The recuperative furnace utilizes the same principle in a different way. The fuel gas and air do not pass through the same chamber through which the waste gases have passed. but through adjoining flues which are heated by being surrounded by those gases on their way to the chimney. Each method has its warm adherents. It is to be said, however. that the recuperative furnace occupies less space, and does away with the necessity of reversing valves. It is claimed also that its heat is more constant, as the regenerating chambers are continually cooling off as the gas takes up the heat from the walls.
In both systems the hot gas and hot air are brought separately to the point where they enter the furnace, and are mixed at the very entrance. A rapid mixing causes a very hot but short and localized flame. A slow mixing produces a longer and farther reaching flame, and is better adapted to a larger furnace. In the regenerating furnace the direction of the flame is changed from one side to the other about every half hour, and this gives a more even distribution of heat. In the recuperating furnace the flame always passes in the same direction, and it is customary to cause it to return upon itself in the form of the letter U.
and to make its exit from the turnace at a point close to its entrance.
In the tank furnace the openings for the entrance of the fuel-gas and air, and for the exit of the waste gases after combustion, are generally placed along the side walls of the furnace just above the level of the molten glass, or at the base of the dome of the roof. For tank furnaces the regenerative type is preferred, as the flame and exhaust are alternated at the several ports, and the heat is thus mdre evenly applied to the melting mass. The proportions of the ingredients used and the method of fusing and handling each special kind of glass are described in detail in the article entitled GLASS, VARIETIES OF.
FusiOn.— The plant being prepared, the ac tual making of the glass is the successful inter melting of the ingredients. The more refrac tory of these are ground very fine; the sand is used in a granular form, freed from dust and coarse grams; the soda and salt-cake, which melt readily at a low temperature, are simply broken into small fragments. The weighing out of the several components is accurately done, according to the scheme of the individual glass-maker, each having his own particular variations from the standard formulas. Where
hand mixing is employed, the mass is dumped into a bin and turned over again and again by shovelers, being finally passed through a sieve. The cullet is then added with more or less uniformity. If the glass is to be melted in a tank, it is merely shoveled in at the melt ing end, which supposedly contains melted glass — as the operation of the tank furnace is con tinuous. The mixture melts more quickly and with less expenditure of fuel in these conditions than in pots, where there is no melted glass to start with. In the crucible furnace practically all the previous contents of the pots have been emptied out, and the new material goes in dry. As there is much foaming dining the melting, the pots cannot be filled at once, but as the melting proceeds more materials are added from time to time, from four to seven instalments, depending upon the size of the pot and the chemical reactions of the mixture involved. The last instalment is usually of cullet only. The frothing of the glass mixture during melt ing has the effect of filling the molten mass with innumerable bubbles, some of which are air, others carbonic acid or oxygen released in the chemical actions which take place. A high degree of fluidity is required to enable these bubbles to make their way to the sur face of the glass. Some of them are got rid of by stirring with an iron rod, but is liable to add iron oxide to the mass, and is highly objectionable in some cases.
To aid in this process certain sub stances are added to the glass mixture to form large bubbles which shall gather up the tiny bubbles and carry them along to the top. Ar senic and sodium nitrate are often used for this purpose. The molten mass being clear, it is finally skimmed to remove the surface which contains many floating impurities. Be fore the glass can be worked it must be con siderably reduced in temperature — that is, from the fluid stage to the plastic stage. In crucible furnaces this result can only be accomplished by cooling down the entire furnace. In the tank process the fluid glass usually is made to flow from the hotter melting end under a tition wall to the working end of the tank, which is kept at the lower, working temperature.