472
MANUFACTURES.
M.mufac- They subscribed a capital or joint stock of eighty shares,tares. 0 f each, and constructed works of considerable
extent at Ravenhead, near Prescot, in Lancashire , whichstill rank among the most important glass-works inthis Country.
The Art of glass-making, having been thus patronizedby the Nobility and Gentry of the Country, soon maderapid progress, and the attention of scientific men beingdirected to this subject, great improvements and facili-ties were introduced in the manufacture ; and it may
now be said that we equal, if not excel, those Conn- Mechanic 11tries which were formerly our instructors, both in the Processes-beauty of the material, and in the variety and eleganceof the various utensils into which it is manufactured. #
The present extent of this Art will be seen from thefollowing Table, giving the quantities of differentkinds of glass which have been charged with duty,and the net produce thereof in each year from 1820 to1831.
Years.
Flint Glass.
Fluxed Materials forFlint Glass.
Plate Glass.
Crown Glass.
Broad Glass
Green Glass.
Total Produceof Duties.
Cwts.
lbs.
Cwts.
Cwts.
Cwts.
Cwts.
£.
1820
50,878
8,855
97,309
7,253
321,470
405,940
1S21
52,016
9,650
97,273
8,704
276,323
439,477
1822
56,583
10,198
103,193
7,921
286,927
502,813
1823
67,299
12,107
119,493
9,172
343,293
549,358
1824
63.442
13,922
141,463
9,300
376,287
635,496
1825
33,367
8,806,234
15,642
160,709
8,386
393,567
660,532
182(3
16,040,182
19,958
135,606
8,118
421,902
623,786
1827
16,819,656
14,807
138,744
7,611
818,225
619,297
1828
18,501,504
16,641
137,950
6,784
421,003
598,640
1829
19,961,504
13,526
105,097
6,181
365,802
653,206
1830
16,232,384
14,296
98,287
5,635
33S,862
550,499
1831
* * I
16,627,408
14,109
103,803
5,609
288,760
531,494
On the Ingredients employed in the Composition ofGlass.
i ds ( 680 -> Glass is produced by fusing silica w ; th adue pro-
emptoveil. P ortion of alkali, which acts as a flux to the silica, andrenders the whole transparent. Besides these two in-gredients, which are absolutely essential to the compo-sition of glass, the following accessory substances areemployed, which, by their chemical action, give pe-culiar properties to the different kinds of glass; viz.borax, carbonate of lime, oxide of lead, oxide of man.-gatiese, white oxide of arsenic, and nitre.
The silicious material most commonly used in thisCountry for the manufacturing of glass is sea sand,which is well known to consist chiefly of quartz; it hasthe advantage of being naturally divided into grains,sufficiently small to be used without any other prepara-tion than that of washing. The finest sand is said tocome from near Lynn, on the coast of Norfolk , andfrom the Western extremity of the Isle of Wight. An-other equally pure kind of silex is the common black flint,which is prepared by heating it red hot and plung-ing it into cold water, which splits it in every direction,and renders it so brittle that it can be ground in millswithout much difficulty. This kind of silex is, how-ever, more commonly used in pottery than in glass-making.
Alkali The alkali used in this manufacture is either soda or
potash, each being apparently equally well fitted for thepurpose. It is preferred in the state of the carbonate,although the carbonic acid is drawn off by the heat ofthe process; the glass being a compound of silex withpur.e alkali, and" not with the carbonate. For fineflint glass the best pearl-ashes, purified by solution andevaporation to dryness, are employed, but the inferiorglasses are made with the coarser alkalies, with barilla,where this alkali is cheap, with common wood-ashes, and
very largely with kelp. Although these last-mentionedalkalies contain many impurities it does not preventtheir dissolving the silex into a very good and perfectglass, for the impurities partly consist of neutral saltsand partly of lime and other earths, all of which assist inthe vitrification. Glass made from these alkalies hasalways more or less of a green tinge, owing to a portionof iron contained in them, as may be observed in thecommon bottle glass. „
Borax is very important in the manufacture of the °finer sorts of glass, from its powerful action as a flux;but it is not used in the common kinds, on accountof its high price, Borax is a compound of soda andboracic acid ; it is found native in several parts of theworld, but is exported into this Country from the EastIndies, under the name tinsel, which is in a very impurestate, and has to be prepared for use by calcination andsolution. Lil0 e.
Lime is employed in the form of chalk as a flux; nis also beneficial in facilitating the operations of theworkmen in fashioning glass, and it has the property ofdiminishing its liability to crack on exposure to suddenand great variations of temperature. Lime is useu msmall proportions in the glass pots, for the escape of thecarbonie acid from the chalk during the fusion causes thematerials to swell to an inconvenient degree, and alsoif the lime is in excess it acts very powerfully on the sidesof the pots.
In making plate glass quicklime is very commonlyemployed; but it is necessary that it should be of the verybest quality. The limestone brought from Gibraltar asballast is found to produce much better lime for the pur-pose than the chalk in the neighbourhood of London ?but it is said that the best lime for the glass business isthat of St. Vincent’s rock, near Bristol . Q r ;jes of
Two kinds of oxide of lead are used in glass-makin Dminium. Litharge will melt by
i) 22 . litharge and