When the glass has stiffened to such an extent that it is no longer possible to continue the stirring, preparations are made for the final cooling-down of the pot of glass. The fire-clay stirrer is sometimes withdrawn from the glass, but this is laborious, and entails dragging a considerable quantity of glass out of the pot with the clay cylinder; more usually, therefore, the stirrer is simply left embedded in the glass.
The next object to be accomplished is that of cooling the glass as rapidly as safety will permit until it has become definitely “set”—the purpose being to prevent the recrudescence of striæ as a result of convection currents or other causes which might disturb the homogeneity of the glass. This rapid cooling is obtained in various ways; in one mode of procedure the furnace is so arranged that by opening a number of apertures provided for the purpose cold air is drawn in and the pot and its contents chilled thereby without being moved. This method has the advantage that the pot containing the viscous glass is never moved or disturbed in any way, but on the other hand the cooling which can be effected within the furnace itself is never very rapid, and the furnace as well as the pot is chilled. Further when the glass has been chilled down to a certain point this rapid rate of cooling must be arrested, as otherwise the whole contents of the pot would crack and splinter into minute fragments. Where the pot has been left in the furnace this can only be done by sealing up the whole furnace with temporary brickwork and lutings of fire-clay, leaving it to act as an annealing kiln until the glass has cooled down approximately to the ordinary temperature, a process that occupies a period of from one to two weeks according to the size of the melting. Such enforced idleness of a melting furnace is of course very undesirable from an economical point of view, and it is generally avoided by adopting the alternative method of drawing the pot bodily out of the furnace as soon as the stirring operation is ended. For this purpose the temporary brickwork forming the front of the furnace is broken down, and with the aid of a long crow-bar the bottom of the pot is levered up from the bed or siege of the furnace to which it adheres strongly, being bound down by the sticky viscous mass of molten glass and half-molten fire-clay which always accumulates on the bed of the furnace. The pot being temporarily held up by the insertion of a piece of fire-brick, the tines of a long and heavy iron fork running on a massive iron truck are introduced beneath the pot; an iron band provided with long handles is then passed around the pot, and the latter is then drawn forward by the aid of suitable pulley blocks. The tines of the fork are then raised, and the pot is wheeled out of the furnace and deposited upon a suitable support. Here it is allowed to cool to the requisite extent, when it is again picked up on the tines of the fork and deposited in an annealing kiln which has been previously warmed to a suitable temperature. It will be seen that this handling of a heavy mass of intensely hot material involves much labour, while there is also a risk of losing the glass if the pot should break before the glass has set sufficiently. Every care is taken to prevent such an accident, the pot being wrapped round with chains or otherwise supported in such a way that a small crack could not readily develop into a large gap.
When such a melting of glass has cooled sufficiently, either in the furnace or in the annealing kiln, to be safely handled, the whole pot is drawn out, and the fire-clay shell, which is generally found cracked into many pieces, is broken away by the aid of a hammer. Under favourable circumstances the whole of the glass may have cooled intact as one solid lump sometimes weighing over half a ton. Unless special care is taken, however, it is more usual to find the glass more or less fissured, a number of large lumps being accompanied by a great mass of small fragments. These are now picked over, and all those which are free from visible imperfections or which can be readily detached from such imperfections by the aid of a chipping hammer are put upon one side for further treatment.
The next step of this treatment consists in moulding the rough broken lump into the shape of plates, blocks, or discs according to the purpose for which the glass may be required by the optician. The plant used for the moulding process varies widely, but in all cases the operation consists in gradually heating the glass in a suitable kiln until it is soft enough to adapt itself to the shape of the mould provided for the purpose. In some cases these moulds are made of fire-clay, and the glass is simply allowed to settle into them by its own weight; in other cases iron moulds are used, and the glass is worked into them by the aid of gentle pressure from wood or metal moulding tools. In yet other cases, particularly where the glass is required in the form of small thin discs or where it is to be formed into the approximate shape of concave or convex lenses, the aid of a press is sometimes invoked.
In all cases the moulding process is followed by the final annealing, which consists in cooling the glass very gradually from the red heat at which it has been moulded, down to the ordinary temperature. The length of time occupied by such cooling depends very much upon the size of the object and also upon the degree of refinement to which it is necessary to carry the removal of small internal strains in the glass. For many purposes it is sufficient to allow it to cool down naturally in a large kiln in the course of six or eight days. For special purposes, however, where perfect freedom from double refraction is demanded, much greater refinements are required, and special annealing kilns, whose temperature can be accurately regulated and maintained, are employed. In these the annealing operation can be carried out so gradually that a rate of cooling in which a fall of 1° C. occupies several hours can be maintained, so that very perfectly annealed glass can be produced even in discs or blocks of large size.
When removed from the annealing kiln the plates or discs of optical glass are taken to a grinding or polishing workshop, where certain of their faces or edges are ground and polished in such a way as to permit of the examination of the glass for bubbles, striæ and other defects in the manner indicated in the previous chapter. As the amount of sorting that can be done while the glass is still in rough fragments is necessarily very limited, it follows that a considerable proportion of the glass which has been moulded and annealed must be rejected as useless when thus finally examined. A yield of perfect optical glass, amounting to 10 or at most 20 per cent. of the total contents of each pot, is therefore all that can be expected, and smaller yields are by no means infrequent—a consideration that will serve to explain the relatively high price of optical as compared with other varieties of glass.
A consideration of the various factors that are involved in the production of a piece of perfect optical glass will make it apparent that the cost and difficulty of its production increases rapidly with the weight of the piece to be produced, so that it is not surprising to find that the price of very large discs of perfect optical glass such as those required for large astronomical telescopes, reaches figures which become prohibitive when very large sizes are considered. Thus, while it is quite possible to obtain say 100 pounds of good glass from a single melting if the glass is to be used in the form of pieces not weighing more than five or six pounds each, it is only rarely that a single block of perfect glass can be found weighing 100 pounds. In the former case the best pieces can be picked, the worst defects can be eliminated by chipping the rough fragments, and at a later stage other defective pieces can be cut off or ground away; not so where a large single block is required. A single fine vein, perhaps too small to be visible to the unaided eye, may be found to run through a whole block in such a way that it cannot be removed without breaking or cutting up the whole piece, and it will be seen that the frequency with which this is liable to occur increases with the volume of the piece required. The difficulties of re-heating and moulding are also increased enormously with the size of the individual pieces of glass that have to be dealt with, and where very large pieces have to be heated and cooled accidental breakage becomes a serious risk. In view of these difficulties it is not surprising to find that the dimensions of our astronomical refractors appear to have approached their limit, but rather are we led to admiration of the skill and enterprise that has pushed this limit so far as to produce discs of optical glass measuring as much as one metre in diameter.