GLASS. 



■which It is to be wronght into platf s. For purifying the 

 pearl-alhes, difTolve them in four times their weight of boil- 

 ing water, in a pot of caft iron, always kept clean from 

 ruft. Let the folution be removed into a clean tub, and re- 

 main there twenty-four hours, or longer. Having decanted 

 the clear part of the fluid from the dregs or fedimeut, put 

 it again in the iron pot, and evaporate the water till the fall? 

 are left perfeftly dry. Preferve them in flone jars, well fe- 

 cured from air and moifture. 



Pearl-alhes may alfo be purified in the highdl degree, fo 

 as to be proper for the manufacture of the moft tranfpa- 

 rent glafs, by pulverizinir three pounds of the bell poarl- 

 afhes, with fix ounces of falt-pctre, in a glafs or marble 

 mortar, till they are well mixed ; and then putting part 

 of the mixture into a large crucible, and expofing it in a 

 furnace to a ftro^g heat. When this is red-hot, th.row 

 in the reft gradually ; and when the whole is red-hot, pour 

 it out on a moiftened ftone or marble, and put it into an 

 earthen or clean iron pot, with ten pints of water ; heat it 

 over the fire till the falts be entirely melted ; let it then Hand 

 to cool, and filter it through paper in a pewter cullender. 

 When it is filtered, put the fluid again into the pot, 

 and evaporate the fait to drynefs, which will then be as 

 white as fnow ; the nitre having burnt all the phlogiilic 

 matter that remained in the peai-l-afhes, after their former 

 calcination. 



As to the fand, it is to be fifted and waflied, till fuch 

 time as the water come off very clear ; and when it is well 

 dried again, they mix it with the fait, paffing the mixture 

 through another fieve. This done, they lay them in the 

 anneahng furnace for about two hours ; in which time the 

 matter becomes very light and white : in this ftate they are 

 caWed/rii, or frht a ; and are to be laid up in a dry clean 

 place, to give them time to incorporate. They lie here for 

 at leaft a year. 



Whi'n they would employ this frit, they lay it for fome 

 hours in the furnace, adding to fome the fragments or 

 (hards of old and ill made glafles ; taking care firft to cal- 

 cine the fliards by heating them red-hot in the furnace, and 

 thus cafting them into cold water. To the mixture mull 

 likewifc be added manganefc, to promote the fution and 

 purification. 



The bed compofition for looking-glafs plates is faid to 

 eonfift of fixty pounds of white fand cleanfcd, twenty-five 

 pounds of purified pearl-aflies, fifteen pounds of falt-pctre, 

 and feven pounds of borax. If a yellow tinge fhould affetl 

 the glafs, a fmall proportion of manganefe, mixed with an 

 equal quantity of arfenic, fliould be added. An ounce of 

 the manganefe may be firll tried ; and if thif proves infuffi- 

 cient, the quantity Ihould be incrcafed. 



A cheaper compofition for looking-glais plate confifts of 

 fixty pounds of the white fand, twenty potmds of pearl-aHies, 

 ten pounds of common fait, feven pounds of nitre, two 

 pounds of artenic, and one pound of borax. 



The materials of the fincll plate glafs, fuch as that of 

 French manufaclure, are white fand, foda, and lime, to 

 which are added manganefe and zaffre, or any other oxyd 

 •f cobalt for particular colouring purpofes. The land is 

 of the fineft and whitell kind, Wiiich ihould be previoufly 

 pafied through a wire fieve, moderately clofe, into water, 

 in which it (hould be well ftirred about and waflied. The 

 (harpeft grained fand is preferred, and it is found that grains 

 of moderate fize melt with the alkali fooncr, than the very 

 fine dull or the larger fragment?. The alkali is always 

 foda, which is preferable to pota'.h, as glalTes made with 

 foda are found to be fofter and to flow thinner when hot, 

 ^d yet to be equally durable whsM cold. Befides, the 

 Vol. XVI. 



neutral falls with the balls of foda whiili ron^ituf :!■ gii!«- 

 gall in this inflance, fuch as the muriat and fulpbut o£ 

 ioda, aj)pear to be didipatcd more readily by the fire tha» 

 the correlponding faltj of potafh. I'lie foda that is ufcd i» 

 confiderably pure, or fuch as is feparatcd from the rough 

 allies of barilla, and other Ioda plants bv lixiviation. Lime 

 adds to the fulibility of the other materials, fupplyiiig the 

 ufe of litharge in the fli:it-glafs ; but excefs of it would im- 

 pair the colour and folidity of the glafs. About I-I5th of 

 the whole is as much as can properly be ufed ; but fome re- 

 duce the quantity to i-24th. The decolouring fubftanc"! 

 are azure, or cobalt blue, and manganefe. IJefiJcs thcfe, 

 there is always a great quantity of the fragments of glafs, 

 colleftcd from the walle of the manufacture, which are 

 made friable by quenching in water when hot, and ufed in 

 this Hate together with the frelh materials. As to the 

 quantities and proportions of the ingredients, much latitude 

 is allowed. The following are faid to produce a very tine 

 glafs ; v'l-z. 50olb3. of fand ; zoolbs. of foda ; 3olb5. of 

 lime ; 32 ounces of mangauefe ; three ounces of a/.:«rc ; and 

 3O0lbs. of fragments of glafs. In the manufaitorv at .St. 

 Gobin fecrecy is obfcrved with regard to the materials ; but 

 it is afiirnied, and with much probability, that borax is ufed 

 in fmall quantity. 



Of the materials now enumerated the fand, foda, lime, 

 and manganefe are firll mixed together with more care thai 

 for ordinary glafs, arid they are fritted in fmall furnaces built 

 for this purpofe, the heat being gradually raifed to a full red- 

 white, and then kept with frequent ftirring till the materials 

 undergo no farther change, nor yield any kind of vapour. 

 The azure and glafs fragments being already in a ftate of 

 perfetl vitrification are not added till juft at the end of the 

 procefs, which lafts about iw hours-. When the n-.ateri.il» 

 are thus prepai-ed, they are fit for platc-glafs, to be formed 

 either by blowing or calling. The largeft glalTcs at St. 

 Gobin are run ; the middle-fizcd and fniull ones are 

 blown. 



Blo-.umg hol'ing-ghfs plain. I'hc work-houfcs, furnaces, 

 &c. uled in the making of this kind of plate-glafs. are 

 the fame, except that they are fmaller, and that the car- 

 quaifles are dil'pofed in a large^ covered gallery, over-againft 

 the furnace, as thofe in the following article, to which the 

 reader is referred. 



After the materials are vitrified by the heat of the fire, 

 and the glafs is fufficiently refined, the workman dips in his 

 blowing iron, fix feet long, and two inches in diainctcr, 

 fliarpened at the end, which is put in the mouth, and wideiK-d 

 at the other, that the matter may adhere to it. By thi» 

 means he takes up a fmall ball of matter, which fticks to 

 the end of the tube by conftantly turning it. He then 

 blows into the tube, that the air may fwell the ainiexed 

 ball: and carrving it over a bucket of water, which it 

 placed on a fupport at the height of about four feet, he 

 ipriiikles tlu? end of the tube to which the matter adheres, 

 with water, ftill turning it, that by this cooling, the mat- 

 ter may coalefce with the tube, and be fit for luftaining a 

 greater weight. He dips the tube again into the fame pot, 

 and proceeds as before ; and. dipping it in the pot a third 

 lime, he takes it out, h)aded with matter, in the fliapc of a 

 pear, about ten inches in diameter, and a foot long, and 

 cools it at the bucket ; at the fame time blowing into the 

 tube, and, with the afiillance of a labourer, giving it a 

 balancing motion, he caufes the matter to leiigthcn ; which, 

 by repeating this operation feveral times, afliunes the form 

 of a cylinder, terminating like a bull at the button), and in 

 a point at the top. The alTill.int is then placed on a ilool 

 three feet and a half high ; and on this ftyul there are two 



T t ujirijjl.t 



