OPTICAL GLASS
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 achromatise 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 impure 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 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.
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.
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