Fig. 732 is a sectional view of one half of an old-fashioned circular furnace ; Fig. 733 is a ground plan of the same taken at the level of the bases of the crucibles. The passage A runs under the grate G of the furnace, and is connected at both ends, either directly or by cross passages, with the outer air. This passage introduces the air necessary for the consumption of the fuel. From this passage, the stoker is able to stir the fire from beneath, and into the passage, fall the cinders and slag, and the lost glass from broken crucibles. P is a small passage, opening at each end of the grate into the main passage, and enabling the stoker to pass from one end of the grate to the other, without going immediately under the furnace. R is an iron rest to support the bars used for stirring the fire. D is a small iron door for charging the grate with fuel, which is thrown in by a shovel, or forced in by a Lang rake. The crucibles C rest upon the bed B. The clinker, when coke is used, is thrown into the furnace by the stoker, in the form of lumps of broken gas-retorts ; but when coal is burnt, it is formed by the coagulation of spent coal. The clinker plays an important part, both in regulating the draught, and in protecting the iron bars of the grate from the heat of the fire. It is one of the stoker's most urgent duties to attend to the condition of the clinker. There must be enough, and not too much ; and what there is must be continually moved, in order to prevent the bottom of the furnace becoming impervious to air. Iron staves I, at each side of every opening of the furnace, carry a continuous iron ring s, upon which the cone of the furnace is supported. Flues F stand on each side of the charging-door and of all the large crucibles. These flues can be cleaned through small doors opening outwards. They are arranged so as to draw the heat, after it has struck the crown of the furnaoe, all round the lower part of the crucibles. The dome or crown Z is kept as flat as possible, in order to economize heat.
Furnaces are usually built of moulded fire-clay blocks ; shaped blocks, for building complete furnaces, are kept in readiness at olay-worke. The blocks are dried, but not burnt, and are there fore comparatively yielding; every block is rubbed against its immediate neighbours, until an impervious joint is formed. In building the crown, the work commences at the outside, and no supports or centres are used. So soon as the blocks forming the outermost ring of the crown have been rubbed into position, they are firm enough to support a man'e weight. The blocks for the next ring aro one by one fitted with clips at top and bottom. A man, standing on the outside ring, holds the upper clip, and a man on the base of the furnace the lower. The block is than sawed up and down against the outer ring, and seems to adhere to it more and more, until it becomes perma nently fixed, In this manner, the crown is built concentrically block by block. In furnaces in wbioh open cruoibles are used, great care must be exercised in the selection of the blocks of which the crown is formed. If use he made of a material which, when exposed to intense heat, crumbles,
even to a email extent, the glass in the crucibles ie liable to be spoiled. In sheet- and plate-glass works, it ie common to employ Dines' silicate blocks, which appear to be practically indestructible. In building a furnace, the only cement or medium is a mixture of finely ground fire-clay and water. After the furnace has been built, it requires to be gradually warmed, before it is fit to withstand the intense heat necessary for the complete fusion of glass. For this purpose, the whole of the aye of the furnace, from the grate•bars to the surface of the bed, is tightly packed with coal or coke dust, which is lighted at the top, and allowed to smoulder. It takes 4-5 weeks to raise a furnace with safety to its full heat.
The crown and the base of the furnace are the parts most liable to wear out. The crown, espe cially when it is very flat, sometimes " gives," and one or more of the crown blocks fall in. When this happens, the opening must be temporarily covered with large fire-clay slabs and moist clay, and another furnace must with all speed be lighted and prepared to receive the crucibles. Dirt and soot accumulate on the outside of the crown, and, by keeping the crown unnecessarily hot, assist the corroding action of the flames within. For this reason, the crowns should be kept clean. The base or bed of the furnace must gradually wear away, and the duration of the bed usually deter mines how long the furnace shall remain at work. A point must sooner or later be reached when the base will be so reduced in width as no longer to afford a safe resting place for the crucibles. It is unwise to shirk the expense and trouble of lighting a fresh furnace, when there arises the slightest risk of losing crucibles and glass. It is customary, whenever a new crucible is set, to effect repairs in a damaged bed, by means of lumps of wet fire-clay, applied at the end of long iron shovels. It is, however, doubtful whether such repairs afford sufficient advantage to compensate for the labour expended on them.
Figs. 734, 735, and 736 are respectively a vertical section, a ground plan, and a horizontal section through its bed, of a furnace, the arrangement of which is intermediate between those of an old-fasbioned furnace and a gas-furnace. The fuel is introduced at the ports A, and makes its own way down to the grate-bars B. By the fuel passing through and under the base, crucible space is gained. The partial combustion of the fuel upon the grate-bars generates a gas, which burns with intense heat, when brought into contact with the air, introduced by the passages P cut through the base, Fig. 736. By reason of the combustion of the fuel taking place beneath the base, it is possible to reduce the size of the eye of the furnace, and, in this manner, to increase the width of the base, and consequently the duration of the furnace. The heat is directed by the flues in such a manner as to "surround the crucibles C, as in the furnace just described.