The intensity of the fire requires that the furnaces and crucibles, should be constructed of materials the least fusible in their nature, and the best calculated to resist the violent and incessant action of heat; or the manufacturer will incur the most serious losses and delays from casualties which, even after the most careful and costly outlay, can not be always averted. The crucibles especially demand attention in this respect, in consequence of the solvent property of some of the materials which are melted in them. These crucibles are deep pots, varying in size according to the extent or objects of the manufacture; and some notion may be formed of the importance attached to them from the fact, that they are not unfrequently made large enough to contain individually not less than a ton weight of glass. Great skill and care are requisite in their structure, so as to adapt them to the temperature in which their qualities are to be tested; and even with the utmost attention that can be bestowed upon them, they are often found to break soon after they are exposed to the furnace, by which heavy losses are entailed upon the manufacturer. Nor is this the only point which must be considered. The size of the crucible should bear a proportionate relation to that of the furnace, or one of two consequences, equally to be avoided, will ensue; either that there will be a waste of fuel, if the crucibles are too small, or an inadequate heat, if they are too large.[13] [Footnote 13: For details see Loysel "Sur l'Art de la Verrerie;" and Lard. Cyclo.] We have now before us the three principal processes--the calcination, by which the materials are prepared in the first instance--the melting down of these materials into glass in the great working furnace, and the annealing of the finished article after it has been fashioned by the workmen. These processes are broad and simple; but that part of the manufacture which is, probably, most calculated to surprise the uninitiated, is the manner in which the red-hot mass of glass, as it is taken out of the crucible, is instantly, so to speak, shaped into form by the dextrous hands and practiced eyes of those men whom you see standing about at tables and stools, twisting long iron rods called _pontils_, blowing through pipes, and performing mysterious evolutions with scissors, pronged sticks, compasses, and other instruments, with a rapidity that baffles the most vigilant observer. From the infinite diversity of objects into which glass is thus moulded, it must be obvious that the operations of these artificers embrace a variety of curious details which it is impossible to enter upon here; but a glance at some of them will enable the reader to form a general notion of the curious manipulations upon which they are so actively employed.