Fig. 730 is a horizontal section of a flint-glass crucible oven, taken through the fire-place, which stands about 18 in. from the ground : F are the openings to flues ; P, the fire-place ; air-tight doors. Fig. 731 represents the iron crucible-carriage. When the crucible is first placed in the oven, the dampers of all the flues should be closed, and the fire be allowed only to smoulder. After 3-4 days, the flues may be gradually opened, and the fire be increased until on the 7th-8th day ; when the heat of the oven is equal to the heat of the furnace, the crucible may be moved into the fur nace. Probably more crucibles are broken by carelessness in the management of the flues of the oven than in any other way.
The removal of an injured crucible from the bed of a furnace, the preparation of the bed, and the setting of the new crucible in the place of the old, are operations which require very great presence of mind, and physical strength and endurance. The temporary brick-work, which acts as a screen in front of the crucible, bee to be removed piece by piece. Finally, the whole arch, tinder which the injured crucible stands, is laid open, and the workmen are exposed to the full heat of the furuace. In order to soften any glass which may have found its way on to the bed of the furnace, and which might tend to seal the broken crucible to the bed, the furnace is driven to its utmost heat. The tools, with which the workmen accomplish their task, are of the simplest description, being merely diamond-pointed or chisel-edged steel crowbars, of various lengths and sizes. The arch having been laid open, the orucible loosened and raised from the furnace-bed, and the parts of the bed on each side of the crucible levelled and smoothed, the crucible is drawn forwards by means of iron bars and rakes ; it is thrust upon the crucible-carriage, removed, and broken up. The levelling of the bed is effected either by a long iron pointed bar, resting upon a ful crum, and worked by several men, or by separate blows dealt by relays of men, each man being armed with a light crowbar. Part of the broken crucible is preserved, and ground, for mixing with the raw clay, in the manufacture of crucibles. The remainder is used for building up the screen in front of the new crucible, which takes the place of the one removed. After the old crucible has been removed, it is customary to repair any defects in the furnace-bed with lumps of moist fire-clay, and to spread a layer of sand for the new crucible to rest on. The doors of the oven are now thrown open,
and the driver of the carriage thrusts its prongs under the crucible, depresses the handle, raises tho orucible from its supports, withdraws it from the oven, and carries it to the furnace. Tho prongs of the carriage bearing the crucible being half-way within the arch of the furnace, the wheels are blocked, and two men, using the upright poet A of the carriage (Fig. 731) as a fulcrum, gradually, by means of bars, force the crnoible into the position which it is intended to occupy. The screen is now rebuilt, partly with fire-clay slabs made for the purpose, and partly with the debris of the broken crucible, moist fire-clay, and fire-bricks.
Before the new crucible is fit to receive the raw materials, it requires to be glazed internally with hot glass. If the process of setting has been at all prolonged, or if the crucible, during its passage from the oven to the furnace, has become chilled, or if the crucible has not attained in the oven to the same temperature as the furnace, there is every probability of its being found to be cranked. The duration of crucibles is very uncertain, they may stand for 6-12 months or even longer, and they may crack in the first week after setting. If the furnace becomes chilled, or if tho grate is insufficiently covered, and allows cold air to rush upon the crucibles from the cave below, the crucibles run a great risk of being cracked. The crack is not usually discovered until the furnace regains its heat, and the liquid glass finds its way through the crack and on to the grate. The bars of the grate have to be widely separated, to allow the glass to fall through ; and if as usually happens, the air be allowed to rush up through the gap, the breakage of one crucible is very likely to be the cause of the destruction of others. Cracks in crucibles may be stopped by exposing them to the air, and allowing the exuding glass to cool and solidify into an effectual lute.
This remedy applies to the whole circumference of an open crucible, but only to the front of a flint-glass crucible, and in neither case is it of any avail for a cracked base. When a new furnace is to be lighted, new crucibles are sometimes placed in position, and heated with the furnace. Such great losses have, however, been incurred by this means, that there are very few manufactories in which this plan is carried out.