• 590
GLASS-MAKING.
I conceive that this glass contains too much lead and potash. Such a mixture will P rt _duce a dull metal, very attractive of moisture; defects to which the French crown-gl aSalso is subject.
The flint-glass leer for annealing glass, is an arched gallery or large flue, about 36 l elong, 3 feet high, 4 wide ; having its floor raised above 2 feet above the ground of 1glass-house. The hot air and smoke of a fire-place at one end pass along this S a '! e . 1 ?gand are discharged by a chimney 8 or 10 feet short of the other end. On the floor of 1vault, large iron trays are laid and hooked to each other in a series, which are drawn J r °the fire end towards the other by a chain, wound about a cylinder by a winch-handle P rjeeting through the side. The flint-glass articles are placed in their hot state intotray next the fire, which is moved onwards to a cooler station whenever it is filled, aan empty tray is set in its place. Thus, in the course of about 20 hours, the gl asS avances to the cool end thoroughly annealed.
Besides colorless transparent glass, which forms the most importantmanufacture, various colored glasses are made to suit the taste of the public.taste at Paris was lately for opaline crystal; which may be prepared by adding to .above composition (No. 2) phosphate of lime, or well burnt bone ash in fine P° w jwashed and dried. The article must be as uniform in thickness as possible, \speedily worked into shape, with a moderate heat. Oxyde of tin, putty, was f° ral j[ Sused for making opalescent glass, but the lustre of the body was always impaired bymeans. , a il
Crystal vessels have been made recently of which the inner surface is colorless, an .the external facets colored. Such works are easily executed. The end of therod must be dipped first in the pot containing colorless glass, to form a bulb of a , cel i aS s.size, which being cooled a little is then dipped for an instant into the pot of colored gThe two layers are associated without intermixture; and when the article is ^ n ' s ie „ a ssits form, it is white within and colored without. Fluted lines, somewhat deeply rut, P^ sthrough the colored coat, and enter the colorless one; so that when they cross, tbctfalone are colored. r j ;e t
For some time past, likewise, various crystal articles have been exhibited in the m ^ ver ywith colored enamel-figures on their surface, or with white incrustations of a s * ( | ielustre in their interior. The former are prepared by placing the enamel object i tbrass mould, at the place where it is sought to be attached. The bulb of glass b e,n ® theinto the mould, and blown while very hot, the small plate of enamel gets cementeasurface. For making the white argentine incrustations, small figures are prep a ' e jj lt )ean impalpable powder of dry porcelain paste, cemented into a solid by means oi a ff j a5 sgypsum plaster. When these pieces are thoroughly dried, they are laid on to’encafwhde it is red hot, and a large patch of very liquid glass is placed above it, so as t0 j ose d;it and form one body with the whole. In this way the incrustation is completely en aS „ect,and the polished surface of the crystal, which scarcely touches it, gives a brilli antpleasing to the eye. tici an '
A uniform flint-glass, free from striae, or wreath, is much in demand forIt would appear that such an article was much more commonly made by nC ymanufacturers ma'riy years ago, than at present; and that in improving the b rI ^ e pea^ scrystal-glass they have injured its fitness for constructing optical lenses, whic ra llelnot so much on its whiteness and lustre as on the layers of different densities being ^ rea tetto each other. The oxyde of lead existing in certain parts of a potful of glass i t ],atproportion than in other parts, increases the density unequally in the same ma ^ ^ te iVjthe adjoining strata are often very different in this respect. Even a P ot u . y jpe sub"uniform glass, when it stands some time liquid, becomes eventually unequable ^ysidence of the denser portions; so that striae and gelatinous appearances begij 1 ,a
themselves, and the glass becomes of little value. Glass allowed to cool slow y ^ (U - thr eethe pot is particularly full of wreath ; and if quickly refrigerated, that is, 1“ j, e uia<l e-hours, it is apt to split into a multitude of minute splinters, of which no u ? e ff C £a thered ° r *For optical purposes, the glass must be taken out in its liquid state, being - t j ie upP erthe end of the iron rod from the central portion of a recently skimmed pot, a _
layers have been worked off in general articles. es that f a j
M. Guinand, of Brennets near Geneva, appears to have hit upon proce . j^rk 8 ®,
nished almost certainly pieces of flint-glass capable of forming good lenses o' cnC y, aa .dimensions, even of 11 inches diameter; of adequate density and transp t out 0nearly free from striae. M. Cauchoix, the eminent French optician, says> e jght aten object glasses, 4 inches in diameter, made with M. Guinand’s flint g a ’j e of ta ,nine turned out very good, while out of an equal number of object glasses f 0 u>i
flint glass of the English and French manufactories, only one, or two at mos ’ „ 0 t be°.
serviceable. The means by which M. Guinand arrived at these results “ sseS sio a 0published. He has lately died, and it is not known whether his son be in phis secret.
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