Above the siege and over the pots is a covering called the crown of the furnace, which is supported by fire-brick pillars. This is built of the most refractory material possible to be obtained, as the hottest flames from the furnace fires beat against this crown and are reverberated downwards upon the surrounding pots. The flames, continuing their course, pass between the pots into small openings or flues leading from the siege floor and passing upwards through the pillars which are situated between each pair of pots, they then escape from little chimneys leading into the outer dome or conical-shaped structure so familiar to outsiders. This outer truncated cone-shaped structure constitutes the main chimney of the furnace. The furnace chamber containing the pots is constructed entirely within this cone. The blocks are carefully shaped, neatly fitted, and cemented together with a mortar made of fine, plastic, raw ground mixed to thin paste with water. The presence of any molten glass which escapes from a cracked pot, and the fluxing action of the fuel ashes, cause severe corrosion of the blocks forming the siege and fire-box, and these necessarily have to be made of extra thickness in order to extend the life of the furnace. When the furnace crown or siege becomes badly corroded away, the furnace has to be put out for repair; so generally an auxiliary furnace is kept at hand, in order that it may be started and the workmen transferred from one furnace to the other whilst the repairs are being done. [Illustration: EXTERIOR VIEW OF ENGLISH GLASS-MELTING FURNACE Pot Trolley in foreground ]