The founder, or glass melter, now takes charge of the pot, and he brings up the mixture of batch and cullet and shovels it into the empty pot until it is filled well above the mouth or level of the opening. The heat of the furnace melts the batch, and after several hours it becomes liquid and shrinks in volume so that probably only two-thirds of the height or capacity of the pot is occupied. The pot is then again filled with more batch materials until it is full of molten metal up to the level of the mouth of the pot.
The furnace is kept going at its full heat until the founder, drawing a small portion of the glass on the end of an iron rod, examines it and finds that it is melted clear and free from seeds or bubbles of gas. When clear, the metal is “plain,” and at this stage is in a very liquid, fluid, and watery state, too liquid to be easily gathered. It is, therefore, allowed to cool off by removing the stopper down and leaving the mouth of the pot open, until the glass becomes more viscid, or of a stiffer nature. The glass is then skimmed by dragging off any scum present on the surface, which is due to undecomposed salts that may have risen during the melting.
The metal is now ready for the glass blowers to begin work. Upon looking into the pot, the ring will now be noticed floating on the surface of the glass. This ring keeps back from its interior any further scum that may arise whilst work is in progress. The glass blower always gathers from within this ring, where the metal is cleanest; and from time to time the metal within the ring is skimmed in order to keep that portion in the best condition. When the greater part of the metal within the pot has been gathered or worked out, the heat of the furnace is raised again and fresh batch materials filled and the process repeated.
The time taken to melt the glass depends upon the heat of the furnace. A gas-fired furnace will melt the batches in eight hours, but the old type of English furnace takes much longer, usually two to three days.
------------------------------------------------------------------------
Owing to the peculiar structure of glass, and its liability to fly or collapse when exposed to sudden changes of temperature, a process of annealing becomes necessary in order to produce a more equal distribution of the tensions throughout the structure of the glass; otherwise, glassware of any thickness would be in such a state of tension as to be extremely liable to fracture when passing through any sudden change in the atmospheric temperature, especially in frosty weather. In this state it is useless or dangerous for general purposes. On this account most glasswares undergo a form of annealing at some time during the process of their manufacture. And in the case of certain goods, such as table glass, lamp glasses, optical glass, etc., special care and time are devoted to this process of annealing. Often in the case of improperly annealed glass, instances are known where its unhomogeneous structure has suddenly given way as the result of derangements set up by internal tension. Friction, or rough handling whilst cleaning, at the ordinary temperature of the atmosphere, is sufficient to cause a rupture. Therefore annealing cannot be too carefully attended to.