Fire-clay is also used in the manufacture of bricks and blocks of various sizes required for the construction of glass-melting furnaces. Here fire-clay is only used in positions where contact with molten glass is expected, as in the walls of the basin or tank proper in “tank” furnaces, or at a level below that of the pot or crucible in pot furnaces; in the latter position leakage of glass from broken pots or overflow being liable to result in an accumulation of molten glass on the floors or walls of the furnace and passages. The fire-bricks used in these latter positions are usually of a much poorer quality of fire-clay than that used for the manufacture of pots, and this is justified in so far as certain of the requirements that apply to crucibles do not apply here--but on the other hand the use of more refractory bricks would result in a longer life for the furnace. Such bricks, it should be noted, are not laid in mortar when used for furnace construction, but are set in a thin paste of fire-clay in water, and these joints are kept as thin as possible. The part of the furnace known as the “siege” (French “siège”), _i.e._, the floor of the furnace upon which the pots are placed, is usually built of very large blocks of fire-clay, made of coarse materials calculated to give great strength. At or near the points where the flame enters the furnace, these blocks rapidly wear away, partly by melting but chiefly by a process of abrasion, for it seems that a rapidly moving flame has an abrading action of a very marked kind.