The temperature of the furnace is controlled by regulating the draught by means of dampers in the main flues, arranged to act so as to carry out the annealing of the glass within the furnace. The regulation of the temperature within the furnace is of the greatest importance; if too hot the glass dissolves the clay of the pot, and if retarded too much it gives difficulty in freeing the metal from seeds, and plaining or fining the glass properly. Small furnaces containing one or two pots give the best results. These furnaces are worked on an intermittent process of first melting the glass and then gradually cooling to anneal the glass within the pots in mass, the furnace being allowed to die out gradually. When cool, the pots are broken away from the glass, which is then cleaved into lumps. Each lump is carefully examined for any defects and the best pieces selected for re-annealing. These are afterwards ground to the desired shape in the form either of a lens or prism. The chances are that not many pieces of perfect glass can be obtained from each pot of metal, and probably out of a whole pot only a fifth would be suitable for use after the process of selection and cleaving has taken place. In the manufacture of optical glass, batch materials are chosen that do not differ greatly in specific gravity. Every effort is devoted to obtain the purest materials possible; the batches are finely ground and well mixed before melting. The glass-melting pots should be made of the purest and most refractory obtainable in order to prevent the solution of any impurities into the glass whilst it is melting. In heating the pots for melting optical glasses every endeavour is made to heat them equally all round the top, bottom, and sides, so as to dissolve all portions of the glass evenly and completely together. At times the melted glass is stirred with a bent iron rod encased in a porcelain tube, and the glass agitated in order thoroughly to mix the components whilst fusing, and keep the composition of the glass as uniform as possible. After the metal has melted and plained clear from all seed and cords, the pot of metal is annealed, and when cooled the glass is extracted in lumps and examined for any defective pieces, which are rejected. The selected pieces are afterwards ground to the desired shape and, if necessary, re-annealed. In this process the pots being used only once, are expensive items, and they considerably increase the cost of production.