475
Manufac-
tures.
some measure necessary to analyze it, with a view toascertain how much real alkali it contains, and, hence,how much sand it will require. When this is known,the calculation is made without any difficulty, for it isfound that one pound of pure soda is sufficient for fourpounds of sand, to produce a hard compact glass, whichcannot be injured by the action of water, or even by thecommon mineral acids. Thus, if the prepared alkalishould contain 40 per cent, of real soda, which is aboutthe usual quantity, the remainder being muriate ofpotash, carbonic acid, and water, the operator shouldtake 160 pounds of sand to every 100 pounds of suchsalt. Upon these data Mr. Parkes gives the followingproportions of the materials to produce a glass suitablefor the best plates.
Lynn sand previously well washed and dried 720 pounds
Alkaline salt prepared as above. 450
Quicklime slacked and sifted . 80
Nitre. 25
Cullet or broken plate glass. 425
1700
If the process be well conducted these quantities ofdifferent materials will produce one pot of metal, con-taining 1200 pounds of good plate glass.
Dr. Ure gives for the best proportion of the materials,
Pure sand. 43
Dry subcarbonate of soda,.. 26.25
Pure quicklime. 4
Nitre. 1 • &
Broken plate glass. 25.0
100
Prom which about 20 parts of good plate glass may berun off.
The lime is of considerable use here, adding much toth e fusibility of the materials, supplying in this respecttlle use of litharge in the flint glass ; too much lime,however, impairs the colour and solidity of the glass.About one-fifteenth of the whole is as much as can beused with propriety; and some use as little as one-
tvve nty-fourth.
The substances added to destroy the colour are cobaltblue, and manganese. The manganese is in sufficientquantity to impart a slight red tinge, which, when inter-'uixed with the blue of the cobalt and natural yellow ofbe other materials in proper proportions, is found tone utralize them so that scarcely any definable tint remains.
Crown Glass.
(686.) The alkali employed for this description of glassmuch
s generally kelp, but being an
alkali which varies sourn 10 ' tS fiuahty, it is difficult to give any defined**“»■ for its use. The following are the most
common :
Irish kelp dried and ground. . 450 pounds
A-ynn sand dried. 325
slacked and sifted lime. 25
In add' ' 800
verv mm ltl0n i to t ^ lese substances, a quantity of arsenic is°r the ? a dded to facilitate the fusion ; and zaffre
correct tbp 6 *1 C0 ' Ja 'f whh ground flint, is also used tocrown fflass y e " ow ’ which the inferior kind of
blue, natural f Ur . a 'y acquires, and by thus adding theglass coloured with this oxide, the Whole
is converted into a soft green light. The green hue Mechanicalthus given is very slight, and not disagreeable to the eye, P rocesses -and is hardly perceptible unless the light pass through '
a great thickness of glass, as happens when a piece isheld edgeways. The quantity of zaffre necessary forthis effect is extremely small, one ounce being sufficientfor 1000 pounds, and when the alkali and sand are goodnone at all is required.
The Orkney kelp is sometimes preferred for windowglass, because it makes it of a better colour than theWestern Isle or the Scotch kelp. Mr. Bowles, the cele-brated maker of crown glass, at Cock-hill, Radcliffe-highway, London , is said always to have used Spanish barilla instead of kelp, and his glass was generally pre-ferred to that of others.
Broad Glass.
(687.) Broad glass, although employed for the purpose Broadof window glass, is quite a distinct manufacture to that glassof the crown glass, both with respect to the materialsemployed and the process of fabrication.
The ingredients are soap-boiler’s waste, kelp and sand,in about ^he proportion of six bushels ol the first to threebushels of kelp, and four of sand ; but from the greatvariability of the qualities of these materials there is noabsolute rule for the proportions. Loysel has stated, asthe result of his own experience, that if coarse sand beused, two pounds of pure alkali must be put to everyfour pounds of sand, or it will not melt; but if the sandwere very fine, one pound of pure soda would be sufficient.
Green Bottle Glass.
(688.) This is the coarsest description, and executed Bottlein greater quantities than any other kinds of glass, so glass,much so, that one half of the entire manufacture of thismaterial in Great Britain is of this kind.
It is commonly made of soap-boiler’s waste and sand,in various proportions, according to the quality of thematerials. Sometimes a portion of kelp is also added,but this is not used in all manufactories. In conse-quence of the rate of duty being only one-eighth of thatlevied upon flint, the manufactories are restricted to theuse of the commonest kind of sea or river sand, lest therevenue should suffer by the superior quality of the less-burdened ware. This use of so coarse a sand occasionsthe necessity of the employment of a large portionof alkaline matter, and in this respect increases thecharges to the glass-blower. The impurity of the alkali,and the abundance of fluxing materials of an earthynature, joined to the very high degree of heat at whichthey are fused, occasion the glass to contain a verysmall proportion of saline matter, for which reason it isbetter qualified than flint glass for the recipientof fluidswhich have any corrosive action ; chemical retorts, sub-limary vessels, and carboys for containing aquafortis,should for this reason be always made with the commonglass, which has this further advantage over the flintthat it will bear a much stronger heat without beingsoftened or undergoing any alteration of shape.
Bottle glass is very hard and well vitrified, and isof less specific gravity than other descriptions. Thecomposition employed in France to produce this mate-rial, as given by Loysel, is
Common white or yellow sand.. . 100 partsCoarse kelp. 39
AW,