The pots within which the raw materials are melted are set within a strongly heated chamber called the glass furnace. The old circular type of English furnace usually contains either six, ten, or twelve pots, and will be described first. The pots stand in a circle upon a form of hob called the “siege,” which constitutes the floor of the furnace. In the centre of this chamber and below the level of the siege is the “eye” of the furnace through which the flames come from the furnace fire below. The burning fuel is contained in a circular or cylindrical-shaped fire-box, about 4 ft. deep and 5 ft. in diameter, and is supported by a number of strong iron bars across the bottom of the fire-box. Passing under the fire-box, and across the whole width of the glass furnace, there is an underground tunnel called the “cave,” each end of which is exposed to the outside air, which is drawn in through the caves by the draught of the chimney cone above the fires. These caves are of sufficient height and width to allow the fireman, or “tizeur,” as he is called, to attend to the stirring of the furnace fires from time to time. Using a long hooked bar of iron, he rakes out the dead ashes and clinkers, as they are formed, and stirs the fire through the bars by prodding the fuel with a long poker. The coal is fed upon the furnace fire through a narrow mouth situated in the glass house leading into a chute which runs under the siege, from the glass house floor level towards the fire-box of the furnace. The fuel is pushed down this chute and falls into the fire-box and is fed at intervals of the half to three-quarters of an hour, according to the heat desired and the draught allowed.

Above the siege and over the pots is a covering called the crown of the furnace, which is supported by fire-brick pillars. This is built of the most refractory material possible to be obtained, as the hottest flames from the furnace fires beat against this crown and are reverberated downwards upon the surrounding pots. The flames, continuing their course, pass between the pots into small openings or flues leading from the siege floor and passing upwards through the pillars which are situated between each pair of pots, they then escape from little chimneys leading into the outer dome or conical-shaped structure so familiar to outsiders. This outer truncated cone-shaped structure constitutes the main chimney of the furnace. The furnace chamber containing the pots is constructed entirely within this cone. The blocks are carefully shaped, neatly fitted, and cemented together with a mortar made of fine, plastic, raw ground mixed to thin paste with water. The presence of any molten glass which escapes from a cracked pot, and the fluxing action of the fuel ashes, cause severe corrosion of the blocks forming the siege and fire-box, and these necessarily have to be made of extra thickness in order to extend the life of the furnace. When the furnace crown or siege becomes badly corroded away, the furnace has to be put out for repair; so generally an auxiliary furnace is kept at hand, in order that it may be started and the workmen transferred from one furnace to the other whilst the repairs are being done.

The action of the glass upon the siege of the furnace is very active, and any leakage quickly destroys the blocks, leaving fissures which gradually increase in size until the blocks are eaten right through. Consequently, every care is taken to preserve the pots from losing metal. If by chance any pot develops a crack through which the metal leaks into the furnace, the glass working is ceased at that particular pot, and every endeavour is made to ladle out what remains of the metal, and so prevent any more running on to the siege and causing further mischief. The metal is ladled out of the pot by means of thick, heavy, iron spoons, with which the hot metal is scooped out of the pot and dropped into a large cauldron containing water. This is very exhausting work, but there is worse trouble still if the metal is allowed to continue to run through the crack in the pot and over the siege into the eye of the furnace, for it then fluxes with the ashes of the fuel, causing them to form into a big mass of conglomerate, which, lying in the fire, interferes with the draught and combustion of the fuel within the furnace, and before the furnace can be got to work properly again has to be cut away, piece by piece, through the firebars whilst hot, until it is all removed. At the sign of any glass running down into the fires and through the bars, the tizeur hurries up to give the word that a pot is leaking in the furnace, and when the pot is isolated the work of ladling the hot metal out into water begins in earnest. A pot which has cracked and leaks is useless for any further work of melting glass, and at a convenient time it has to be withdrawn from the furnace and a new pot must be substituted. Glass-melting pots form a very expensive item in the glass manufacturer’s costs; consequently, every care is taken to prevent the pots within the furnace from getting chilled by inadvertently allowing the fires to burn too low or allowing cold air to rush through the bars, through unskilful clinkering and inattention to the furnace fires. Sometimes these furnaces are fitted with a Frisbie Feeder. This is a mechanical firing arrangement fitted underneath the furnace bars, by which the fuel is fed upwards into the furnace box, so that all smoke given off by the fuel baitings has to travel through the hot fuel above, and thereby is more completely consumed, giving better combustion than when the black fuel is thrown on the top of the hot bed of fuel. A mechanically operated piston pushes up small charges of fuel from within a cylindrical-shaped box, which works on a swivel backwards and forwards as the fuel is fed into it.

In the old type of English furnace containing twelve pots, each 38 in. diameter and holding about 15 cwts. of metal, the furnace would be capable of melting 7 to 8 tons of glass a week, taking 40 tons of best fuel. The more up-to-date glass-melting furnaces are constructed upon a much better principle than the coal-fired old English type of furnace just described. These are usually producer gas-fired and give more economy and greater convenience in every way.

In these better types of modern furnaces some form of regeneration or recuperation of the waste heat is usually adopted. These furnaces are much smaller and more compact; being gas-fired, they give much higher temperatures, more complete combustion of the fuel, greater ease in regulation, cleaner conditions, and far greater production than the older types of English furnaces. Considering the reasonable initial cost that the latest types of these modern furnaces can be built for, it appears incredible that so many of the old out-of-date English furnaces remain in use in this country.

As examples of the types of regenerative and recuperative furnaces, a description will be given of the Siemens Siegbert Gas-fired Regenerative Furnace and the Hermansen Recuperative Furnace for glass-melting, which are extensively used on the Continent and are giving remarkably good results.

In the Siemens Siegbert type, the furnace may be a rectangular or an oval-shaped chamber, approximately 18 ft. by 9 ft., the crown of which is about 4 ft. 6 in. high. No outer cone-shaped dome exists, and the pots within the chamber are arranged much closer together and practically touching each other round the furnace. The furnace chamber is heated by a mixture of producer gas and heated air, the gas being generated in an independent gas producer situated outside the glass house and some little distance away from the furnace. At either end of the furnace, beneath the floor of the siege, are two blocks of regenerators. These are deep rectangular chambers containing an open lateral arrangement of fire-brick chequers, through which the air or products of combustion pass on their way to or from the furnace. Port-holes are situated directly above these regenerators which lead the gases through the floor or siege into the furnace chamber. The draught is induced by a tall stack, which draws the gas from the gas producers through a duplicate arrangement of flues to the port-holes at one end of the furnace, where it is mixed with the air which has been drawn and heated in its passage through the regenerator beneath. This gaseous mixture, whilst in combustion, is drawn across the furnace chamber to the other end of the furnace. The flames playing across the tops of the pots on either side pass down through the port-holes and regenerator at the opposite end. The hot gases or products of combustion, in passing through the lateral channels of this regenerator, leave behind their heat by the absorptive or conductive capacity of the fire-brick chequers through which the hot gases have passed on their way to the stack. The direction of the current is reversed at intervals of half an hour or less by using an arrangement of valves situated in the gas and air flues, so that the currents are made to travel on the contrary direction, the air necessary for combustion then being drawn through the hot block of regenerators which was previously heated by the exit gases. On its way through these lateral channels the air becomes intensely heated, and, when it is admixed with the coal gas at the porthole, this pre-heated air accelerates the combustion and calorific intensity of the gaseous mixture. The direction of the current is continually being reversed at the interval of half an hour or less by the manipulation of the valves, so long as the high temperature is desired.

In practice, however, the regenerators are only used whilst the batch materials are being melted during the night, and by morning, when the metal is melted and “plain,” the heat is brought back, or retarded, by using the gas from the gas producers and cool atmospheric air under natural draught, instead of the regenerated hot air. This cooler mixture, naturally not being so active in combustion, maintains just sufficient temperature for working the metal out during the day. Later in the day, when the pots are emptied and refilled with batch, the regenerators are re-connected and the founding proceeds again through the night, and the metal is again got ready for the workmen coming in next morning.

It will be seen that this method of melting and working out the metal does away with night work, the furnace man alone remaining in charge during the night. All firing is done outside the glass furnace room, which is well lighted, clean, and free from coal dust, totally different conditions from those existing in many English glass houses of to-day.

A Siemens Siegbert furnace taking ten open crucible pots, and filled each day, turns out 15 to 18 tons of metal a week. The crucibles are about 30 in. in diameter and have a capacity of 5-1/2 cwts. of metal each. The amount of fuel consumed is about 18 tons a week. This type of furnace costs about £1,600 to £2,000 to build. In the miter’s opinion, a disadvantage of this furnace is that, during the reversing in the direction of the fire gases, the greatest heat is suddenly brought to bear on the cooler pots, resulting in a short life for the pots. The temperature of the incoming air is not so constant as with the recuperative type of furnace; however, with proper control, these defects may be obviated to some extent.

By the kindness of Messrs. Hermansen, the patentees, I am permitted to illustrate their Recuperative Glass-melting Furnace, eight pot type.

The Hermansen furnace, like the Siemens furnace, is producer gas-fired. The gas producer is built within the body of the furnace, (P) below the glass house floor. On either side of this gas producer the recuperators are situated. These are constructed by an arrangement of tubes, designed to give two distinct continuous channels, the one horizontal and the other vertical. The vertical channels are connected with the atmosphere and supply the air necessary for combustion. The horizontal channels (R) are the flues through which the hot waste products of combustion are continually being drawn from the furnace by the stack. It will be evident that, the horizontal channels being intermediate to the vertical tubes, the waste heat is continually being absorbed by the air travelling inwards. In other words, the air is pre-heated by passing through flues which are surrounded by the hot waste gases. Therefore, in this type of furnace there is no necessity for reversing the currents to procure the necessary pre-heated air for combustion, and the regulation of the furnace heat becomes a simple matter of controlling the draught by means of the dampers provided in the main flue. In this type of furnace the glass is melted nightly; open or covered pots may be used, the capacity of which varies between 5 and 12 cwts., according to the class of glassware manufactured. The furnace is designed in four, six, and eight pot types, and several are now working in this country. These Hermansen furnaces are capable of producing 20 tons of metal, with a fuel consumption of 16 tons.

The Hermansen Continuous Recuperative Furnace is the most efficient furnace known to the writer. It is easier to control than the regenerative types. Being compact, it takes up little space and is easy to repair, and its life well surpasses other types. The initial outlay and cost of erection varies from £850 to £1,200. The combustion in this type of furnace is so perfect that it is used with open crucible pots for melting lead crystal glasses. On the Continent this furnace is in general use for all types of glassware, and, from the amount of glass it will melt, its efficiency is greater than the regenerative type.

Tank Furnaces are at present used for the melting of the commoner and cheaper types of glass. They are so constructed as to contain a single rectangular-shaped compartment, or tank, about 18 in. to 2 ft. deep, and from 30 to 100 ft. long. The bed and retaining walls of this tank are constructed of specially selected fireclay blocks; no pots are used. Tank furnaces are simple and melt the glass economically, but the metal produced is not nearly so good a quality as pot metal.

Tank furnaces are chiefly used for making the cheaper glasswares, such as wine, stout, and beer bottles, gum bottles, ink-pots, sauce bottles, and like goods, where a large production is essential. Improvements are continually taking place in the design of this type of furnace, and much finer and clearer metals are being produced. It is quite probable that in the future tanks will be preferred for making cast plate and sheet window glass, as a larger body of metal is held by them when compared with pot furnaces. Like the Siemens and Hermansen furnaces, they are gas-fired, but the port-holes by which the gas and air are introduced and the products of combustion are withdrawn from the melting chamber, are situated on either side, above the level of the metal, whilst the glass blowers work at one end of the furnace. The melting and working of the metal is continuous. The tank is divided by a shallow bridge, which is partially submerged and situated midway between the two ends of the furnace, dividing it into two sections, respectively the melting and working compartments. This bridge keeps back all unmolten material and allows only that portion which is melted to travel forward to the working compartment. The tank is crowned or arched over, and at the working end openings are provided to enable the glass workers to gather the metal from within. Small rings, or syphons, are used, which, floating on the metal, serves further to refine the glass as it is gradually used. The batch mixture is filled through a convenient opening near to the port-holes. Tank furnaces vary in capacity. Some have been constructed to give an output of 300 tons of glass a week. This pace can only be kept up with the aid of automatic bottle-making machinery; in which case hand labor is practically eliminated.

Liquid fuel or oil-fired glass furnaces have not proved a success, being very costly in repairs on account of the local heating effects of the flames issuing from the burners vaporizing the oil.

Electric furnaces for glass-melting have been tried with partial success. These are expensive in maintenance compared with their efficiency in producing glass.