21
So that in this case the silica contained about fourtimes more oxygen than the bases, and the quantity°f oxygen in the lime and potash, the two principalbases, was nearly the same.
As the beauty and value of this glass depend uponits absolute limpidness, the most careful selection ofmaterials, both for the mixture and the pots, and aprotracted and assiduous process of fining are re-quired. It is not easy to arrive at a certainty ofsuccess in its manufacture. To obtain it withoutcolour, potash must be used, and not soda, and nooxide of lead; but lime must be added, since by form-ing a glass with merely a potash base, the glasswould be soluble in boiling water, and consequentlyhygrometrical. This would be a serious disadvan-tage ; for spectacle-glasses or other lenses so formedWould constantly tarnish by the deposit of a film ofaqueous vapour, and in the space of a few yearswould altogether lose their polish. This incon-venience is avoided by adding to glass with a baseof potash a certain quantity of lime ; but then arisesa not less serious evil, a proneness to devitrification.Glass with a base of potash and lime, submitted tothe prolonged annealing which thick masses intendedfor large lenses require, is apt to assume the milkyaspect which indicates a commencement of crystalli-zation in the mass. Thus the manufacture ofcrown-glass, as well as that of flint-glass—the twoglasses indispensable for the preparation of achro-matic objectives—both present the most seriousdifficulties, although from very different causes.
Window-Glass . —The glass which has long beenin common use for window-panes is that which isgenerally known as English crown-glass , in the manu-facture of which a large globe is first blown at theend of the pipe, and this is converted by a rapidrotatory motion into a circular plate or disc thickenedat the centre. Until a comparatively recent periodthis was the only method employed in the manufac-ture of window-glass in this country; but latterlythe Continental method has been extensively intro-duced, which consists in forming the globe into acylinder, and then, after cutting it up in a directionparallel to the axis, flattening it out into a broadsheet, from which it has received the designation ofBritish sheet-glass. Plate-glass , or that which is castinto sheets by pouring the liquid metal on a flatsurface, is now much used for the same purpose.English Crown and Sheet-glass are composed ofprecisely the same materials, and differ only in themechanical operations by which they are broughtinto form. Chemically speaking they are the same,except that the nature of the crown-glass manipula-tion requires a smaller quantity of lime.
The materials employed for the manufacture ofEnglish crown and sheet-glass—in other words, ofwindow-glass — are chiefly silica, soda, and lime.This glass differs, therefore, from the foreign crownand Bohemian glass in the circumstance, that soda,as the cheaper alkali, is generally substituted forpotash. Sometimes, however, though rarely, a mix-ture of the two is employed. Alumina, oxide ofiron, and oxide of manganese are also found in
window-glass, but only as accidental ingredients.It is true, indeed, that when iron is present, as isalmost always the case, the binoxide of manganeseis purposely introduced to neutralize its effects; anda little arsenic is generally added to promote thedecomposition of the other ingredients.
To lay down any standard proportions for window-glass is almost impossible, as no two manufacturersuse the same; and even in the same works the meltingpowers of the furnaces may so far differ as to rendernecessary, for the production of the same glass, varia-tions in the proportions of its elements. The followingare mixtures used in some English manufactories:—
Lbs. •
Lbs.
Cwts.
. 560 ...
.. 448
. 16
Chalk, .
. 154 ...
. . 146
. 5
Carbonate of soda,
. 119 ..
. . 168
. 5
Sulphate of soda. .
63 ..
17
. 1-25
Arsenic, .
. 2 . .
2
.... 0
Gullet .
. 448 ...
.. 448
. 16
The introduction of carbonate of soda, prepared fromsalt, into the glass manufacture of England, datesfrom the year 1831. Previously to this kelp wasemployed as the source of alkali.
The following is a French composition, whichgives a window-glass of fine quality:—
Parts.
Sand,. 100
Chalk. 35 to 40
Dry carbonate of soda,. 28 to 35
Broken glass,. 60 to 180
Binoxide of manganese,. 0*25') Sometimes in
Arsenic,. 0*20 j larger quantity.
The oxygen of the bases in this glass is generallyto the oxygen of the acid as 1 to 4.
Bastenaire gives the three following compositions,which Dumas considers too rich in alkali, and toopoor in lime :—
Parts.
i.—White sand,. 100
Good potash,. 65
Lime slaked in the air,. ‘6
Fragments of white glass,. 50
Arsenious acid,. 1
Oxide of manganese,. 0*30
ii.—Very white sand,. 100
Good soda,. 90
Lied oxide of lead or minium,. 5
Broken white glass,.. 100
Carbonate of lime,. 5
Oxide of manganese,. 0*40
in.—White sand,. 100
Good soda,. 80
Carbonate of lime,. 8
Broken glass,. 110
Oxide of manganese,.. 0*20
Oxide of cobalt,........ 0*10
A beautiful window-glass is produced by thefollowing composition by Dcjmas :—
Parts. Parts.
g an ^ 5 . 100 .... 100
Dry sulphate of soda,. 44 .... 58 to 75
Charcoal in powder,. 4 .... 4*5 to 5*5
Slaked lime,.. 8 .... 13 to 15
Broken glass,. from 20 to 100_25 to 100
Plate-Glass.—Dodd justly remarks that a bettername than plate- glass would be cosi-glass, since theformer is vague in its acceptation, and might be