GLASS. 



wood-aHies, and kelp, are employed. The allies of fci-n caufes the purple hue to vanilTi, with vi llight cffcrvefceiir/? 

 will alio yield a fall, which will make excellent glafs ; and, (if the glafs, and cfcape of numerous fmall air-bubbles. Oir 

 moreover, the afhes of tin- cods and ftalks of beans, as alio the other hand, if a fmall quantity of nitrt is added to glafe 

 thofc of cok'worts, bramble bufh, millet-flalks, ruflies, cy- containing manganefe, the purjile colour h reflored, or, 

 perufes, and many other plants, may be ufcd for the like if prefent, the difcolouring effeft of the charcoal is prc- 

 p\irpofe, and after the fame manner. vented, till the nitre becomes alkalized bv th;^ beat, and 

 Tiiere are other fluxes nfed for different kinds of glafs, mixes with liw other ingredients of t lie crucible. For the ex- 

 and for various purpofes. Lime, in the form of chalk, is planation of thefe phenomena, it is oblerved, that the oxyd 

 employed in the manufafture of glafs ; hut this muft be of manganefe gives the purp'e colour only fo long as it re- 

 nfed only in fmall proportions; for an cxcefs would aft mains in its higher Hate of oxygenation ; but when in con- 

 powerfully on the fides of the glafs-pots, in confcqucnce tacfl with charcoal, the latter partially deoxygenates it, car- 

 •f the efcape of the carbonic acid from the chalk during bonic acid gas is formed, the caufe of the bubbles obferved 

 liie fufion, and, befides, it would render the glais opaque and on this occalion, and the colour is now loll. Nitre, on t\\<r 

 milky in cooling, however clear it might be when hot. It other hand, is known to give out oxvgen largely as foon a* 

 is known by experience, that to lOO parts of filex and the red-hot; and hence the manganefe iuunediately retakes from, 

 jft-quifite quantity of alkali, no more than about fix or feven this fource the oxygen of w Ivich the charcoal had deprived 

 parts of quick-lime, or chalk, can be added, without af- jl, and refumcs its colouring power. The other fubltancef 

 fefting the clearnefs of the glafs. Borax is another very which take away tlie colour from glafs, tinged red with. 

 ■Jaluable flux ; but its high price refiriels the ufe of it to manganefe, are all the falts with the bails of fulphuric acid,, 

 the fiiiell kinds of glafs, and to tkSTe vhich are required to fuch as gypfum, fulphat of foda, &c. and alio fulphur itfelf; 

 lie free from fpecks and bubbles. A very fmall quantity likcwife tlie oxyds of tin and iron, and of fome other metals. 

 »f borax will correct a.ny deticicM:cy of flre:ogth in the Neverthelels thefe fubftances have this power only when in 

 -Jkali. contact with charcoal. The tinging power of manganefe » 

 Of the oxyds of lead, litharge and minium are found perfectly dellroyed by the addition of arfcnic in any form. 

 So be of fingular ufe in the manufafture of glafs. Litharge Thus, a mixture of oxyd of cobalt and oxyd of manganefe, 

 is a powerful flux, and imparts to glafs the valuable qualities of in tlie colouring (late, is of a dark purple ; but on the addv 

 greaterdenfity andgreaterpowcrofrefradtingtheraysof light, tion of any arfeniat, or of white arfenic, the manganefe is 

 and of bearing fudden changes from heat to cold, without made inaftive, and the proper cobalt-blue alone appears. 

 being fo liable to crack, and alfo greater tenacity when Hence we perceive the necelfity, when the red colour of 

 red-hot, fo that it is more eafily wrought. A confiderable manganefe is wanted, to avoid anything arfenical ; and nitre 

 quantity of this oxyd is contained in the finer glaffes ; fuch is alio generally added to keep the manganefe always at tire- 

 as the London flint glafs, and that which is ufed for the proper ftate for imparting its colour. The oxyd of man- 

 Uible, for luftres, for artificial gems, and for moil optical ganefe is a very powerful flux for all earthy matters ; and 

 purpofes. Glafs, however, that contains much lead, is this, as well as lead, gives a greet denfity to glafs. The 

 extremely foft ; and liable to be in'iured by hard bodies white oxyd of arfenic is another powerful and cheap flux 

 that come into contaft with it ; and it is alfo very fuiible. in the making of glafs ; but it (hould be very moderately 

 It is alfo liable to be corroded by very acrid liquors. Be- ufed; for it takes a longer time to mix intimately with glafs, 

 fides, the ufe of lead renders it difficult fo to unite the filex and to allow it to be perfectly clear, than any other of the 

 and alkali, that a piece of glafs fliall be throughout of uni- additions commonly employed. For want of this the glafs 

 form denfity. Another ingredient occalionally ufed in has a milky hue, which inereafes with age; and when the 

 glafs is the black oxyd of manganefe, called " glafs-foap," arfenic is in excefs, the glafs becomes gradually foft, and 



from its ufe in clearing the glafs from any accidental foul- 

 iiefs of colour, and more efpecially from the green tinge, 

 owing to the prefence of iron. Schecle and Bergman in their 

 refpeitive " Effays," have illuftrated many curious cireuni- 

 ftances that attend the ufe of manganefe in glaffes, wiiich 

 are particularly detailed in Aikiii's Di(£lioiiary. The man- 

 ganefe fhould be chofen of a deep colour, and free from 

 fpecks, cf a metalline appearance, or a lighter caft : and 



is decompofed. Beiides, glafs of this kind is unfafe to be 

 ufed in the form of drinking veficls. 



Nitre is alfo ufed, in fmall quantities, in the manufaftu^ 

 of glafs, and is defigned to anfwer particular purpofes, fome 

 of v.'hich we have already i'pecified. It not only fcrves to 

 dellroy the llrong tinge of yellow which is found in glafs, 

 prepared with lead as a flux ; but in faline glafs, it is requi- 

 iite, in a fmaller proportion, to render it fufficiently tranf- 



it requires ta be well calcined in a hot furnace, and then parent, as in the cafe of looking-glaffes, and other kinds of 



to undergo a thorough levigation. The effect of manganefe 

 in deftroying the colours of glafs is accounted for by M. 

 Montamy, in his " Traitc des Couleurs pour la Peinturc 

 rn Email,'' in the following manner ; the manganefe de- 

 flroys the green, olive, and blue colours of glafs, by add- 

 ing to tl)em a purple tinge, and by the mixture producin 



plates. For an account of Mr. Dollond's excellent contri- 

 vance for deltroying the colours in the objeft-glaffes of tele- 

 fcopes. See. fee AuEilK.M'iOV. 



With regard to the feveral fluxes above enumerated, we 



may obfcrve, in general, that the more calx of lead, or 



jj other metallic earth, enters into the coinpofition of any 



A blackilh brown colour ; and as blacknefs is caufed merely glafs, fo much the more fufible, foft, coloured, and denfe 



by an abforption of the rays of light, the blackilb tinge this glafs is, and reciprocally. 



given to the glafs by the mixture of colours, prevents the The colours given to glafs by calces of lead are fliades 



refleftion of fo many rays, and thus renders the glafs lefs of yellov.': on the other hand, glaffes tiiat contain only 



col.vjred than before^ But the black produced by this faline fluxes partake of the properties of falts; they are lefs 



fubilance fuggefts an obvious reafon for ufing it very fparingly heavy, lefs denfe, harder, whiter, more brilliant, and more 



in thofe compofitions of glafs, which arc required to be brittle than the former ; and glaffes, containing both faline 



very tranfparent. and metallic flaxes, do alfo partake of the properties of both 



Thi.^ purple colour may be correfted by charcoal, or in thefe fubftances. Glaffes too fahne are eafily fufceptible of 



the glafs-houfe, by tlirufting a billet of wood down into the alteration by the adtion of air and water; efpecially thofe in 



welted glafs, wliich becomes charred by the iiitenfe heat, and which alkalies prevail j and thefe are alfo liable to belnjured 



8 by 



