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|THE STORY OF THE LENS||Manufacture of Optical Glass|
|HE manufacture of optical glass is the most difficult task of the glass maker's art. It presents problems requiring, for their solution, the technical training of the research chemist, the mathematical ability of the physicist and the skill of the expert craftsman. From the time of Galileo to the Great War optical glass was made|
entirely in Europe—principally in Germany. There, under the
direction of Professor Abbe, the early master of optics, and
Doctors Schott, Carl and Zeiss, this art had been brought to a high state of
perfection. Every wearer of spectacles as well as any user of any optical
instrument was dependent upon the furnaces beyond the Rhine for the glass
from which these instruments were made.
But back in 1853 John J. Bausch went to Rochester and began, in a very small way, the grinding of superior lenses that soon attracted the notice of New York opticians. A most disconcerting feature of his work, however, was the fact that all of his glass had to be imported. Long and persistently he sought to solve the problem of making glass that would meet the high requirements of his art. Then came the war, and what had been only a highly desirable goal, at once, became a necessity and a patriotic duty. After three years of diligent research, interspersed with many temporary failures, Bausch, now nearly ninety years old, assisted by his son, William, succeeded in making optical glass of a very high grade.
When the United States entered the war it became imperative that the Government should have large quantities of glass for range finders, gun sights, periscopes, search-light mirrors, photographic lenses and binoculars. Immediately the Bureau of Standards and the Geophysical Laboratory at Washington took up the problem and Major F. E. Wright, Ph.D., was detailed to take charge of the work. A number of glass manufacturers throughout the country placed their plants at the service of the Government.
There are six fundamental requirements of good optical glass: (1) Correct optical and physical properties; (2) freedom from striæ; (3) freedom from bubbles; (4) high light transmission; (5) freedom from color; (6) freedom from strain. To meet all these requirements there were necessary a stirring device that would eliminate the striæ, or tiny grooves, melting pots more resistant to corrosive fluxes and freer from iron and sulphur than any then being made in the United States, and the purest "batch" materials. Most essential, too, were skilled workmen of whom there were none in this country at that time. As the
result of an amount of technical and practical experimentation
unprecedented in the history of the glass industry, these conditions
were successfully and quickly met.
The two principal kinds of glass used in optical instruments are crown glass, consisting essentially of silica, potash, soda and and lime, and flint glass, consisting of silica, potash and lead oxide. Small quantities of other substances, as boric acid, magnesium, zinc, barium and antimony are also used. Silica is ordinary sand and is always contaminated with iron oxide, from which it must be freed. This is accomplished first by passing a huge magnet over it, and then washing the sand with acid and water.
The first step in the manufacture of optical glass is to preheat the clay pot through a period of seven days to a temperature of 2500 degrees F. Into this are weighed at intervals of fifteen minutes for one hour 100 pounds lots of cullet, i.e., optical glass of inferior quality from a previous melt. This is followed with pure batch materials in successive lots of 400 pounds each at intervals of two hours accompanied by hand stirring for periods of fifteen minutes each. The mass is then machine stirred for four hours, when the pot is removed from the furnace, covered with earth and allowed to cool, or anneal, slowly for four days. The pot is now broken away and the chunks of glass sent to the examination room. All portions not having the qualities of good optical glass are sent back to be used as cullet for another melt, while the good glass goes to the press room. In the press room the chunks are slowly preheated until soft, and then pressed into slabs in hydraulic and pneumatic machines. After annealing, these slabs are laid in frames on a grinding table covered with fine sand or emery and ground to a smooth surface. They are then polished by driving to and fro over their surfaces an iron block covered with felt and rouge. Rigid inspection for striæ and bubbles follows. The latter are detected by looking through the slabs toward a dark cloth while a narrow ray of light enters the glass at right angles to the line of sight. Defective places are marked and removed by sand blast. All accepted glass is reheated, pressed and very slowly annealed.