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 so as to exclude all air spaces between them, and continuously works 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 levelled 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 round 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 round 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 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.1 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.
1 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 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.
Fine ground strong Fire-clay
Fine ground mild Plastic Fire-clay
Ground burnt Chammotte
Ground selected Potsherds
The fusion point of the mixture should not be less than Cone 32, or 1710° Centigrade.