Various methods have been tried for working the clay into pots, by using moulds, &c; but the method that has been most generally practised, is to knead the clay, while in the state of a paste, till it is nearly as tough as the put ty used by glaziers. It is then made into rolls, and wrought one layer upon another, and pressed together with the greatest care, so as to make a compact body, quite free of any vacuity, and generally into the shape of an inverted cone, or into a cylindrical form.
The pots used for bottles and window glass are gene rally made about 40 inches diameter at top by 30 at bot tom, and about 40 inches deep, and are termed open pots. Those for flint glass are covered over, and termed rapt pots; they are made of various sizes and shapes. Bottle and crown house pots are made from three to four inches thick ; and flint house pots from two to three inches thick. After the pots are made, it requires a good deal of atten tion to bring them to that state of dryness requisite for their being taken to the annealing furnace. Before pots are set into the furnace, they are heated up with the great est caution, in an arch or vault built for the purpose, to a perfect white heat. This operation requires four or five days, or longer, if they are not very dry previous to their being used ; and when they are completely annealed, as it is termed, they are carried with the utmost expedition from the annealing arch, and set into the working furnace. The setting of pots is deemed the severest labour abottt a glass house, from the great heat attending it; and is described as follows by Mr Blancourt. Ile observes, that the rough est work in this art is the changing the pots when they are worn out or cracked. In this case, the great working hole must be uncovered, the faulty pot must be taken out with iron hooks and forks, and a new one must be speedily put in its place through the flames, by the hands only. For this work, the man guards himself with a garment made of skins, in the shape of a pantaloon, that covers him all but his eyes, and is made as wet as possible. The eyes should be defended with a very thick plate of glass.
The bottle-house furnace, represented in Plate CCLXXV. Fig. 1, (see description of Plates,) is generally an oblong square chamber, arched over with the same material.
Some give it a very flat crown, as it is called ; and others raise it high and of a barrel shape. This furnace is erect ed in the centre of the building, on the top of the cave or vault, and is divided into three spaces in the inside by the grate, and on each side of that is the site for the pots or crucibles, which is a bank of the same material as the fur nace, generally about a foot high, and three broad.
In this furnace there is a hole about a foot diameter for each pot, called working holes, at which the workmen put in the materials, and take out the liquid glass. At each angle of the furnace there is also a hole about the same size, which communicates with the calcining fur nace ; and the flame that issues from the main furnace, which otherwise would be lost, reverberates on the mate rials in this furnace, and calcines them. There are gene rally eight other furnaces or arches in a bottle-house. Six are used for annealing the bottles after they are made, and two for annealing the pots, previous to setting them into the main furnace.
The materials used for bottle glass are of the coarsest kinds. Government will not allow any but the commonest sea or river sand, mixed with soap boiler's waste, which is done in the proportion of three of soap waste to one of sand, according to the quality of the soap waste : this soap waste is generally calcined in what is termed the coarse arch. Two of the calcining arches are kept for that pur pose at a led heat for 24 to 30 houri, the time required to melt and work the metal or glass that is made by filling the pots at one time; this is called a journey ; after that the sonp-waste, now termed ashes, are taken out and bi uis ed, and mixed with the sand in the proportions already mentioned. The mixture is then put into the fine arch, where it is again calcined during the working journey, which is generally 10 or 12 hours more. When the work ing journey is over, the pots are again filled with the red hot materials out of the fine calcining arch. In about six boors it is melted; the pots are again filled up, and this se cond filling requires about four hours to melt.