The furnaces used for the production of optical glass vary very much in type in different works. In some the old-fashioned conical coal furnaces are still used, the disadvantages attached to their employment being outweighed--in the opinion of the manufacturers--by their simplicity and ease of regulation. In other works gas-fired regenerative furnaces of the most recent type are installed, and in these also optical glass of the highest quality can be produced. As a rule, however, optical glass furnaces differ from other pot-furnaces found in glass-works in this respect--that the former are usually constructed to receive one pot or crucible only, while in other glass furnaces from four to twelve or even twenty pots are heated at the same time. The reason for this restriction in the capacity of the furnaces lies in the fact that since the mixtures used for optical glass cannot be adjusted to suit the furnace, the latter must be worked as far as possible in such a way as to suit the mixture to be melted in it, and this implies that every pot will require its own adjustment of times and temperatures, and this it would be difficult, if not impossible, to secure if more than one pot were heated in the same furnace. It is further to be remembered that the amount of care and attention required during the melting of a pot of optical glass is out of all proportion to that needed with other varieties, so that little would be gained by having a number of pots in one furnace, since several sets of men would be required to tend them. In addition to the single-pot melting furnace, a very important part of the equipment of the optical glass works is formed by a number of kilns or ovens which are used for the preliminary heating, and sometimes for the final cooling of the various crucibles or pots. Similar kilns are used in other branches of the industry, but in those cases the pots, once introduced into the furnace, are expected to last for a number of weeks, or even months. In optical glass manufacture, on the other hand, a pot is used once only, so that fresh pots are required for every new melting. The kilns in which these pots are heated up before being placed in the melting furnace are thus in very frequent use. As a rule they are simply fire-brick chambers provided with sufficient grate-room and flue-space to be gradually raised to a red heat in the course of four or five days, while for the purpose of gradual cooling they can be sealed up like the annealing kilns used for polished plate-glass.