As soon as the empty pot has been put into place, the melting furnace is carefully sealed up by means of temporary work built of large fire-bricks, the whole being so arranged that the mouth of the hood of the pot is left accessible by means of an aperture in the temporary furnace wall. This aperture can be closed by one or more slabs of fire-clay, and when these are removed an opening is left by which the raw materials are introduced, and through which the other manipulations are carried out. When this stage of the process is reached, the wagons containing the mixed raw materials are usually wheeled into place in front of the furnace, but the introduction of the materials themselves into the pot is not begun until several hours later, when the furnace has been vigorously heated and an approach to the melting heat has been attained. When the furnace and pot have attained the necessary temperature, but before the raw materials are introduced, a small quantity of the cullet, which has been reserved for this purpose, is thrown into the pot and allowed time to melt, and then only is the first charge of mixture put into the pot. The object of this proceeding is to coat the bottom and part of the walls of the pot with a layer of molten glass which serves to protect it from the chemical and physical attack of the raw materials during the violent action which takes place when they are first exposed to the furnace heat. The gradual filling of the pot with molten glass is now carried out by the introduction of successive charges of raw material; as the mixture not only occupies more space than the glass it forms, but also froths up a good deal during melting, the quantities introduced each time must be carefully adjusted so as to avoid an overflow of half-melted glass through the mouth of the pot. As the pot is more and more nearly filled, the space left for the raw materials is proportionately diminished, and the later charges are therefore much smaller than the first few.