The method of transferring the glass from the melting-pot to the rolling table differs somewhat in different works. In many cases the melting-pots themselves are taken bodily from the furnace and emptied upon the bed-plate of the rolling machine, while in other cases the glass is first transferred to smaller “casting” pots, where it has to be heated again until it has freed itself from the bubbles enclosed during the transference, and then these smaller pots are used for pouring the glass upon the rolling slab. The advantage of the latter more complicated method lies, no doubt, in the fact that the large melting-pots, which have to bear the brunt of the heat and chemical action during the early stages of melting, are not exposed to the great additional strain of being taken from the hot furnace and exposed for some time to the cold outside air. Apart from the mechanical risks of fracture, this treatment exposes the pots to grave risks of breakage from unequal expansion and contraction on account of the great differences of temperature involved. Where smaller special casting-pots are used, these are not exposed to such prolonged heat in the furnace, and are never exposed to the chemical action of the raw materials, so that these subsidiary pots may perhaps be made of a material better adapted to withstand sudden changes of temperature than the high-class fire-clay which must be used in the construction of melting pots. On the other hand, the transference of the glass from the melting to the casting-pots involves a laborious operation of ladling and the refining of the glass, with its attendant expenditure of time and fuel. Finally, the production of plate-glass in tank furnaces could only be attempted by the aid of such casting-pots in which the glass would have to undergo a second fining after being ladled from the tank, and this would materially lessen the economy of the tank for this purpose, while it is by no means an easy matter to produce in tank furnaces qualities of glass equal as regards colour and purity to the best products of the pot furnace.