THE INDUSTRY OF ALL NATIONS.

rendered opaque either by oxyd of tin or antimony, and so fusible as to be easilymanaged by the heat of a table lamp.

The manufacture of glass is divided into a great number of distinct branches,founded on differences of composition and of use, from which is derived the follow-ing classification:

A. Window glass, including sheet glass, crown glass, and colored sheet glass.This glass is composed of silica, soda or potash, lime, and alumina.

B. Painted and other kinds of ornamental window glass. Composition muchthe same as section A.

0. Plate glass, whether cast, pressed, or rolled. Composed of silica, soda orpotash, lime, and a little alumina; and differs from section A only in the greaterpurity and colorlessness of the materials employed.

D. Bottle glass, including,

а. Ordinary bottle glass, consisting of silica, potash or soda, alumina, and oxydof iron.

б. Medicinal bottle glass, composed of silica, potash, lime, some alumina, and atrace of protoxyd of iron.

c. White bottle glass (in a limited sense,) for bottles, tumblers, tubes, andchemical glass, &c., and composed of silica, soda or potash, and lime, veryinfusible.

E. Flint glass, or crystal, usually composed of silica, potash, and oxyd of lead;and used for ornamental table glasses, chandeliers, lamps, beads, Venetian glassweights, aventurine, glass mosaic, and when peculiarly pure, for the basis of imi-tative gems.

F. Optical glass, both flint and crown, the former composed of silica, or bora-

cic acid, potash, and more lead than is usual in flint glass; the latter composed ofsilica, or boracic acid, potash or soda and lime, these materials being of the great-est purity. (

The limited use of glass for windows both from its greater rarity and costin olden times has been already alluded to. Sheets of transparent gypsum,and plates of mica, have been used for windows in countries where theseminerals are found in pieces of sufficient size. It appears that as late as the closeof the 17th century, common houses in Great Britain were unprovided with glass,and even in the palaces of nobles it was regarded as an article of splendid luxury.The venerable Bede, in his history of the planting of the church in Britain, givesa particular account of the ornamental glazing with painted glass of the churchesand monastic houses of Yarrow and Wearmouth, by artists whom the AbbotBenedict brought over from Italy for that purpose in the latter part of theseventeenth century (see Howitt’s Visits to remarkable Places, article Bede). Thisis probably as early as this art was practised in any part of Europe.

In the sixteenth century the diamond was first employed to cut glass, andthis circumstance has probably exercised a controlling influence upon the generaluse of glass for architectural purposes. Indeed, it is hard for us to imagine howthe manufacture and use of window glass could be carried on at all without thediamond to cut it with ease and certainty to a required size.

Window glass is chiefly of two sorts, named, in allusion to the mechanicalprocesses employed in their manufacture, viz., 1 . Sheet-glass formed by the flat-tening of blown cylinders, and 2. Crown-glass, formed from a blown sphere by theeffect of centrifugal force.

Before describing these two processes and their results, let us briefly advertto a few facts, familiar to all who are acquainted even slightly with the pro-cesses of the glass house, but which may not be so generally known as to ren-der some allusion to them unimportant. The materials of which glass is formedare mingled in weighed quantities, and in a dry state, upon a floor prepared for thepurpose. The melting pots, which are designed to hold from 500 to 2000 poundsof materials, are formed of the most refractory fire clay, to which is added a cer-tain quantity of the pulverized fragments of old pots. They are fashioned withthe greatest care, the clay being tempered for months, and have the form of acylinder or frustum of a cone. Several of these pots are set in a circular fur-nace heated by wood or bituminous coal, and sustained on strong flat arches.The opening of each pot is directed outwards for convenience of charging theraw material, observing the progress of the fusion, and withdrawing the product.It is also important that the products of combustion, and the smoke of the fireshould have no access to the materials in the pots, hence their tops are arched, andthe fire plays only on their exterior. The heat is raised until the pots are fully redhot before the charge of weighed materials is introduced. This is accomplishedin several small portions added successively, an interval being allowed after eachaddition for the mass to become fully heated before another is made. The chemicalaction of the materials upon each other under the influence of heat is very simple.The alkali employed is almost always in the form of carbonate of soda or carbonateof potash. Silica has the property at a high temperature of acting the part of apowerful acid, and when the proper degree of heat is attained it drives out thecarbonic acid before combined with the alkali, while the silica and alkali unite toform a salt (glass). This action is by no means soon over. The viscid masshas so pasty a consistence at first, that the expelled carbonic acid escapes very

slowly, filling the whole mass of materials with numberless cavities and air

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cells, so that even at the end of 24 hours the glass in the pot resembles rather a loafof light bread than the transparent material we are wont to see. When the mate-rials are incautiously added, or the heat raised too suddenly, this escape of carbonioacid sometimes occasions the frothing over of the pot. From time to time theworkman withdraws a portion of the glass upon the end of his iron blow piperod, and fashions it into some form from which he can judge of the progress ofthe fusion. The glass blower always speaks of the melted glass as “ metal .” Thetools which are used to fashion glass are of wonderful simplicity, the art of glassblowing being chiefly one of manual dexterity. The blow pipe, a hollow rod ofiron, protected by a wooden covering over part of its length, a pair of rude scis-sors, with a spring back like the sheep shearing scissors, a knife, a flat surface ofiron (the marver) on which to roll the molten glass, and a solid rod of iron (thepunty rod or pontil), are the chief implements required by the glass blowers. Ofmoulds, now so much used to fashion vessels of all sorts in flint glass, we shall speakmore particularly by and by.

When by trial the metal is found to be sufficiently refined, the heat is some-what reduced to permit the glass to assume that pasty consistence, resemblingthick honey, which is essential to enable the blower to manage it with ease. Wewe will suppose that sheet glass for windows is the object to be formed, and thatof the best quality, perfectly white. The materials that have been found bestfitted for this purpose are 100 lbs. sand, 52^ lbs. of purified potashes, 14| lbs. ofchalk, f lb. of peroxyd of manganese, and 125 lbs. of broken glass of a former oper-ation. The lime is required to prevent the glass from corroding when exposed tothe atmosphere. The most colorless window glass when seen edgeways hasalways a yellowish tinge.

The workman now introduces his pipe into the pot of metal, and collects asufficient quantity to form the cylinder he is about to blow. The ponderous globeof solid glass thus withdrawn is rounded on the marver, and pushed forward onthe rod by means of a knife, so as to be attached to it by a grooved neck. He isaided in this process by placing the glowing mass in a globular or pear-shapedcavity in a block of wood kept moist by water. The mass, reheated at the fur-nace, is now inflated until a considerable cavity is formed, and the mass has a pearshape. By a rapid motion the workman next raises the mass over his head, stillinflating it. Gravity causes the plastic metal to assume a flattened form, and thepressure of inflation, which now distends the sides only, is continued until thediameter of the flattened bottle is equal to that of the intended cylinder. Anotherrapid downward movement lengthens the heated and now pendulous mass with-out diminishing its diameter, and now the workman swings his pipe from side toside like a bell clapper, inflating from time to time, until under the united influ-ence of gravity, inflation, and incessant motion, a perfect cylinder is formed.Often it is requisite in the course of these operations to reheat the glass severaltimes, but sometimes an adroit workman will carry forward the operation to itspresent stage at one heat. Next he presents the end of the newly formed ves-sel to the fire, resting it in a crotch, on which he can revolve the work beforethe flame. A strong blast, or even the expansion of the air imprisoned by thethumb closing the opening of the pipe, will occasion the heated end to puff out, andthus to form an irregular opening. The cylinder thus opened, the aperture is maderegular by an assistant who cuts the ragged edges with scissors, while the work-man fashions the still pliant glass with the edge of his scissors, revolving it all thetime into a perfectly symmetrical form. The blow pipe and its attached cylin-der is then revolved adroitly over his head, and with great speed through an en-tire circle several times, by which it is cooled before it loses its regular form.The application of a thread of red-hot glass to the cooled surface of the cylindernear the end of the blow-pipe, occasions a neat separation of the parts by cracking.We have now a cylinder of glass open at both ends, uniformly thick, and of afine lustre. Good specimens of these may be seen in the Holland Court of theExhibition, Class 24, No. 2. It now remains to open the cylinder and flatten itinto a square sheet. For this purpose it must be carefully reheated in a furnaceof peculiar construction. At the moment when the cylinder has been brought tothe proper temperature, it is opened lengthwise by applying a drop of wateror by a cold iron, and the workman adroitly opens the cylinder, and spreads itupon a hard table, by gently pressing against its sides with a rule. The surface ofthe pliant glass is then flattened with a polisher of iron or wood, and the sheet ispassed into another chamber where it is slowly cooled and tempered.

Such is a brief account of the method of blowing cylinder, spread, sheet, orbroad glass, for it has all these names. It is afterwards cut up by the diamondinto any required sizes. This sort of glass is recommended by its cheapness anduniform thickness, &c. As the process is now conducted it is equal to any blownglass. When carelessly made, however, it has a very wavy, uneven surface, anda deficiency of lustre.

We have dwelt with more particularity on the steps of this process, as theyare essentially the same with the operation of blowing vessels of every sort.Thus the cylinder of glass in its various stages of progress represents a variety ofvessels, and should the operator stop at one of them he would form a bot-tle, at another stage a chemical vessel or air bell; and it is only the last operationof opening the cylinder which distinguishes it from the usual glass blowing pro-