OPTICAL GLASS INDUSTRY—GLAZE 
of the mirror. Because of this simpli- 
fication the largest telescopes are 
reflectors. 
Except for two disks, one 23 inches 
in diameter made in 1889 and one 
60 inches in diameter made in 1895, 
the manufacture of telescope mirror 
blanks was also considered a European 
monopoly until 1926. In that year, 
Ohio Wesleyan University asked the 
National Bureau of Standards to make 
a blank 70 inches in diameter. Sucha 
disk, assuming the thickness to be one- 
Melting Furnace 
and Pot 
223 
When arrangements were completed 
for tapping the pot, the water-cooled 
plug was removed and the glass flowed 
by means of a trough into the mold- 
annealing furnace (fig. 2 and pl. 3). 
After practically all the glass was in 
the mold, the trough was removed 
and the top of the annealing furnace 
placed in position. 
Thirty-three hours after pouring, 
the insulation over the top of the 
annealing furnace had been built up 
to the 12 inches that covered the rest 
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Annealing Furnace Cover 
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Annealing Furnace 
Ficure 2.—The essential features of the equipment for the 70-inch disk, shown in use in 
plate 3. 
sixth the diameter, would be 11 inches 
thick and weigh about 4,000 pounds. 
The type of glass decided on was 
an ordinary borosilicate crown. The 
batch was melted in a pot holding 
approximately 5,000 pounds of glass, 
near the bottom of which was a hole 
stoppered with a water-cooled metal 
plug. After the batch had com- 
pletely melted, stirring was started 
_and continued for 6 hours, after which 
the temperature was reduced from 
1400° to 1350° C. While the pot 
was filled, current was applied to the 
heating elements of the combination 
mold-annealing furnace. 
817369—49—17 
of the furnace. The loss of heat be- 
came so slow then that no current 
was necessary for 4 days; it was not 
until 8 days after casting that the 
average temperature of the furnace 
reached 600° C. During the next 2 
months, the temperature of the furnace 
was gradually reduced to the anneal- 
ing temperature of 461° C., which was 
then maintained for 41 days. 
Assuming now that the disk was free 
from strain, it was still necessary to cool 
it so slowly that no great amount of 
strain would be present at the end of 
the process. Although the data of L. 
H. Adams and E. W. Williamson indi- 
