The Manufacture of Glass

The silica is introduced in the form of white sand, large quantities of which are obtained from Paris and Belgium. Flints were formerly employed to avoid the use of sand containing iron, hence the name "flint glass," but this has been discontinued. Lime may be introduced as slaked lime, chalk, or limestone, and soda chiefly as sodium sulphate, although carbonate is sometimes used. " Cullet," or waste glass, is also added to hasten the reaction by acting as a flux. The finely ground materials are mixed with a wooden shovel or a mixing machine, and introduced little by little into a previously heated pot. The actual melting occupies ten to twelve hours, depending on the quantity of glass and the type of furnace. There is a large evolution of gas, chiefly carbon dioxide and water vapour, which helps in the mixing, and a froth or scum is formed on the surface and is removed. This scum, known as " sandiver " or " glass gall," consists chiefly of sodium and calcium sulphates with a certain amount of glass. The glass is then refined or planed. Sometimes potatoes or apples are thrown in, or it is stirred with a stick of green wood at the end of which a fire-clay cylinder is attached. At the beginning of this process the glass should be intensely heated so as to be very fluid and mobile. The temperature sinks during the stirring, so that the glass is finally quite viscous, when it is ready for drawing and blowing.

Several pots are heated in one furnace, or, for the coarser kinds of glass, a single large tank may be used. Lead glass and optical and other special glasses must be melted in covered pots, to prevent access of reducing gases in the case of lead glass, or the dropping in of brick-work or other impurities from the roof of the furnace. The glass cannot then be stirred, but it must be removed, broken up, and the cleanest fragments returned to the pot for remelting in order to produce a homogeneous mass.

Great care must be exercised in the construction of both pots and furnace, as they must be capable of withstanding for some months at a time the action of a corrosive mixture at a temperature which may reach 1400° C. or even more. A highly refractory clay, for example Stourbridge clay, consisting mainly of aluminium silicate, is used. The pots must be carefully annealed, and, when once heated up, must never be allowed to cool down again.

Gas-fired furnaces, being more economical than coal-fired ones, are more frequently used. There are also several types of electric furnace, for example the Sauvageon, Voelker, and Becker furnaces, in which the heat is developed by the resistance of the glass itself, or, on the other hand, those in which the furnace chamber is heated by the passage of an electric current through resistant material surrounding it.

There is scope for much greater economy in the working of glass furnaces. Travers has calculated that, in three different glass furnaces, the heat energy actually utilised in melting the glass was 14.5, 12, and 9 per cent, respectively of the total consumed. The rest was lost either outside the furnace or through the walls.

For fuller details of the processes of manufacture and the construction of furnaces and of machinery for the drawing and blowing of glass the reader is directed to the literature mentioned in the subjoined references.