GLASS, 



fli-oiig lieat to diffipale the whole of it, or othcrv.-ife tlic 

 gliifs would be full of bubbles, unfound, and having a 

 cloudy gelatinous apiior.raiice. It is obfcrved, that glafs 

 from potalli is more likely to fuffcr from glafs-gall tlian the 

 foda-glafs is, becaufe the potalh glaflfes arc harder, and do 

 not run fo thin as the other, and the glafs-gall from them 

 does not fo eafily difiipate in the fire. 



Durinf this procefs famples for examination are drawn 

 out of the pots witii an iron rod ; and the glafs gradually 

 becomes more and more flexible, dcnfe, and Icfs brittle, and 

 at lull the glafs-gall is entirely diffipated. Whilll the heat 

 is continued, the glafs which was full of fpecks and bubbles 

 is relincd, and becomes beautifully clear, tranfparcnt, and 

 colourlefs ; and this procefs, which goes on from the cefia- 

 tion of the vapour of tlie glafs-gall and its entire removal to 

 the time when the glafs is altogether clear and free from 

 bubbles, is called the "refining." After this the glafs 

 is complete ; but being too thin for working, it is cooled, 

 by Hopping the draught of fire round the pot which con- 

 tains it, and in cooling it thickens to a fit Hate for being 

 wrouglit. For glafs that is call into plates, Icfs cooling is 

 neceflary, as it is required to flow very thin and hot. On 

 an average it takes about 48 liours for the fine flint glafics, 

 from the time when the pots are firfl filled till tiie glafs is 

 ready for \^■orking, in which ftate it is of a very full red 

 colour, and polTelfes a fingular kind of confidence and tena- 

 city. It is jufl foft enough to yield with cafe to any ex- 

 ternal impreffion, even to the force of 'die breatli urged 

 pretty llrongly in the centre of the glowing mafs, and may 

 be bent and fluiped in every poffible way ; and fuch is its 

 tenacity, that it extends uniformly without any cracks or 

 fiffures ; but when llretched out to the utmoll, it forms a folid 

 firing, the diameter of which is confl;antly decreafing till 

 it feparates from the mafs in a thin capillary thread. It 

 lliffens as it cools, and becomes perfectly brittle and alfo 

 tranfparcnt. As melted glafs adheres very feebly to polilhcd 

 metal, it is very eafily wrought with bright iron tools. 



Glass, luork'mg or blowing round. — Every kind of glafs, 

 plate-glafs excepted, is formed from a hollow globe that has 

 been produced by blowing. For this purpofe the operator 

 takes his blowing-iron, which is a hollow tube, about four 

 or five feet long, and dipping it in the melting-pot, turns it 

 about there till the metal adheres to the iron like fome glu- 

 tinous or clammy juice ; he then holds it near the ground, fo 

 that the mafs is extended by its own weight, and blows 

 ftrongly into the tube. With his breath tlius penetrating into 

 the centre of the red-hot mafs, he enlarges it into an uniform 

 hollow globe of the requifite thicknefs and bulk, keeping 

 the force of his breath upon it for a few feconds till it fl:iifens 

 by coohng, and thus preventing its finking by the com- 

 preflion of the denfer external air. This globe, adhering by a 

 neck to the iron rod, is formed by the dexterity of the 

 workman, and by a variety of ingenious manoeuvres into all 

 the common utenfils. As a fpecimen of his art, we may 

 inftance a common tumbler. The hollow globe already 

 mentioned is taken ofl" the iron rod by the following fimple 

 procefs : An alliftant dips the end of a Ihort folid iron rod 

 into the glafs-pot, and, bringing out at its extremity fome 

 of the melted glafs, thrulls it immediately againll the hollow 

 of the globe at the part direftly oppofite to the neck, to 

 which it firmly unites, and thus the globe is cemented by 

 the melted glafs to the fecond rod. The workman then wets 

 a fmall piece of iron with his mouth, and lays it on the neck 

 of the globe, which is extremely hot, and this, in a fecond 

 ■or two, cracks it round ; fo that with a flight pull it comes 

 off and detaches the hollow rod, leaving the globe open at 

 ?the neck, and transferred to the fecond rod at the oppofite 



fide. The open globe is again foftened by holding it a fe\v 

 feconds ovlt '!■;■ mouth of the glafs-pot, and is cut away 

 fi uni tiie opon end to the form of a cup by iron fliears. The 

 operator, when employed in fafliioning the globe, ufually 

 fits upon a kind of arm-chair, with its arms floping forwards , 

 and covered with a flat fmooth iron-plate ; and by laying 

 the iron rod llraight before him, refting on both the arms of 

 his feat, and twirhng it backwards and forwards, the hot 

 glafs at the end is made to revolve like clay on a potter's 

 lathe, and thus is opened, widened, or compreffed at plea- 

 fure by any fimple iron inftrument that is prefled againfl it. 

 The globular cKp is thus extended eafily into a cylinder, or 

 made into the fhape of a barrel, if this form be required, 

 and is fmoothed up at the edges. In order to feparate it 

 from the iron-rod, it is wetted as before at the point of at- 

 tachment, and the tumbler drops off complete. Tliis lall 

 operation leaves that burr or roughnefs, with fliarp frag- 

 ments, which is feen at the bottom of all glafs-veffels, 

 unlefs it be taken off by pohfhing. The next operation is 

 that of cooling the veffel very gradually, called *' an- 

 nealing." See Anneal'wg of Glass. 



Gl.\s.'5, dtfirent kinds of. The manufaftured glafs now in 

 ufe may be divided into three general kinds ; white tranf- 

 parcnt glafs, coloured glafs, and common green or bottlc- 

 glafs. Of the firil kind, there is a great variety ; as the 

 flnit glafs, as it is called with us, and the German cryllal 

 glnfs, which are applied to the fame ufes ; the glafs for 

 plates for mirrors or looking-glaffes ; the glafs for windoivs 

 and other lights ; and the glais for pliials and fmall veffels. 

 And thefe again differ in the lubftances employed as fluxes 

 in forming tiiem, as well as in the coarfenefs or finenefs of 

 fuch as are ufed for their body. The flint and cryftal, 

 mirror, and bell window glafs, not only require fuch purity 

 in their fluxes, as may render it practicable to free the glafs 

 pcrfedtly from all colour : but for tlie fame reafon hkewifc, 

 either the white Lynn iand, calcined flints, or white pebbles, 

 fliould be ufed. The others do not demand tlie fame nicety 

 in the choice of the materials ; though the fecond kind of 

 window glafs, and the bell kind of phial, will not be fo 

 clear as tiiey ought, if either too brown fand, or impure 

 fa'ts, be fuffercd to enter into their conipoiition. 



Of coloured glafs there is a great variety of forts, dif- 

 fering in their colour, or other properties, according to the 

 occalions for which they are wanted. The diflerences in the 

 latter kind depend on tlie accidental preparation and manage- 

 ment of the artills by whom they are manufadtured. 



Gl.AM';, Cryjlal. Foreigners ufe this term for our 

 flint glafs, and for making it they give the following direc- 

 tions: Take of the whitelt tarfo, pounded fmali, and fearced 

 as fine as flour, two hundred pounds ; of the fait of pol- 

 verine, a hundred and thirty pounds : mix tliem togetiiei", 

 and put them into the furnace, called tlie calcar, firll heat- 

 ing it. For an hour keep a moderate fire, and keep flirring 

 the materials with a proper rake, that they may incorporate 

 and calcine together ; then increafe the fire for five hours ; 

 after which take out the matter ; which, being now fufli- 

 ciently calcined, is called frit. From the calcar put tiie frit 

 in a dry place, and cover it up from the dull for three or 

 four mouths. 



Now, to make the glafs, or cryftal: Take of this cryftal 

 frit, called alfo boU'tlo ; fet it in pots in the furnace, adding 

 to it a due quantity of manganefe : when the two are fufed, 

 caff the fluor into fair water, to cleai- it of the fait, called 

 fandiivr ; which would otherwife make the cryftal obfcure 

 and cloudy. This lotion muil be repeated again and again, 

 as often as needful, till the cryftal be fully purged ; or, this 

 fcum may be taken off by means of proper ladles. Then fet 



it 



