OPTICAL GLASS INDUSTRY—GLAZE 
heat-resisting alloy. The anchors 
were provided with adequate air 
cooling. This maintained the tem- 
perature safely below that at which 
the tensile strength of the anchors 
could not withstand the buoyant force 
of about 300 pounds on the largest 
cores for the 7 hours necessary for the 
casting operation. 
The pouring oven over the mold had 
three ladling holes and was heated 
with natural gas to keep the glass 
sufficiently fluid to flow to all parts of 
the mold. The three ladles used in 
casting had a capacity of 750 pounds 
each and the ladle skins were returned 
to the tank in order to conserve glass. 
After pouring was completed on 
December 2, 1934, the mold was 
moved on rails to the annealing 
position by means of a hand-operated 
drum and cable. The entire upper 
portion of the annealing furnace, in- 
cluding top and side walls, was sus- 
pended like the pouring oven and was 
electrically heated (pl. 4, 2.) The 
lower portion of the annealing furnace 
was built on the screw hoist beneath 
the mold. The two portions tele- 
scoped for some distance as the disk 
was raised into the annealing position. 
During the soaking, or constant- 
temperature period of annealing, 
which required about 50 days, con- 
trollers automatically maintained the 
temperature at the desired value. 
For the 200-inch disk, 26 inches in 
thickness, the cooling rate was main- 
tained at 0.72° C. per day from the 
initial temperature of 500° C. until 
300° C. was reached. Below the latter 
value, a more rapid cooling rate was 
possible. About 10 months were re- 
quired to reach room temperature. 
To some extent, the task of making 
these disks was lessened by the choice 
of a Pyrex-type of glass. Its co- 
efficient of expansion, only 2.5 x 
10-* cm. per °C. per 1 cm. length 
againstes.o. x) 10s°l cm, per °C. for 
ordinary borosilicate crown, greatly 
225 
reduced cooling stresses and shortened 
annealing times. On the other hand, 
it added materially to the difficulty of 
melting and working. The successful 
fabrication of large disks cannot be 
ascribed wholly to the use of low- 
expansion glass. The same methods 
of manufacture outlined here would 
produce disks of ordinary crown glass 
with greater ease of melting and 
casting. The time required to anneal 
such glass, however, would be in- 
creased nearly fourfold, and greater 
demands would be made on the 
temperature-control equipment to 
maintain the corresponding slower 
rate of cooling. Mirror blanks of 
ordinary crown would be heavier and, 
because of the higher coefficient of 
expansion, would require a much 
longer time for grinding and polishing. 
This took a total of 3 years for the 
200-inch disk. 
From the foregoing it can be seen 
that there is no need for astronomers 
of the United States to go to Europe 
to fill their needs for optical glass. 
There is in this country the “know 
how” to fill their wants, fantastic 
though they may seem. The field 
has been thoroughly explored. Prac- 
tically no money or time need be 
spent in experimental work to develop 
a satisfactory method of procedure. 
REFERENCES 
Adams and Williamson, Journ. Franklin 
Inst., vol. 190, pp. 597, 835, 1920. 
Chance, Journ. Soc. Glass Techn., vol. 27, 
p= Lisi 943: 
Encyclopaedia Britannica. 
Finn, Ind. and Eng. Chem., vol. 21, p. 744 
1929; Journ. Opt. Soc. Amer., vol. 28, 
jon ws GIS ete). 
Heindl, Massengale, and Cossette, Glass Ind., 
vol. 27, p. 177, 1946. 
Joint Army-Navy Specification, Glass, Opti- 
cal, Jan. 30, 1945. 
McCauley, Journ. Soc. Glass Techn., vol. 19, 
ps 156u8, 1935: 
Ordnance Department Document No. 2037, 
The manufacture of optical glass and of 
optical systems. 
