GLASS-MELTING POTS AND THEIR MANUFACTURE
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.
Footnote 3:
These rings, floating on the metal, are used by the glass makers to
keep back the scum of the glass away from the middle portion from
which he gathers.
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.
MIXTURE FOR POT-CLAY
_By
volume._
(=Base=) Fine ground strong 5 parts
Fire-clay
(=Binder=) Fine ground mild Plastic 4 parts
Fire-clay
(=Grog=) Ground burnt Chamotte 2 parts
(=Grog=) Ground selected Potsherds 1/2 part
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.
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