The Indianapolis times. [volume] (Indianapolis [Ind.]) 1922-1965, 1929-08-24
A Marvel in Glass
BY DAVID DIETZ
Scripps-Howard Science Editor

THE largest block of optical glass ever cast in
l. the United States has just been produced in
the United States bureau of standards in Washing
ton. The block is in the form of disk seventy inches
in diameter and eleven inches thick 1t weighs two
tons.
1t is to be used to form the great mirror for the
Seventy-inch rejecting telescope at Ohio Wesleyan
university, Delaware, o.
The great disk has been cast by new method
developed at the United States bureau of standards
according to Dr. A. N. Finn, director of the bureau
The method opens up field for American
manufacturers, Dr. Finn declares.
Optical instruments require glass of exceptional
grade. The larger the instrument the more perfect
the glass must be. slight defect which would be #
no Importance in a small instrument is sufficient to
render large instrument worthless for scientific
purposes.
When America entered the World war, it was
brought face to face with the fact that it was de-
pendent for optical glass upon Europe chiefly Ger
many.
The bureau and other scientific agencies inme-
diatey tackled the problem of establishing American
independence in that field.
The great disk just cast at the bureau represents
the climax of researches which have gone on from
the days of the World until the present
DR FINN points out that there is not much optical
IL glass manufactured. He says that there are
only ten plants in the whole world turning it out.
1t is, however, very fundamental industry, since
there is not great industry in existence which
where in its processes does not require the of
scientific measuring instruments which contain optical
glass.
Scientific and industrial research in particular are
dependent upon exact measuring instruments.
The new process developed at the bureau of
standards, therefore, is Of great Importance to
American industry
Dr. Finn discusses the process in Industrial and
Engineering Chemistry" official organ Of the Ameri-
can Chemical Society.
THE most serious difficulties met in producing
large pieces of glass include properly melting the
amount of glass required, transferring it to mold
to- produce the desired shape while maintaining the
necessary quality, and finally, cooling it at rates such
that it does not crack and is free from disturbing
internal stresses" he says
Several processes tried before success was
attained. In each of the earlier attempts the glass
cracked while being cooled.
A COMBINED mold and annealing furnace was
"a constructed and placed in hole dug in the
ground at the foot of the melting furnace.
Temperatures Of both the melting furnace and the
annealing furnace were controlled electrically.
The melting furnace was heated slowly the tenn
perature being increased each day until the Thrift
eth day, temperature of 1925 degrees reached.
The glass was then introduced into the furnace
at the rate of ITO pounds an hour
When more than two tons Of molten glass were
in the melting furnace, a plug removed, per-
hitting the glass to flow into the mold.
The temperature allowed to fall in the mold
until it dropped to 600 degrees. 1t then cooled
very slowly for sixty days at the end of which time
a temperature of 461 degrees was reached.
1t was kept at this temperature for annealing
purposes for fortyone days. The purpose of annea-
ing is to reduce the brittleness and increase the
toughness of the glass.
Following the annealing, the temperature was re-
duced gradually each day, until four and half
months had elapsed. BY that time, the glass was
completely cooled of
The long slow cooling process is necessary to pre-
vea cracks and the development Of internal strains
and defects.