The best form in which flint is met with for the purpose of making glass, is in the state of sea-sand; and those kinds best adapted to the purpose in England, are procured from Lynn, in Norfolk, and from Alum Bay, in the Isle of Wight.

The first process consists in what is called fritting, that is, placing the materials in a crucible, and submitting them to the action of a violent heat, for the purpose of driving off any moisture they may contain, and to prevent their swelling too much when placed in the melting-pots, which are formed of a kind of clay that is able to endure the action of fire without melting. When the materials are sufficiently fritted, they are thrown into these pots with clean iron shovels, the fire being previously raised to its greatest intensity, to prevent the whole furnace from being chilled, and to save time. As the fritted materials are much more bulky than when they fall into a thin flowing glass, 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 full, the opening in the pot is closed with red clay, except a small hole for examining the work, and when the glass is well refined, and about to be worked off, this opening is again enlarged by the removal of the clay.

The crucibles, or pots, in which the glass is melted, are of an upright form, with the opening on the side near to the top; they are arranged in a circle, and built into a conical furnace, the walls of which are in the form of a sugar-loaf.

The kinds of glass manufactured are of various qualities, the commonest being that of which wine bottles are made.

Windows and plate-glass are both considerably harder in texture and more brittle than that which is called fluid-glass, of which wine-glasses and decanters, lampglasses, &c., are made. Various contrivances are made use of for the purpose of forming them into different shapes, but the material parts of the operations are

much the same as those already mentioned. In some instances, brass moulds are employed, into which the different articles are blown, and in this manner many excellent imitations of cut glass are formed.

1. Flint Glass.-This was originally named from the flint formerly used in its manufacture, but which is now superseded by fine sand, selected with care from various districts. Sand, pearlash, and litharge, are the materials generally employed for the production of flint glass; but different manufacturers use various proportions of these substances, as their scientific knowledge or experience may suggest. Some skilful glass-makers fuse together one hundred parts of Lynn sand, sixty parts of litharge, and thirty of purified pearlash.

When the materials have been submitted to the action of the fire for a certain time, varying from thirty to thirty-six hours, they have become perfectly liquid; the fire is then damped, and the glass in this fluid state, is suffered to cool to such an extent as to become sufficiently thick to be taken up on the end of an iron rod. When in this state there is perhaps no substance in nature so ductile, or so easily moulded into any form that may be required.

2. Crown Glass.-This, though not so rich as the preoeding, must not be passed over in silence, being the best species of window glass, and therefore contributing to the comforts of all those numerous families who inhabit the better class of houses. It is also composed of different materials from flint or plate-glass, for, whilst much metal enters into these, little is allowed to mix with the ingredients from which crown glass arises. It is, therefore, much lighter and harder than those kinds into which so softening and heavy a substance as litharge (oxide of lead) enters. The substances used by different manufacturers vary exceedingly in their proportions, each having his own pet system of working. The best French crown glass is formed from one hundred parts of fine white sand, added to the same quantity of broken crown glass, and with these elements twelve parts of carbonate of lime, and four times that

amount of carbonate of soda, are mingled. But in this country the following proportions are frequently used:

Sand,

Kelp,
Lime,

Broken crown glass,

200 lbs.

330

15 200 39

When superior glass is required, other proportions are employed, whilst pearlash and saltpetre are substituted for the kelp. What is this kelp, which we have not hitherto had occasion to mention? It is the ashes of sea-weeds, which were formerly gathered in large quantites along the shores of Ireland and Scotland, and in some places cultivated by the landowners with the greatest care. But the alkaline matter, resulting from the burning of kelp, was too coarse and impure for use when a superior glass was required, and it is now rarely employed except in cases when fine material is not the object. The introduction of Barilla* at a moderate duty from abroad, and the reduction of the duty on salt, from which alkali for the glass-works is now made, have freed our manufacturers from the necessity of using kelp in their operations.

The wonderful ductile property of glass, cannot, perhaps be exhibited to greater advantage than when the workman avails himself of it in the manufacture of crown glass. His mode of proceeding is, in the first place, to dip a long iron tube, called panten or the punt, the end of which has been previously heated, into the melting-pot, and to take out as much glass as will adhere to it; this operation he repeats until his judgment tells him he has sufficient metal on the end of the tube to form the table of glass he is about to blow. He then moulds the metal into a regular form by rolling it on a smooth iron table; when this is accomplished, he blows through the tube and forms the glass into a hollow vessel, shaped like a pear. He continues to enlarge this

* A carbonate of soda, imported in large quantities from Spain, Italy, Sicily, and the Canary Isles, obtained from two plants, one of which is called Barilla.

form by alternately heating the glass, and blowing through the tube; at the same time lengthening the neck, by rolling it on an iron rod, fixed to a kind of seat. It is now carried to the mouth of a larger furnace, called the bottoming-hole, in front of which a low wall is built to protect the workmen from the heat; it is here again heated, and whirled round with a slow and steady motion. The effect of this is to flatten the wide end of the glass, owing to the tendency of the metal to fly from the centre on which it revolves, in the same manner as the water does from a wet mop when twirled on the wrist.

A second workman now comes to the assistance of the first, and having taken up a portion of the metal on the end of an iron rod, smaller and lighter than that first used, he applies it to the centre of the flat bottom of the glass held by the other, to which it adheres. The first workman now touches the glass where it is attached to his blowing tube with a cold iron rod which has been dipped in water, this causes the glass to crack, and the blow-pipe is easily detached. Taking hold now of the smaller rod, he presents the broken end of the glass to the heat of the furnace, and continues the whirling motion; the effect of this is to enlarge the opening or mouth of the vessel. As the work proceeds, the workman impels the glass round with greater rapidity, till at length the blazing metal flies out with a jerk, with great force, and with a loud ruffling noise, like the rapid unfurling of a flag in a strong breeze. This part of the process is called flashing the glass. The sudden extension of the glass in this part of the process is sure to strike the spectator, who sees it for the first time, with surprise, and induces him to expect that the glowing mass will be torn to pieces by the violence of the shock. It is now moved more slowly round, till it is sufficiently cool to retain its form; the rod is then broken off in the same manner as before, and the circle of glass is carried to the annealing-furnace, where it is gradually cooled. The knot which is found in window-glass, shows the spot from which the iron was broken.

PART III.

There is a kind of glass called German plate, (much thinner than the genuine plate-glass) which is made by blowing the metal. The metal is first blown into a long pear-like shape, one end of this is pierced with an iron instrument, and the opening is afterwards enlarged by the same means. A slit is now made with a pair of shears through half its length, and the iron instrument having been dipped in the melted metal, is applied to the open end. The iron rod is now broken off, and the opening enlarged to the same size as that at the opposite end; the form is now a perfect cylinder like a roll of paper. The slit is now continued the whole length, and being laid upon a smooth iron table, it falls flat like a sheet of paper.

Plate-glass.-The production of this costly material demands far greater care than the preceding kinds, both in the selection of the ingredients, and in the careful fusion of the mass and rolling of the plates. To make 1,200 lbs. of plate-glass 1,700 lbs. of material is required, made up of the following substances mixed in proper proportions:

It will be seen that exactly one quarter of the whole is old glass, without which the quality required cannot be produced. When heated to a liquid mass, the whole is laded out into a vessel called a cuvette, from which, after being further heated, it is poured on to the surface of a long table, made of copper, when a heavy polished copper roller is passed over its surface, reducing it in