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fcrimftone half an ounce ; let thefe be well powdered and | mixed, and put them, by degrees, into the glafs at fix times, waiting a little while between each putting in ; when the | whole is put in, let the glafs boil and fettle for twenty four hours, then make a little glafs body of it, which put in the furnace many times and fee if the glafs be enough, and whe- ther it have on the outfide veins of blue, green, red, yel- low, and other colours, and have befide thefe veins, waves like thofe of of the chalcedonies, jafpers and oriental agates, and if the body kept within looks as red as fire. Neri's Art. of Glafs, p. 73.
When it is found to anfwer thus it is perfect, and may be worked into toys and veffels, which will always be beauti- fully variegated ; thefe muft be well annealed, which adds much to the beauty of their veins. Maffes of this may be polifhed at the lapidary's wheel as natural ftones, and appear very beautiful. If in the working the matter grow tranf- parent, the work muft be flopped, and more tartar, foot, and crocus martis, muft be put to it, which will give it again the neceffary body and opacity, without which it does not fhew the colours well. Neri, p- 74- Glass of lead, a glafs made with the addition of a large quan- tity of lead, of great ufe in the art of making counterfeit gems. The method of making it is this. Put a large quantity of lead into a potter's kiln, and keep it in a ftate of fufion with a moderate fire till it is calcined to a grey loofe powder, then fpread it in the kiln, and give it a greater heat, continually ftirring it to keep it from running into lumps ; continue this feveral hours till the powder become of a fair yellow, then take it out and ftft it fine ; this is called calcined lead.
Take of this calcined lead fifteen pound, and cryftalline, or other fritt, twelve pound, mix thefe as well as poflible together, put them into a pot, and fet them in the furnace for ten hours ; then call the whole, which will be now per- fectly melted, into water ; feparate the loofe lead from it, and return the metal into the pot, and after Handing in fu- fion twelve hours more, it will be fit to work. It is very tender and brittle, and muft be worked with great care, taking it flowly out of the pot, and continually wetting the marble it is wrought upon. Nn-i's Art of Glafs, p. no. Glafs of lead is capable of all the colours of the gems in very great perfection. The methods of giving them are thefe : for green, take polverine fritt twenty pound, lead cal- cined fixteen pound, fift both the powders very fine, then melt them into a glafs feparating the unmixed lead, by plunging the mafs in water ; after this return it into the pot, and add brafs thrice calcined fix ounces, and one penny weight of crocus martis made with vinegar ; put this in at fix different times, always carefully mixing it together, let it finally fet- tle an hour, then mix it together and take a proof of it; when the colour is right, let it ftand eight hours, and then work it. If inftead of the calcined brafs the fame quantity of the caput mortuum of the vitriolum veneris be ufed, the
freen is yet much finer. Neri's Art of Glafs, ibid, and p. 112. or topaz colour, take cryftal fritt fifteen pound, calcined lead twelve pound, mix them well together by lifting the powders through a fine fieve, then fet them in a furnace not too hot, and feparate the fuperfluous unmixed lead, by cafting the whole into water ; repeat this twice, then add half gold yellow glafs, and let them incorporate and purify, and they will be of the true and exact colour of the orien- tal topazes. Neri p. 113.
For fea green, take cryftal fritt fixteen pound, calcined lead ten pound, mix and fift them together, and fet them in a pot in a furnace, in twelve hours the whole will be melted ; then caft it into water and feparate it from the loofe lead, put them into the furnace again for eight hours, then fepa- rate the loofe lead by wafhing a fecond time, and return it to the pot for eight hours more. Neri, p. 1 14. Glass porcelain, the name given by many to a modern inven- tion of imitating the china ware with glafs. The method given by Mr. Reaumur, who was the firft that carried the attempt to any degree of perfection, is delivered by that
Pentleman in the memoirs of the academy of fciences of aris to this effect. Memoirs Acad. Paris 1739. The mixing glafs reduced to powder, with other lefs eafily vitrifiable fubftances for the forming a pafte to be afterwards made into a fort of porcelain, has been a contrivance long practifed, but it is very troublefome, and the refult fubject to many faults ; but this new ware is made of glafs alone, and that with much lefs trouble, and without the re- ducing it to powder. By this art, vefTels of glafs are changed into vefTels of a fort of porcelain, without altering their form, and the meaneft glafs ferves as well as the beft for that purpofe ; our common coarfe green quart bottles, or the great bell glaffes with which gardeners cover their melons, cffc. being by this means changeable into a beautiful white fort of porcelain ware ; and this is to be done in fo eafy a manner and with fo fmall expence, that it requires no more trouble or charge, than that of baking a common veflel of our courfe earthen ware, and for this reafon the vefTels of this fort of ware may be afforded extremely cheap.
It is very certain that all procelain ware is a fubftance in the ftate of femivitrification ; and in order to bring glafs, which is a wholly vitrified fubftance, into the condition of porce- lain, there requires no more than to reduce it to a lefs per- fectly vitrified ftate.
The queftion which would naturally be ftarted on this oc- cafion, is, whether it be poflible to reduce glafs to a lefs vi- trified ftate, it having already undergone what is elteemed the laft change by fire. But when we confider that the glafs of antimony, the vitrifications of many of the metals, as the glafs of lead, and the counterfeit gems coloured by the metals, are more or lefs eafily reduced again by chy- miftry to metals, &c. the reducing fand, flints, &c, after they are vitrified, at leaft a little way back toward their na- tive or priftine ftate, may appear not wholly impracticable ; and the attempts which Mr. Reaumur made on this occa- fion, were what gave him the firft hints of the glafs por- celain.
The method of making it is this. The glafs vefTels to be converted into porcelain, arc to be put into a large earthen veffel, fuch as the common fine earthen difhes are baked in, or into fufficiently large crucibles ; the vefTels are to be filled with a mixture of fine white fand, and of fine gypfum or plaifter Hone burnt into what is called plaifter of Paris, and all the interftices are to be filled up with the fame powder, fo that the glafs vefTels may no where touch either one another, or the fides of the veflel they are baked in. The veffel is to be then covered down and luted, and the fire does the reft of the work ; for this is only to be put into a common potter's furnace, and when it has flood there the ufual time of the baking the other vefTels, it is to be taken out, and the whole contents will be found no longer glafs, but converted into a white opake fubftance, which is a very elegant porcelain, and has almoft the properties of that of China. Memoirs Acad. Sciences Par. 1739. The powder which has ferved once, will do again as well as frefh, and that for a great many times j nay it feems ever fo often.
GLASs-balls, a term ufed to exprefs circular or otherwife fhaped hollow vefTels of glafs, coloured within fo as to imi- tate the femipellucid gems. The method of doing it is this ; make a ftrong folution of ichthyocolla or ifinglafs in common water by boiling, pour a quantity of this while warm into the hollow of a white glafs veffel, lhake it thoroughly about, that all the fides may be wetted, and then pour off the reft of the moifture. Immediately after this, throw in red lead, fhake it and turn it about, throw it into many places with a tube, and the moifture will make it ftick and run in waves and pretty figures. Then throw in fome of the painter's blue fmalt, and make it run in waves in the ball as the red lead, then do the fame with vcrdigreafe, next with orpiment, then with red lake, all well ground ; always cafting in the colours in different places, and turning the glafs that the moifture within may run them into the waves. Then take fine plaifter of Paris, and put a quantity of it into the ball, make it alfo nimbly about, this will every where ftick firm- ly to the glafs, and give it a ftrong inner coat, keeping all the colours on very fairly and ftrongly. Thefe are fet on frames of carved wood, and much efteemed as ornaments in many places. Neri's Art of Glafs, p, 167.
Glass Pots, the veffels in the glafs trade ufed for melting the glafs. Thofe for the white glafs works are made of a to- baccopipe clay, brought from the Ifle of Wight, which is firft well warned, then calcined, and afterwards ground to a fine powder in a mill ; which being mixt with water, is then trod with the bare feet till it is of a proper coniiftence, to mould with the hands into the proper fhape of the vefTels. When thefe are thus made, they are afterwards annealed over the furnace. Thofe for the green glafs work are made of the nonfuch, and another fort of clay from Stafford- Ihire ; they make thefe fo large as to hold three or four hundred weight of metal. And befides thefe, they have a fmall fort called piling pots, which they fet upon the larger, and which contain a finer and more nice metal fit for the niceft works. Neri's Art of glafs, p. 245.
Glass Tubes, laid before the fire in an horizontal pofition, and their extremities fupported, have been obferved to have a rotatory motion about their axis ; and alfo a progreifivc motion towards the fire, even when their fupports are in- clining from the fire, fo that the tubes will move a little up hill. See Phil. Tranf. N°. 476. § 1.
When the progreffive motion of the tube is flopped by any obftacle, its motion about its axis will ftill continue. When the tubes are placed in a nearly upright pofture leaning to the right hand, the motion will be from eaft to weft ; but if they lean to the left, the motion will be from weft to eaft; and the nearer they are placed to the perfectly upright pofture the lefs the motion will be either way. If the tube be placed horizontally on a glafs plane, the frag- ment for inftance of coach tvindozv glafs, inftead of moving towards the fire, it will move from it, and about its axis, in a contrary direction to what it had done before. Nay it wi43 recede from the fire and move a little up hill, when
the
