476

MANUFACTURES.

Manufac- Lixiviated earth of ashes . 160

hires. Brush wood or other ashes. 30

Brick day. 80

To which is added a quantity of broken glass, generallyto the amount of 100 parts.

At Newcastle-upon-Tyne , where the manufacture ofbottle glass is much encouraged, by the excessive cheap-ness of small coal or slacks, the manufacturers employ amixture of lime and sea sand. The sand is frequentlymelted with sea water, which, on evaporating, depositsits salt, of which the soda contained is the only alkaliemployed. When combined with silica and expbsed toa high degree of heat, lime appears to be endowed withthe property of decomposing common salt; its presenceis therefore essential to the success of this operation.

On the Fusion of Glass.

. (689.) The operations ofthe glass-house admit of being

"■lass' 10 divided into two distinct classes; viz. those connectedn with the fusion or formation of the material itself, and

those connected with its manufacture into the variousarticles for which it is employed. Under the presenthead we speak simply of those processes necessary tobring the glass into a proper state to be operated uponby the glass-manufacturer.

Frittin". The first process in glass-making is that of pre-paring the materials for the glass pots, which is termedfritting, and consists in calcining them to a pointof semivitrification, in which they strongly adhere, andbegin to become pasty, but are still opaque, and not yethomogeneous. This process is sometimes performed insmall furnaces adjoining the glass furnace, and heatedbj the same fuel alter the chief force has been spentupon the glass pots, although generally small furnacesor ovens are constructed for this express purpose.

I he uses of this operation are to drive off the moistureIrom the materials, which might endanger the destruc-tion of the glass pots, and to expel the carbonic acidfrom the alkalis and chalk, by which the swelling of thematerials is prevented.

This calcination has also the further advantage ofcausing a chemical union to commence between thealkali, silex, and metallic oxides. For if the raw mate-rials were immediately exposed to the intense heat ofthe glass pots, the alkali tvould flow like water, andthe grosser particles ofthe sand, and the heavier oxides,would settle to the bottom, leaving the alkali abovenearly disengaged, and thereby liable to act on the cru-cibles ; besides which a large portion would be rapidlyvolatilized to mere waste. The glass materials wouldthus have an excess of sand from the loss of the alkali,and a portion would remain at the bottom unvitrified.

These operations do not apply to the preparations ofmaterials for flint glass, the fusibility of which is muchgreater than that of any other description, owing- to thepresence of so large a portion of lead. For this reason,manufacturers of this kind of glass apply the process ofcalcination to the sand alone, with,a view of separatingfrom it all carbonaceous impurities, previously to its ad-mixture with the remaining materials.

When the process of calcination or fritting has con-tinued for a sufficient length of time, which varies from sixto twenty, or even to a greater number ot hours, depend-ing upon the materials employed, tue substance, whichis in a softened state, is either marie into the form of, square cakes, known to the workman by the name of

frit, to be preserved for future use, or thrown at once

into the crucibles in the furnace, where the fire has been . roe ^ . -

previously raised to its greatest intensity. As the fritted

materials are much more bulky than when fused, the

pots receive their full charge by two or three successive

portions, the last added being always thoroughly melted

down before a fresh charge is thrown in. When the

pots are full, the side opening in the furnace, by which

the materials are admitted, is closed up (except a small

opening for examining the work) until the complete

fusion and refining of the materials has taken place.

(690.) As soon as the frit begins to feel the action of the Glass g il]i-fire in the glass pots, it sinks down into a soft pasty mass,and gradually increases in tenacity until a perfect fusionis effected. It is still, however, opaque at first, owingto the rising of a quantity of white porous scum, knownby the name of sandiver, or glass gall. This substancehas engaged a good deal of the attention of scientific men.

It appears to be a confused mass, consisting of all thesalts contained in common alkalies, which readily meltat a somewhat less heat than glass, and, being naturallyvery volatile, and having little, if any, affinity for silex,do not unite in the composition. When laded out andcooled, it forms a white crumbly mass, sometimes quitewhite, and at others brown and foul, and stronglysaline, but not very uniform in its composition, beingsometimes very bitter, which is probably owing tothe sulphate of potash. Glass gall is very volatilein a strong fire, so that it is constantly dispersingfrom the surface of the glass in a dense vapour, whichvery powerfully corrodes the top of the crucible in itspassage. If the fusion were continued a sufficient lengthof time, the whole might be dispersed in this way ; butit is generally scummed off with iron ladles, to be soldto the metal refiners, to whom it is very valuable as aflux. Some ofthe alkali itself is also dissipated by thecontinuance of the fire, partly before it can unite withthe silex, and partly from the glass after vitrification, towhich the corrosion of the pots must in some measurebe owing ; it unites also in some measure with the glassgall, which renders it slightly deliquescent. An abun-dance of glass gall is one of the greatest inconveniencesthat a glass-maker can meet with, as it requires a con-siderable lime and a very strong heat before the wholecan be dissipated ; and if the glass be wrought beforeit is thoroughly purged of this material, it will be fullof bubbles, unsound, and of a cloudy appearance. Glassmade from potash is more likely to sutler from this impu-rity than the soda glasses, being harder and not so thinas the others, and the glass gall does not so readily dis-sipate in the fire.

There is another hetereogenous substance also calledsandiver, which sometimes is found at the bottom of thepots, and is taken out when the whole is worked up.

The nature of this is very different, from the other, con-sisting apparently of a vitrified mass of arsenic another impurities.

As the heat in the furnace is continued, the g* asgradually becomes more flexible, dense, andbrittle, the glass gall is at last entirelyand the vapours which it occasioned areto be perceived. The glass is now full -■ ,

specks and bubbles, which the heat causes to expanand burst at the top, till at last it refines beautifully c ea ^transparent, and colourless, as may be seen by the san \pies which are taken out by -an iron rod and coo eThis latter part of the process. Irom the cessation o

less

dissipated,

no longerof minute