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.