This article has no pretension to being a general treatise on the manufacture of glass, which would here be out of place, and which is already given in a very complete manner in Spons' 'Encyclopaedia'; it aims at conveying a number of useful hints to the many who use glass, as much as to the few who make it.

Breaking

Easy method of breaking glass to any required form. Make a small notch, by means of a file, on the edge of a piece of glass, then make the end of a tobacco pipe, or a rod of iron of about the same size, red - hot in the fire; apply the hot iron to the notch, and draw it slowly along the surface of the glass in any direction you please; a crack will be made in the glass, and will follow the direction of the iron. Round glass bottles and flasks may be cut in the middle by wrapping round them a worsted thread dipped in spirits of turpentine, and setting it on fire when fastened on the glass.

Coating On Metals

The following method has been suggested for coating metallic surfaces with glass: - Take about 125 parts (by weight) of ordinary flint - glass fragments, 20 of soda carbonate, and 12 of boracic acid, and melt. Pour the fused mass out on some cold surface, as of stone or metal, and pulverize when cooled off. Make a mixture of this powder with soda silicate (water - glass) of 50° B. With this coat the metal to be glazed, and heat in a muffle or other furnace until it has fused. This coating is said to adhere very firmly to steel or iron.

Coloured

(1) It is supposed that any one of certain metals, if its condition of oxidation or its proportion be varied, will, in combination with glass, produce the several effects of colour into which white light can be decomposed. Thus, copper, when suitably treated, will produce the effects of blue, green, and red. Metals enter into combination with glass in various ways. The effect of avanturine - glass is due to the suspension in the body of the glass of minute particles of metallic copper. When gold oxide is used as a colouring agent, it often happens that some oxide is reduced to the metallic state, and the result is a glass which, when viewed by reflected light, appears to be of a dull opaque red colour, but by transmitted light, yields a beautiful opaline blue. Opacity is probably due to an insoluble excess of metallic oxide held in suspension in the glass. White opacity is obtained by the use of arsenic trioxide, tin dioxide, lime phosphate, powdered talc, or cryolite. The effect of blackness is obtained by the oxides of iridium, manganese, cobalt, copper, or iron in excess.

Gold to be used in colouring glass is first dissolved in aqua - regia; the solution, together with oxides of antimony and tin, is added to the ordinary ingredients of flint - glass. The ruby colour is in a great measure due to the reducing action exercised upon the gold salt by the stannous oxide. Ruby-glass i. usually gathered from the crucible in the form of lumps, weighing 1/2-1 lb. As it is gathered from the crucible, it is perfectly colourless, and only acquires its colour after it has been chilled and reheated in the annealing - kiln. The ruby lumps, after having been annealed, are reheated as they are required, and used for casing the flint - glass. Articles are never made of solid ruby - glass, partly on account of its cost, but chiefly because the colour is so powerful that an almost invisible film imparts a rich colour to the article upon which it is spread.

The red colour of copper ruby - glass is due to cuprous oxide, and all substances liable to part with oxygen, and to convert the cuprous into cupric oxide, must be avoided in its preparation. In addition to avoiding oxidizing agents, such as red - lead and manganese oxide, it is necessary to add reducing agents, to counteract such effects of oxidation as are unavoidable. Stannous oxide and iron scales or filings are for this purpose mixed with the raw materials. The ruby colour produced is intense, and can only be used as a casing for colourless glass. The ruby - glass, when gathered from the crucible, is of a pale greenish - blue colour, and, like the gold ruby, requires to be partially cooled and again heated before the red colour appears. If reheating is carried too far, the red is replaced by a dull brown tint. If copper and iron scales be added in great excess, an opaque red mass is obtained.

Cupric and cuprous oxides, when used without reducing agents, produce peacock - blue or green; the result apparently depends on the quantity rather than on the state of oxidation of the copper. A very minute proportion of cupric oxide will give a distinctly blue tint. Ferric oxide (Fe2O3), in the presence of manganese dioxide, which parts with its oxygen, and thereby tends to maintain the oxidation of the iron, produces a rich yellow. Ferrous oxide (FeO) gives a dull green; it is obtained either by the oxidation of metallic iron in the crucible, or by the reduction of ferric oxide. Manganese dioxide by itself and in large quantity gives violet. If the mixture be heated too long, the oxygen is driven off, and the glass is rendered colourless. A red is obtained by a mixture of manganese dioxide and ferric oxide. A minute trace of cobalt oxide imparts a deep purple - blue. Nickel oxide produces a deep red - brown. The oxides of chromium are very slightly soluble in glass; a minute quantity gives an emerald - green or yellow colour; any excess remains in the form of glistening crystals in the body of the glass, and tends to its disintegration. Antimony trioxide imparts a faint yellow tint; excess tends to produce opacity. Oxide of cadmium gives a pale yellow.

Uranic sesquioxide produces a bright yellow, but its peculiar property of fluorescence, already referred to, gives to the glass, when viewed by transmitted light, a bluish - green effect. Silver oxide, in common with cuprous oxide, possesses the power of staining glass, when applied as a pigment to its surface, and heated. This is a more convenient way of obtaining the yellow colour which silver oxide gives to glass, as, when mixed with the raw materials of glass, and placed in a crucible, it is only with the greatest difficulty that the oxide can be prevented from becoming reduced. If reduced, metallic silver sinks to the bottom of the crucible, and the glass remains colourless.