220 
and cullet (waste glass from previous 
melts) (pl. 1, 2, right). The time 
necessary to fill a pot with batch 
varies’ front “5 “to 10 “hours.’Phe 
temperature of the melting furnace 
during this operation may be as low 
as 1350° C. or as high as 1450° C., 
depending on the difficulty with which 
the batch melts. After the batch has 
melted down and foaming has ceased, 
the stirring of the molten glass can be 
started. This is accomplished by 
means of a thimble made of the same 
materials as the pot, actuated by 
means of a water-cooled rod driven 
by a horizontal rotating drum and 
working over a pulley or a guide asa 
fulcrum (pl. 1, 2, left). 
As soon as the molten glass is reason- 
ably free of seeds or bubbles of gas, it 
is gradually cooled to the temperature 
at which the pot of glass should be 
removed from the furnace. As the 
molten glass cools down, the rate of 
stirring must be decreased so that air 
will not be sucked into the glass in the 
wake of the stirring rod. Here is 
another example of compromise. If 
the rate of stirring is too fast, a seedy 
but homogeneous or striae-free glass 
is generally obtained; if it is too slow, a 
seed-free but striated glass results. 
The pot of glass is allowed to cool in 
the melting furnace until the viscosity 
increases to such an extent that any 
further stirring might ruin the glass. 
This temperature is the most critical 
and may vary from 950° to 1100° C., 
depending on the type of glass. The 
time required to produce a pot of glass 
at the National Bureau of Standards 
varies from 19 to 28 hours. 
After the pot of finished glass is 
removed from the melting furnace 
(pl. 2, 1), the bottom of the pot is 
cooled by means of a blower until the 
glass is stiff enough so that there will 
be no movement of the contents due 
to convection currents. ‘The pot is 
then covered with a thermally insu- 
lated can so that the glass will cool 
slowly enough to crack into fairly 
large chunks. ‘The pot is then broken 
open, the pot body processed for grog, 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
and the chunks of glass saved for 
further processing. Any imperfections 
present are trimmed from the chunks 
of glass by means of steel hammers or 
diamond saws (pl. 2, 2). These 
chunks are then broken or sawed to a 
convenient size for molding into the 
blanks on order. 
These pieces of glass are now put 
into a preheating furnace where they 
are gradually heated up to a tempera- 
ture just below the softening point of 
the glass. They are picked up by the 
molder on the end of a steel rod (punty 
rod) and then heated well above their 
softening point in a molding furnace 
and worked into the proper shape for 
molding. When properly shaped and 
at the proper temperature, each gob 
of glass is held over the steel mold and 
the proper amount of glass is cut 
off by a pair of shears and pressed into 
shape. Upon removal from the mold, 
each blank is placed in a cooling lehr 
and cooled slowly to prevent cracking. 
The above molding procedure is the 
best for medium-size and large blanks. 
In the case of small blanks it is best to 
mold slabs of proper thickness, break 
each slab into cubes weighing a little 
more than the desired blank, and then 
adjust to correct weight on a grinding 
wheel, especially rounding off all 
corners and sharp edges. The pieces 
of glass are fed into a small paddling 
furnace, gradually brought up to 
molding temperature, worked into 
approximate shape by means of pad- 
dles, and pulled into the steel mold and 
pressed. To keep them from cracking 
the blanks are then placed in an oven 
attached to the paddling furnace. 
In the case of blanks too large to 
work by hand, the glass pieces are 
placed in a ceramic mold and gradu- 
ally brought up to a sufficiently high 
temperature so that they will soften 
and flow into the shape of the mold. 
They are then slowly cooled so they 
will not crack (a rough annealing). 
In any case, the blanks are then in- 
spected. They are immersed in a 
liquid of index of refraction similar to 
that of the glass. In such a liquid the 
