Glass house pots are large hollow vessels made of refractory in which the glass manufacturer melts the materials of which his glass is composed, and which retain the molten metal whilst in a state of fusion for the workmen’s use. In the case of the lead crystal glass, the materials, whilst being melted, require protection from the flames, smoke, and fuel ash present in the old English types of furnace chambers, which would otherwise reduce the lead present to a metallic state and spoil the glass; therefore, such glasses are melted in covered or hooded pots and thus protected from the direct action of the flames. Consideration has to be given to the extra amount of heat required from the furnace to find its way through the hood of the pot. For crown plate and chemical glassware, the metal is usually melted in open or uncovered pots. In this case the fusion is facilitated by allowing the heat of the furnace to come into direct contact with the materials within the pots.

Pots which are covered or hooded have an opening cut out in the front, in a position just above the level of the molten metal. Through this opening the workman gathers the hot metal. In the case of open pots, the crucible is set in a similar position within the furnace, but the working hole or mouth is built to form part of the construction of the furnace in front of the crucible.

Good pots are of the greatest importance to the glass manufacturer, and upon their life much of the success of glassmaking depends. They have necessarily to resist the corrosive action of the raw materials and molten glass within, and, at the same time, withstand the very intense heat of the furnace without giving way under the great weight of the glass within them. Should a pot of metal give way whilst in the furnace, the loss is considerable and very serious, for not only has the metal been wasted, but much of it has flooded the floor of the furnace and siege, and, finding its way into the fire-box, attacks the furnace walls, fusing and melting with the fuel ash, checking the draught, and causing endless trouble.

Glass house pots are very difficult and expensive to manufacture, and upon an average each pot has cost £10 by the time it is set within the furnace; therefore every care is taken to extend their life by procuring the best possible materials for their manufacture.

Only the best selected pot-clays available are used, and every endeavour is made to keep them clean and free from foreign contamination. Only the best portions of the seam are taken for this purpose, and a considerable amount of diligence and stringent precaution is taken to procure the best qualities. As the clay is raised from the mine, clay pickers look over the lumps and select out the best portions. A foreman of long experience is stationed at the head of the mine, and it is his duty to supervise the clay pickers and see that every care is exercised to guard against any unfortunate results which would naturally attend any indiscriminate or indifferent selection. The best portions having been selected and placed aside, the lumps are scraped on the surface to remove any dirt, and broken into pieces about the size of an egg, which are again carefully examined on all sides and cleaned from foreign matter such as pyrites or bluish parts. If this is carefully done, and the clays analysed and tested from time to time, a good pot-clay is obtained.

The clay for burning is treated similarly and dried. It is then burnt to a very high temperature and taken to the mill to be ground to the necessary fineness of grain. All pot-clays are well seasoned and weathered before use. They are first ground to a very fine flour and then mixed with a ground-burnt clay, or “chamotte.” The proportion of raw clay to burnt varies with most manufacturers but depends very much upon the plasticity or binding property of the raw pot-clay used. The burnt clay is preferable if ground to a size about 1 to 1-1/2 mm., being sieved to take out any coarser particles. Some clays are more plastic than others, so the proportions in the pot-clay mixtures may vary from six parts of burnt clay to five of raw, down to one part of burnt clay to three of raw clay. The proportions are reckoned by volume, not by weight. The mixture is sieved into a trough and mixed with water to form a stiff paste, and removed into a large tank, where it is allowed to soak for some time. It is then well-tempered by treading with the bare feet until the whole mass becomes plastic and tough. The clay mass is turned and trodden several times, in order thoroughly to consolidate the clay particles. Many efforts have been made to do this work mechanically, but without success. The fact remains, and experience has proved that, in the process of treading, the clay is more consolidated than by any mechanical method of preparation. The tempered and toughened clay is then allowed to sour and mature for a few weeks before use. It is then ready for the pot maker to begin the work of building the pots.

The room in which the pots are to be made is kept evenly warm using a series of hot water circulating pipes arranged around the outer walls. Usually a temperature of between 60 to 70° Fahr. is maintained.

Double doors are provided at the entrance, with a porch, so as to prevent sudden inrushes of cold air and prevent draughts in the pot-making room. All unauthorised persons are prohibited entrance, and only those who work therein are allowed free access. They are made responsible for keeping the place clean, as well as looking after the clay and taking care of the pots whilst they are being made.

The usual shape of a pot is of round section, 38 in. in diameter and 42 in. high, but many other shapes and sizes are used, according to the class of goods being manufactured. Thus, for colours, a very much smaller pot, less than one-third this size, is used, three of them, taking the position of one large pot, being set within one arch. For sheet and optical glass, a covered pot with a very large mouth or working opening is used.

In some instances, as in the Hermansen furnace, the pots are oval or egg-shaped. These are used on account of their larger capacity in relation to the space occupied in the furnace. Other pots have an interior division, which has a syphonic refining action upon the glass; such pots permit of continuous melting and working, instead of the intermittent process adopted when the regular or common shape is used. For plate glass, open crucible or bowl-shaped pots are used.

In regard to the manner in which the pots are made, and their subsequent treatment in annealing, the utmost care and control is necessary. In making the pots, the pot maker begins by making the pot bottom first, working the plastic clay paste into rolls about the size of a large sausage. He takes these rolls and applies them one after another in a circular form upon a round level board, the size of the bottom of the pot. This board is supported on a low table. As he applies each roll, he presses them together to exclude all air spaces between them, and continuously work the rolls on the top of each other in circles, until he gets a circular flat slab of clay in thickness about 4 in. and the width of the pot bottom. He then has the necessary thickness and size of the pot bottom formed as a clay slab, which is smoothed and leveled over the face with a knife or straight piece of wood. The slab of clay is then reversed upon another board, covered with a strong hurden cloth and a layer of ground-burnt clay, which prevents the clay from sticking to the board, and facilitates drying of the pots.

The first board is then removed, and the pot maker begins to build the sides or walls of the pot upon the circular clay slab by working the clay in rolls around the circumference of the slab to a thickness of 3 in., which gives the thickness of the pot walls. As he works and presses on each roll with his right hand, he supports the inside of the curve with his left hand, and presses roll after roll around the circumference of the slab of clay, increasing the height of the walls until he attains a height of about 6 in. The height of this wall is increased by about 6 in. every other day or so; these time intervals allow each section built to stiffen a little before beginning upon the next section.

The workman passes from one pot bottom to another, building up these sections until he builds each to a height of about 30 in., when he places within each pot a clay ring about 18 in. in diameter, which he has previously made.^[3] After placing these rings within the pots, the pot maker begins to form the hood or dome of the pot by working on the clay rolls, and at the same time drawing the sides inwards towards the middle, lessening the thickness of the walls and gradually diminishing the open space until it is covered and sealed in. Whilst the clay is still soft, the mouth or working opening is worked on and cut out of the dome, and the whole finished and smoothed by means of wooden tools.

The pots are now completed and are left to dry gradually at a moderate heat, which is increased a little at the end of a few months in order to thoroughly dry them. They are then removed from the boards and are ready for the furnace.

Crucible pots are made in a similar way, except that at the height of about 27 to 30 in. the pot maker finishes off the top edge of the walls and leaves it in that form to be dried.

Many efforts have been made to manufacture pots by other methods. One which has been tried with a fair amount of success is to cast the whole pot or portions thereof by using a plaster case mould and pouring in liquid clay slip. Another method which has been tried is to press the form by means of a hydraulic press and mould. Other mechanical contrivances have been used, but few of them have given such satisfactory results as the hand-made pots.

The fusion point of the mixture should not be less than Cone 32, or 1710° Centigrade.

Strong fire-clays are those coarser and harder grained, and are usually more silicious and less plastic than the mild fire-clays. Mild fire-clays are very fine-grained, plastic, and easily weathered clays. They act as the binder portion in fixing the burnt grog used in pot-clays.

The raw clays should be ground very fine and separately from the burnt clays. The ground burnt should be crushed from hard and well-burnt fire-clays, and should pass a sieve of ten meshes to the linear inch.

The mineralogical composition of the fire-clays for making pots is important. The presence of pyrites renders fire-clays unsuitable as pot-clays. Some indication as to the subsequent behavior of a can be obtained by submitting it to a petrographic examination, and the usual pyrochemical and physical tests carried out in testing refractory materials. In this country, Stourbridge pot-clays are chiefly used for pot-making, and so conservative are the majority of glass manufacturers that they will not use other clays, although, in the writer’s opinion, many better clays exist in Great Britain, and have now been introduced and used successfully by some firms for pot-making.

Ground potsherds are selected pieces of old broken pots, cleaned from any adhering glass. These selected pieces are crushed and ground in a similar way to the burnt clay, and sieved to the same degree of fineness before use.

Plumbago glass house pots are sometimes used. These are made from mixtures of graphite, or plumbago, and raw . They are very refractory and withstand the attack of very basic glasses, where such have to be manufactured.

Pot rings are made by taking a long roll of clay about 3 in. in thickness and shaping it around a circular frame. The two ends are joined and finished smoothly, the frame took away, and the ring dried. A ring is placed in each pot.

Stoppers are the lids used to close the mouth of covered pots whilst the metal is being melted. These are made in plaster case molds by pressing a bat of clay into the desired shape and releasing the outer case by turning the whole upside down upon a board and lifting off the mold. An indentation is made in the middle, forming a small hole. An iron rod can there be inserted, by which the stopper can be lifted away from the pot mouth whilst hot. Stoppers are burnt before use and are made in various sizes to fit the mouths of different pots.

It is always advisable for the glass manufacturer to make his pots and prepare his clay, as he then knows exactly what he is using, and he is not dependent upon outside firms for his pots as he has them ready at hand when needed. The conveyance of pots from one district to another by rail or road is always accompanied by considerable risk, as the vibrations are given they in such journeys often cause mischief. As they are very heavy and fragile, their loading and unloading into the wagons is often attended with a mishap. As often as not, latent strains are caused, which only develop when the pot is put in the furnace.

Annealing and Setting the Pots in the Furnace. The pots, when made and dried, being of raw clay have to be carefully annealed before they can be introduced into the hot furnace. In doing this, the pot is removed from the drying rooms and placed within a small auxiliary furnace called a pot arch, which is constructed purposely to anneal them and get them hot before placing them in the glassmaking furnace. The pot is moved by picking it up on a long three-pronged iron trolley, made purposely to lift and move them about. The pot is set within the pot arch, resting upon two or three rows of fire-bricks, which allows the trolley to be removed and brought away, leaving the pot in a raised position in the pot arch. The doors of the pot arch are then closed and sealed with a stiff clay paste or mortar, and slow fires started which gradually heat the pot, until at the end of a week it is got to a white heat, and the pot is ready to be removed and set within the furnace for melting the glass.

At a convenient time, arrangements are made for setting the pot. All other work about the glass house has to cease, as all hands are required to help in the strenuous and arduous work. The old pot in the furnace, which has done work for several months, has to be withdrawn from the furnace and the new pot from the pot arch has to take its place. We see gangs of men here and there. Some are pulling down the wall of bricks from the front of the old pot, making an opening in readiness to remove it. Another gang of men advance with long, heavy, strong iron crowbars, sharpened at the points, with which by heavy blows and levering they endeavour to loosen the old pot from the floor of the siege, to which it has become firmly cemented by the heat and any leakage of glass which may have taken place. Eventually, by their combined exertions, they succeed in loosening the pot, and then, levering it up, they place the low iron pot trolley under it and drag it out of the furnace, whence it is taken away and thrown aside.

The old pot having been removed from the furnace, the glowing heat radiates more intensely than ever into the faces of the men at work, who endure it in relays whilst they work clearing away the old bricks and preparing the siege for the new setting. When this is done, a gang of men open the pot arch doors, and, placing the iron trolley under the new pot, convey it to the opening in the glass furnace from which the old pot has been removed. Facing the terrific heat, they struggle to push the new pot into its place in the furnace, with the aid of crowbars, and working in relays, in turn face the heat till at last it is got into position. Naturally, everything has to be done in a hurry, so that the new pot may not be chilled before it is got into the furnace by being exposed too long to the outside air. The whole work proves very exhausting to the men, as there is little protection from the heat. After the pot is set in its place, the trolley is brought away and the wall of bricks rebuilt up in front of the pot to protect it, clay being daubed over the exterior of the brick wall to prevent any inrushes of air, which would cause the pot to crack by finding a way through the joints in the brickwork.

The furnace, during these operations, is driven and worked to its full capacity, so as to allow for the very considerable loss of heat which takes place whilst the opening is being made and the pots removed.

The above is a description of the usual method of pot setting. In more modern and up-to-date works a travelling chain screen is used. This screen is like a curtain of loose chains, which is adjusted to hang in front of the open arch of the furnace and protects the workmen from the fierce heat. At the same time it permits the workmen to see and carry out the work of pot setting with greater ease and convenience. In using this screen arrangement whilst setting, the pot is pushed through the chain screen, which closes upon it after it has passed through. The workmen are thus enabled to get closer to their work by manipulating the crowbars through the screen as the heat is not radiated full upon them.

The newly set pot is allowed to stand empty in the furnace for a day or two to regain heat before it is filled with batch. It is first glazed on the inside by a workman taking a gathering of glass from another pot and plastering or covering the inside all round with the hot metal, which flows down and glazes the surface of the pot, giving it a certain amount of protection from the attack of the raw batch materials which are to be introduced later.

The founder, or glass melter, now takes charge of the pot, and he brings up the mixture of batch and cullet and shovels it into the empty pot until it is filled well above the mouth or level of the opening. The heat of the furnace melts the batch, and after several hours it becomes liquid and shrinks in volume so that probably only two-thirds of the height or capacity of the pot is occupied. The pot is then again filled with more batch materials until it is full of molten metal up to the level of the mouth of the pot.

The furnace is kept going at its full heat until the founder, drawing a small portion of the glass on the end of an iron rod, examines it and finds that it is melted clear and free from seeds or bubbles of gas. When clear, the metal is “plain,” and at this stage is in a very liquid, fluid, and watery state, too liquid to be easily gathered. It is, therefore, allowed to cool off by removing the stopper down and leaving the mouth of the pot open, until the glass becomes more viscid, or of a stiffer nature. The glass is then skimmed by dragging off any scum present on the surface, which is due to undecomposed salts that may have risen during the melting.

The metal is now ready for the glass blowers to begin work. Upon looking into the pot, the ring will now be noticed floating on the surface of the glass. This ring keeps back from its interior any further scum that may arise whilst work is in progress. The glass blower always gathers from within this ring, where the metal is cleanest; and from time to time the metal within the ring is skimmed in order to keep that portion in the best condition. When the greater part of the metal within the pot has been gathered or worked out, the heat of the furnace is raised again and fresh batch materials filled and the process repeated.

The time taken to melt the glass depends upon the heat of the furnace. A gas-fired furnace will melt the batches in eight hours, but the old type of English furnace takes much longer, usually two to three days.