Glass making may seem simple, but great skill is required on the part of the workman. This is espe cially true in the manufacture of bottles by the old method described above. There are bottling machines which are entirely automatic, the siphoning of the molten glass being accomplished by means of a vacuum, the molding by an automatic process, and the blow ing by compressed air. Only one operator is required and he merely removes the bottles as they are made.
A glass factory using oil as a fuel will now be de scribed. The glass furnace as shown in Fig. 57 is called the regenerative type of furnace ; in it, the air for combustion passes up through one flue to the com bustion chamber, through the furnaces, and down through the other flue. Thus the inlet chamber, which is made of checker-work brick, imparts its heat to the incoming air, while the escaping gases are heating the checker-work brick in the other chamber. The air is reversed at intervals of twenty minutes.. Thus the air required for combustion is always well heated. The oil is fed by means of a pumping system installed by the Tate-Jones Company. The oil burners were de signed by the same company, and use compressed air at 40 lb. pressure to atomize the oil, which is supplied at a temperature of 140 deg. F. and a pressure of 40 lb.
Three weeks are required to bring the furnace illustrated up to its proper working temperature, in order to prevent cracking of the walls as a result of uneven expansion. The finely ground raw materials, together with some broken glass are then introduced until the capacity of the furnace is reached. When the "batch" reaches its "fusion" point, 2500 deg. F., the temperature of the furnace is raised to 2700 deg. F.
"Floaters" of fine clay are held in place by the current of melted glass flowing towards the revolv ing tank. The liquid passes under the floaters, which skim off the impurities that have arisen from the "batch" during the melting and refining process. • This leaves the glass in the revolving tank with a clean sur face, free from bubbles. The temperature of the revolv ing tank is much lower than that of the main tank.
The molten glass is taken by the "bottle machine," and made into bottles. The bottles are then passed through the annealing furnace, Fig. 58. They enter
at the door C, and by means of an endless chain gear are carried to D where they pass to a revolving table and are conveyed through B. This is the hottest part of the furnace, usually heated to 1200 deg. F. It will be observed that the bottles come into direct con tact with the flame at this point, and when removed are quite clean, excepting for a cloudy film. It was first thought that this film was caused by poor atom ization of the oil. The chemist made some observa tions and stated that it was not an oil soot. but some unknown composition possibly due to the vaporization of the water in the oil. This film is readily removed by wiping with a damp piece of cloth. At E the bottles are again passed to the endless chain gear and pass out the door F. This operation requires about 36 hours. The bottles are then examined, cleaned, and packed for shipment.
It is estimated that about six tons of bottle glass are melted and refined in this furnace with an oil con sumption of 840 gallons barrels, or 140 gallons of oil per ton of glass. Blown window glass may also be made in this style of furnace.
The common pot furnace is used by some of the smaller manufacturers. It is similar in construction as the crucible steel furnace shown in another part of this book. A small special furnace called the "glory hole" is also used in many factories for the purpose described in making window glass. There are also small square furnaces about 18 in. x 18 in. x 24 in., inside dimensions. The flame passes down through the top to a combustion chamber, and the heat of the flame passes over into the furnace. Such a furnace is kept at a low white heat. In some glass factories, small furnaces are used to dry the sand and to prepare the limestone. Air is almost universally used for ato mizing the oil.
Many large glass factories are now using fuel oil because of its cleanliness, in spite of the fact that it costs more than other fuels. The flame comes in direct contact with the glass without producing any discolorization or deterioration and the temperature may be maintained absolutely uniform, thus saving many articles formerly ruined by the fluctuations in temperature caused by unsteady firing.