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How To Make Glass

Glass is a combination of sand, flint, spar, or some other silicious substances, with one or otherof the fixed alkalies, and in some cases with a metallic oxide. Of the alkalies, soda is commonly

preferred; and of the silicious substances, white sand is most in repute at present, as it requires no

preparation for coarse goods, while mere washing in water is sufficient for those of a finerquality. The metallic oxide usually employed, is litharge, or some other preparation of lead. Ironis used in bottle-glass.

The silicious matter should be fused in contact with something called a flux. The substancesproper for this purpose are lead, borax, arsenic, nitre, or any alkaline matter. The lead is used in

the state of red-lead; and the alkalies are soda, pearlash, sea-salt, and wood-ashes. When red-lead

is used alone, it gives the glass a yellow cast and requires the addition of nitre to correct it.

Arsenic, in the same manner, if used in excess, is apt to render the glass milky. For a perfectlytransparent glass, the pearlash is found much superior to lead; perhaps better than any other flux,

except it be borax, which is too expensive to be used, except for experiments, or for the best

looking-glasses.

The materials for making glass must first be reduced to powder, which is done in mortars or by

horse mills. After sifting out the coarse parts, the proper proportions of silex and flux are mixed

together, and put into the calcining furnace, where they are kept in a moderate heat for 5 or 6hours, being frequently stirred about during the process. When taken out the matter is called frit.

Frit is easily converted into glass by only pounding it, and vitrifying it in the melting pots of the

glass furnace; but in making fine glass, it will sometimes require a small addition of flux to thefrit to correct any fault. For, as the flux is the most expensive article, the manufacturer will rather

put too little at first than otherwise, as he can remedy this defect in the melting pot. The heat in

the furnace must be kept up until the glass is brought to a state of perfect fusion; and during this

process any scum which arises must be removed by ladles. When the glass is perfectly melted,the glass-blowers commence their operations.

For the best flint-glass, 120 lbs. of white sand, 50 lbs. of red-lead, 40 lbs. of the best pearlash, 20lbs. of nitre, and 5 oz. of manganese; if a pound or two of arsenic be added, the composition will

fuse much quicker, and with a lower temperature.

For a cheaper flint-glass, take 120 lbs. of white sand, 35 lbs. of pearlash, 40 lbs. of red-lead, 13

lbs. of nitre, 6 lbs. of arsenic, and 4 oz. of magnesia.

This requires a long heating to make clear glass, and the heat should be brought on gradually, or

the arsenic is in danger of subliming before the fusion commences. A still cheaper composition ismade by omitting the arsenic in the foregoing, and substituting common sea-salt.

For the best German crystal-glass, take 120 lbs. of calcined flints or white sand, the bestpearlash, 70 lbs, saltpetre, 10 lbs.; arsenic, 1/2 lb., and 5 oz. of manganese. Or, a cheaper

composition for the same purpose is 120 lbs. of sand or flints, 46 lbs. of pearlash, 7 lbs of nitre, 6

lbs. of arsenic, and 5 oz. of manganese. This will require a long continuance in the furnace; as doall others where much of the arsenic is employed.

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For looking-glass plates washed white sand, 60 lbs.; purified pearlash, 25 lbs.; nitre, 15 lbs.; and7 lbs. of borax. If properly managed, this glass will be colorless. But if it should be tinged by

accident, a trifling quantity of arsenic, and an equal quantity of manganese, will correct it; an

ounce of each may be tried first, and the quantity increased if necessary.

The ingredients for the best crown-glass must be prepared in the same manner as for looking-

glasses, and mixed in the following proportions: 60 lbs. of white sand, 30 lbs. of pearlash, and 15

lbs. of nitre, 1 lb. of borax, and 1/2 lb. of arsenic.

The composition for common green window-glass is, 120 lbs. of white sand, 30 lbs. of

unpurified pearlash; woodashes, well burnt and sifted, 60 lbs.; common salt, 20 lbs.; and 5 lbs. ofarsenic.

Common green bottle-glass is made from 200 lbs. of wood-ashes and 100 lbs. of sand, or 170

lbs. of ashes, 100 lbs. of sand, and 50 lbs. of the slag of an iron furnace; these materials must be

well mixed.

The materials employed in the manufacture of glass, are by chemists reduced to three classes,namely, alkalies, earths, and metallic oxides.

The fixed alkalies may be employed indifferently; but soda is preferred in this country. The soda

of commerce is usually mixed with common salt, and combining with carbonic acid. It is properto purify it from both of these foreign bodies before using it. This, however, is seldom done.

The earths are silica (the basis of flints), lime, and sometimes a little alumina (the basis of clay).Silica constitutes the basis of glass. It is employed in the state of fine sand or flints; and

sometimes for making very fine glass, rock crystal is employed. When sand is used, it ought, if

possible, to be perfectly white, for when it is colored with metallic oxides, the transparency of

the glass is injured. Such sand can only be employed for very coarse glasses. It is necessary tofree the sand from all the loose earthy particles with which it may be mixed, which is done by

washing it well with water.

Lime renders glass less brittle, and enables it to withstand better the action of the atmosphere. It

ought in no case to exceed the 20th part of the silica employed, otherwise it corrodes the glass

pots. This indeed may be prevented by throwing a little clay into the melted glass; but in thatcase a green glass only is obtained.

The metallic oxides employed are the red oxide of lead or litharge, and the white oxide of

arsenic.

The red oxide of lead, when added in sufficient quantity, enters into fusion with silica, and forms

a milky hue like the dial-plate of a watch. When any combustible body is present, it is usual, in

some manufactories, to add a little white oxide of arsenic. This supplying oxygen, thecombustible is burnt, and flies off, while the revived arsenic is at the same time volatized.

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There are several kinds of glass adapted to different uses. The best and most beautiful are the

flint and the plateglass. These, when well made, are perfectly transparent and colorless, heavyand brilliant. They are composed of fixed alkali, pure siliceous sand, calcined flints and litharge,

in different proportions. The flint glass contains a large quantity of oxide of lead, which by

certain processes is easily separated. The plate glass is poured in the melted state upon a table

covered with copper. The plate is cast 1/2 an inch thick or more, and is ground down to a properdegree of thickness, and then polished.

Crown-glass, that used for windows, is made without lead, chiefly of fixed alkali fused withsilicious sand, to which is added some black oxide of manganese, which is apt to give the glass a

tinge of purple.

Bottle-glass is the coarsest and cheapest kind, in this little or no fixed alkali enters the

composition. It consists of alkaline earth and oxide of iron combined with alumina and silica. In

this country it is composed of sand and the refuse of the soap-boiler, which consists of the lime

employed in rendering this alkali caustic, and of the earthy matters with which the alkali was

contaminated. The most fusible is flint-glass, and the least fusible is bottleglass.