The manufacture of optical glass forms a very important section of the glass industry, and presents some of the most difficult problems the glass maker has to deal with. It is in this section of the glass trade that applied physical and chemical science becomes of the utmost importance to the manufacturer. The production of optical glass is impeded by any defects which become evident in the structure of glass when examined under a polariscope. The presence of any striae, seeds, or stresses within the structure of the glass disqualifies it for any important optical work. It is a difficult matter to get pieces of optical glass only a few inches in diameter of the right optical constant and refractive index that are homogeneous enough to allow of the light rays passing without some dispersion when set up for use. It becomes necessary, therefore, to achro-matise one glass with another in the form of doublets to correct aberration. A high degree of transparency and durability is necessary in all optical glasses.
The persistent evidence of stresses developed in the solidification of the glass upon cooling, even when the glass is slowly and carefully annealed, is a most difficult factor to deal with. In annealing optical glass, the various temperatures and time periods have to be delicately adjusted and controlled, or big losses result. Even then many efforts may be made before a suitable piece of glass is obtained, and the costs keep accumulating with each attempt, and some idea of the amount of labour involved in the undertaking to produce optical glass at once becomes evident. The use of decolorizers and many other materials is not permissible, on account of the absorption and consequent resistance to the passage of light rays. The annealing, instead of occupying one or two days, is sometimes extended over a course of ten or fifteen days, in order gradually to relieve any stress present. The pots in which the glass is melted may only once be used, as the glass is usually allowed to cool down gradually and undergo the process of annealing within the pot.
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 two or three 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 fire-clay 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. Sometimes 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.
Before the war the optical glass trade was confined to a few firms in this country, who supplied only a fraction of our needs. We have been dependent mostly upon continental supplies of optical glass, and it is only quite recently that Government state assistance has been forthcoming in giving scientific aid to manufacturers by investigating and reorganising this section of the glass industry. It is to be hoped that this state assistance will continue, and that the optical branch of the glass trade will be perfected to such an extent that we may in future be independent, and produce for ourselves all the optical glass requirements of our navy and army. It is to be regretted that this industry did not receive state assistance before the war. If it had, we should certainly have been better prepared and equipped than was the case at the start of the Great War.