Evening star. [volume] (Washington, D.C.) 1854-1972, 1929-08-11
GIANT GLASS DISC OPENS NEW
FIELD TOU. S. OPTICAL INDUSTRY
Serious Difficulties Beaten
in Making 1900. Pound
Reflector.
Need of Plants for Such Work
Is Outlined by Dr..
A. N. Finn.

Production by the United States
Bureau Of Standards OF a 4,000-pound
reflector disc, 70 inches In diameter and
IL inches thick-the first Of such gil
gantic proportions produced in this
country-is heralded by DR.. A. N. Finn
of the bureau as opening q new field
to the American optical industry.
Development of the process by which
the disc was manufactured Is an
nounced in the current number OF In
dustrial and Engineering Chemistry,
the journal Of the American Chemical
Society. The great disc produced by
the Bureau of Standards is to be used
in the rejecting telescope Of Ohio
Wesleyan University.
The manufacture Of glass for OPP
tical instruments is well established
industry, but is limited, there being,
according to DR.. Finn, fewer than 10
optical glass plants in the world.
Cites Importance of Work.
'"This industry is. however. necessary
to the maintenance and development
Of our commercial and scientific enter
prises," he wrote in commenting on
the process. Lack Of suitable glass
would greatly hamper all work in
which such instruments are used.
Difficulties which are Of minor i'm
portance in making glass for smaller
optical instruments become more Sc.
rious as the size of the instrument,
and consequently the size of the glass,
is increased.
1.-THE most serious difficulties met in
producing large pieces of glass include
properly melting the amount OF glass
required, transferring 1t to q mold to
produce the desired shape while main
tanning the necessary quality, and,
finally, cooling it at rates such that it
does not crack and Is free from dis
turbing internal stresses.
THE proper cooling or annealing OF
large pieces Of glass is probably the
most exacting phase OF the work. and
without knowing the thermal prop
critics of the glass and adequately con-
trolling the temperatures of the anneal-
because of the lack Of definite in
formation on methods of making salts
factory pieces of glass which could be
used for large telescope rehectors. the
Bureau of Standards became interested
in the problem and undertook to make
the 70-inch disc"
4. 10 Tons Of Plaster Used.
A number Of unsuccessful attempts,
in which the glass cracked during the
period of cooling, LED to the abandon
ment of the first procedure attempted
for the production Of the enormous re-
Hector. OF the method finally used,
DR.. Finn says:
1T was first necessary to make pots
sufficiently large to hold the amount Of
glass required. The pots were cast in
plaster OF paris mold by a method
developed at the bureau. About 10 tons
OF the plaster were used In making the
forms, the model and the finished
mold. Each pot was made from 1880
pounds Of domestic clayS and feldspar.
approximately 5000 pounds Of
batch were used, requiring 81 hours
to all the pot. The glass was stirred
BY hand with water-c cooled iron STIR
ring rod for six hours, while current
was applied to the annealing furnaces
to heat and to hold II at about 400
degrees centrigrade.
When all arrangements were COM
pleted for tapping the pot, the SUPP
ports for the water-cooled plug were
released and the plug removed. The
glass in contact with this plug vas
practically solid, and the molten glass
did not begin to flow from the pot yn.
til the plug of cold glass became soft
enough to yield. It then flowed very
rapidly.
"As its level in the mold rose, the
ends Of the channel were adjusted so
that one end remained practically fixed
at the hole In the pot and the other
was not permitted to touch the glass in
the mold. When practtcally 81 the