An examination of a small experimental crucible, in which borax bad been fused, discovered a succession of rings eaten into the body of the crucible. It is known that if red-lead be heated by itself in a crucible for a long time, it will gradually eat its way through the crucible, extracting from it silica, and forming silicate of lead.
In order to strengthen crucibles against internal corrosion, a curve should be formed at the junction of the bottoms and wall, and sharp angles be carefully avoided. It is also advisable, before filling in the raw materials, to throw into the crucible some broken glass, which melts, and protects the bottom from corrosion. The only legitimate death of a crucible is by gradual corrosion. Fig. 729 is a view of a fragment of a bottom of a crucible, through which the glass has gradually eaten a way. The backs and bottoms of crucibles which have done good service are completely honeycombed.
After the burnt and unburnt clays have been carefully mixed through fine sieves, the mixture is placed in a large bin, and water is gradually added. After the mixture has been thoroughly wetted, it is subjected to the process of " treading." Many machines have been tried for combining the moistened clay, and increasing its tenacity ; but none have at present been found capable of com peting with the action of the naked feet of men. The " treader ' holds on to a rope attached to the ceiling of the clay-room, nod, first with one bare foot, and then with the other, presses upon the clay, which is supplied to him little by little, until the whole mass has passed under his feet nt least three times. In addition to elasticity, softness, and warmth, the human foot possesses suffloient sensitiveness to detect the presence of any hard foreign matters in the body of the The crucible-maker receives the clay, as he wants it, from the treader, and fashions it into small tapering rolls with the palms of his hands. A stone slab, larger in diameter than the intended crucible, is first covered with a thin layer of sand. Upon this sand, the potter places the founda tions of the crucible.
The rolls of clay are carefully pressed together, until one complete layer is formed ; the surface of the first layer is scratched across and across by the potter's fingers, so as to make a bond for the second layer, and the process is repeated layer by layer until the bottom is suffi ciently thick. The bottom is consoli
dated by prolonged beating with a wooden mallet, and is finally smoothed with a wooden straight-edge. Ridges are scratched round the edge of the bottom, to form a bond for the walls ; and the walls are slowly built up, roll by roll and layer by layer. The chief object in the whole process is to drive out the air from the substance of the clay. The potter presses the rolls principally with his fingers and the cushions of his hands, and works them towards the middle of the wall of the crucible, both from the inside and the outside. If the outside rolls are worked round the circumference from left to right, the inside rolls are worked from right to left. The building of one crucible is not carried on continuously : several crucibles are kept in band at the same time, and the potter passes from one to the other, leaving one to set whilst at work upon another. However, before leaving one, be takes the precaution to cover the edge with moist cloths. The dome of a flint-glass crucible is made complete ; the opening or mouth is cut out, and the lip and hood are put on after wards. Flint-glass crucibles, although smaller than open crucibles, take longer to make, as the upright wall must be allowed partially to set, before the building of the dome can be commenced. A good potter, with three or four assistants, can finish three or four sheet-glass crucibles in one week, whereas it would not be safe to finish flint-glass crucibles in less than two weeks. Crucibles are allowed to dry in the same room in which they have been built, the temperature being maintained, by means of gas or hot-water apparatus, continuously at about 15° (60° F.). All draughts should be carefully excluded ; with this object, the floor, windows, and doors should be double.
When the clay is firmly set, the crucibles are cut from their stone slabs by means of a wire, and are placed upon low wooden trucks, so that they can be readily moved without jarring. Crucibles ought to stand for twelve months or more, before they are fit for the furnace. A crucible, before being placed in the furnace, requires to be gradually raised to the same temperature with the furnace. For this purpose, it is placed in an oven, constructed for the purpose, and is lodged upon three fire-clay blocks, in such a way that a low iron carriage can be thrust under it for its removal.