OPTICAL GLASS INDUSTRY—GLAZE 
There are no accurate figures for 
the production of optical glass for the 
years 1917-37 inclusive. However, it 
is safe to say that it did not appreciably 
exceed 61,000 pounds (1938 produc- 
tion) for any year during that period. 
Optical glass, as used in lenses, 
functions as a medium to refract the 
rays of light from any distant object 
so that they will converge to a single 
corresponding point in the image. 
This requirement is extremely difficult 
to meet and demands that the glass 
in each lens (or prism) element be of 
uniform quality throughout and that 
its optical constants agree very closely 
with those of certain standard types of 
glass. ‘To manufacture on a large 
scale a series of different types of glass 
of this degree of perfection requires 
close attention to details. 
The characteristics of good optical 
glass are: 
1. Homogeneity. 
a. Uniformity of chemical composition, 
including freedom from streaks of 
different compositions within the 
glass mass (striae). 
b. Freedom from seeds, or bubbles. 
c. Freedom from included fragments of 
undissolved material or crystal- 
lites within the glass mass (stones). 
d. Freedom from cloudiness. 
2. Definite refractive indices for different 
wave lengths of light. 
3. Freedom from color. 
4. High degree of transparency. 
5. High degree of chemical and physical 
stability. 
a. Resistance to action of weather and 
certain chemical agents. 
b. Toughness and hardness. 
The art of making optical glass of the 
above characteristics is the subject of 
the following necessarily general ac- 
count. 
Practically all optical glass in the 
United States is made in pots which 
are used only once. Hence, the first 
requisite is a pot of the necessary 
refractoriness. And here we run into 
one of the main compromises necessary 
in connection with optical glass pro- 
duction. The denser or less porous a 
pot body is, the less it is attacked by 
the glass or its batch. But, the denser 
the pot body is, the more sensitive it 
219 
is to thermal shock (sudden tempera- 
ture change). So the type of body 
finally selected is somewhat less than 
the ideal both as to corrosion resistance 
and resistance to thermal shock. 
The pots are made up essentially of 
four constituents: feldspar, ball or 
bonding clay, kaolin, and grog (old 
pot body). The porosity of the pot 
body is controlled by the particle sizes 
of the grog used. In this country all 
pots are cast in a mold made of plaster 
of Paris. For the more corrosive 
glasses, pots are made more resistant 
by means of a dense lining. This pro- 
vides the resistance to shock of a porous 
body and the resistance to corrosion 
of a dense body. After the pot has 
dried sufficiently so that it can be 
handled, it is removed from the mold 
and air-dried for 3 weeks or more. 
It is then ready to set in the pot arch 
for burning (pl. 1, 1). This burning 
takes anywhere from 48 to 100 hours, 
depending on the density of the pot 
body, for pots of approximately 7 
cubic feet capacity such as are used 
at the National Bureau of Standards. 
Of course, the denser the pot body, 
the more slowly it should be burned. 
The pot can be fired completely in 
the pot arch or it can be partially 
fired in the pot arch and the firing 
completed in the melting furnace. 
The transfer of the pot from the pot 
arch to the melting furnace is accom- 
plished by means of a pot carriage. 
This is equipped with a pair of massive 
tongs operated from the rear of the 
carriage, and with a counterweight 
which can be moved along the carriage 
to balance the weight of the pot of 
glass. The melting furnace is gener- 
ally regenerative—that is, built over 
two chambers of checkerwork which 
are alternately heated by the gases of 
combustion and then used to heat the 
incoming air necessary for combus- 
tion. The fuel usually used is gas, 
preferably natural gas. 
After the pot has been completely 
fired at the melting temperature of 
the particular glass it is designed to 
hold, it is ready to receive the batch 
