Old Glass and How to Collect it

by J. Sydney Lewis

Varieties of Glass.—As we shall frequently find it necessary to refer to the various kinds of glass, it may be as well at the outset to attempt to give a clear idea of their differences and of the meanings of the various terms employed in describing them.

As regards quality, the chief kinds are crown glass, flint glass, plate glass, bottle glass, and crystal glass, and the differences in composition may be conveniently expressed in the form of a table:—

	Silica.	Potash.	Soda.	Lime.	Alumina.	Oxide of Lead.	Oxide of Iron.
Crown.	67	21	—	10	2	—	—
Flint.	44	12	—	—	1	43	—
Plate.	78	2	13	5	2	—	—
Bottle.	59	3	—	25	6	—	7
Crystal.	56	9	—	3	—	32	—

Cheapest of all glass is bottle glass, where the base is mainly lime. The metal used for medicine bottles contains more potash and is purer and clearer. The use of potash and soda makes the glass more easily fusible; alumina has the opposite effect; lime makes a harder glass; lead gives lustre, increases fusibility, and heightens the refractive power. Hence in glass which is to be cut and polished the employment of lead in sufficient quantity is a factor of the highest importance. This is a point to be specially noted in connection with English glass. Lead—chiefly the oxides known as litharge and minium—in small quantities has long been employed, the introduction of the metal serving as a flux, but lead glass was generally avoided as being too brittle. Merret, writing in 1662, remarks that could this glass be made as tough as crystalline, “it would far surpass it in the glory and beauty of its colours.” It will be noted that the two kinds of glass in which lead is used in quantity are flint glass and crystal. The larger the amount of lead the greater the beauty and brilliancy of the product, a result due, as previously intimated, to the increase in refractive power that is brought about by its addition.

Flint glass derives its name from the fact that in England the silica, which is the main constituent of all glass, was procured from flints which were calcined and pulverised. Being highly refractive it is extensively employed in the manufacture of optical instruments—telescopes, microscopes, etc. Quartz and fine sand are now used in the place of flints. The glass is soft, and hence easily scratched and dulled. It is essential that only the purest materials be employed, and special furnaces and pots are needed. Flint glass was known in quite early times. It was probably discovered by accident that certain stones were fusible, for fossil glass is found in many places where great fires have been. Volcanic glass—obsidian—is a well-known substance, while there exist in Scotland ancient forts, the stones of which have been fused together by the action of heat. The Venetians used quartz in preference to sand, since the latter was liable to contain impurities, and the Venetian craftsmen who settled in England were accustomed to ensure the purity of their silica by calcining flints. Crown glass is the finest sort of ordinary window glass. Plate glass is the superior kind of thick glass used for mirrors, shop windows, etc. It will be noted that it is the only kind of glass which contains soda.

The process of glass manufacture comprises three stages, mixing, melting, and blowing. The various ingredients are first finely ground and then thoroughly mixed by the aid of a mixer, forming what is known as the “batch.” This is placed in melting pots. These are crucibles of fire-clay, i.e. clay capable of withstanding the action of heat. The clay must be of the finest quality, and be carefully freed from extraneous matters which might affect the quality of the glass. Hence the manufacture of the “pots” is itself an industry of some importance, and as each costs some £10, they form an important item in the expense of manufacture, especially as the pots are short-lived, some eight to ten weeks being the average life of one of them.

The ordinary pot is an inverted section of a cone, the apex being closed. For flint glass a covered pot is essential, the form ordinarily adopted being a bell-jar closed at the bottom and with an arched opening at the top. Each pot holds from ten to fifteen cwt. of the “batch.” When full, the pots are placed in specially constructed furnaces, holding from five to fifteen pots, and capable of producing a temperature of from 10,000° to 12,000° F. The details of the firing are intricate and interesting but have no direct bearing on our purpose; their object is to produce complete fusion, to allow for the removal of all impurities, and to ensure the homogeneity of the product.

The final stage with which we are concerned is that of blowing, since all table glass, worthy of being called table glass, is blown. In other words, every decanter, vase, tumbler, and wine glass of the better sort begins its existence as a bubble of molten glass at the end of an iron tube—the glass-blower’s tube—and owes its form to the delicate touches of simple tools held in a skilful hand and guided by a trained eye. It is this fact which gives glass its individuality. There is no hard-and-fast rigour of line, no mechanical uniformity of shape, such as is associated with machine-made goods; even the simplest wine glass is an individual thing, which the taste of the craftsman has endowed with artistic distinction whilst retaining its simplicity of form.

It is a matter for regret that the glass-blower’s art is seriously threatened in these latter days of hurry and competition. The demand for cheap glass has led to the introduction of blowing machines, in which the bubble of molten glass is taken up by one of many blowing tubes, and placed inside a mould, air being driven by machinery through the other end of the tube and inflating the bubble until it touches the sides of its mould. The budding craftsman thus loses the practice of blowing these simpler forms, and as he is now forbidden to work at the furnaces until he is over fourteen, he often fails to acquire that lightness and dexterity of hand which are the mark of the first-rate craftsman, and which can be most readily gained in early life. There is, of course, no reason why common vessels should not be produced in this way, and tumblers, decanters, and lamp glasses are so manufactured in large numbers.