore, much harder than flint glass, and would consequently be more difficult to fashion, if it were desired to give it any other form than that of a plane surface.
The composition of crown glass varies considerably. Loysel has given several different recipes for its formation. That which he most recommends, stating that
it is employed at the extensive works of St. Gobain, consists of Fine white sand
100 parts. Carbonate of lime
12 Carbonate of soda, calcined, 45 to 48.
Clippings of crown glass 100; with such additions of manganese and oxide of cobalt, as may be required to correct impurities and remove the colour which they occasion. By proper carefulness in selecting and purifying the ingredients, the employment of these metallic bodies is rendered unnecessary.
Crown glass is. generally made in France of 100 parts of fine white sand, 50 to 65 parts potash, 6 to 12 parts dry slaked lime in powder, and from 10 to 100 parts of broken glass of similar quality : this composition is frequently employed in that country for the manufacture of common drinking vessels, as well as of window panes.
In England, this material has usually been composed of fine Lynn sand, kelp, and slaked lime; the proportions of these ingredients varying according to the quality of the kelp, some kinds of which contain a greater amount of alkali than others. That from Orkney is considered to be the best, not only on this account, but also because the glass of which it is made proves of a better colour than where Scotch or Irish kelp is employed. The proportions when the kelp is of the best quality are,
Fine sand 5 bushels or 200 pounds' weight,
15; which ingredients are fritted in the calcar as already described, preparatory to their fusion. When put into the crucible, about half its weight of broken glass, or, as it is called in the manufactory, cullet, is added to the frit. This compound requires to be kept in fusion at a high degree of heat during thirty-five to forty hours, in which time an intimate union of all the parts takes place; the glass refines itself by throwing off all its sandiver, and becomes perfectly transparent. It is not 60
advisable to use a larger proportion of broken and refuse glass than that just mentioned, because, by its long ex. posure to heat when in fusion, glass is made to give up a portion of its alkali, becoming harsher and less fusible. The quantity of glass usually employed is serviceable by sooner bringing the pot to a working state; but any larger quantity would, for the reason just stated, sensibly alter the quality of the whole contents of the crucible. The fragments, before they are used, are first heated, and then suddenly plunged into cold water, which renders it easy to reduce them to powder.
A very superior quality of crown glass is made by the mixture of 120 parts by weight of White sand,
Arsenic. If the colour should prove yellow, this is corrected by the addition of a small quantity of manganese.
· Another composition, cheaper than the foregoing, consists of 120 pounds of White sand,
Common salt, 10
Arsenic, and 3 ounces of Manganese. This produces glass of a good and useful description, much employed in the manufacture of apothecaries' phials.
The late M. Gehlen, who was well experienced in the art of glass-making, composed crown glass of the following materials *: Sand