The San Francisco call., 1910-01-01

and soda melted and run together.
The sand is made up very largely of
white powder called silica, some kinds
of sand having more of it than others.
The world silica comes from slick,
which means flint.
Glass can be made of silica and soda,
but it would melt away in water, and
that is why lime is put in to harden
it. You may have seen biscuits in
the making of which the soda has been
carelessty used so that they had a
greenish tinge; cheap glass bottles
have this color, but small quantity
of arsenic or something similar is
added before melting to whiten the
better quality
You will remember that salt is COM
posed Of a poisonous gas and metal;
it is from this metal, sodium, in our
salt that we get the soda for our
glass. In glass factory the first
thing to interest is the batch When
we speak of batch we usually refer
to a mixture of -four and yeast from
which bread is to be made; but this is
a san1like powder. with sometimes
pieces of broken glass in 1t which they
call cullet. This batch is put into the
furnace to be melted, and very inter
esting indeed would be a story of the
melting. pots, how the clay, of which
they are made, must be mixed and
ready for use; but that is another story.
Twenty-four hours of melting are re-
quired for this batch; then two to four
hours must be given to clear itself of
bubbles, and about half an hour to
cool slightly; then it has become sticky
and is ready to be worked. There are
three methods of working glass-cast-
ing, pressing and blowing.
Have you ever seen a glass blower
at work? Common window glass is
made by this method: 1t Is first blown
into long cylinders, the usual size be-
ing about IL inches in diameter and
SO inches long; the blower fiattens this
cylinder into pear shape, cuts off
the tapering end and throws it back
into the batch to, be remelted. The
cylinder is then split from end to end
and placed split side uppermost in what
Is called fiattening oven, where the
flame passing over 1t soon opens IL,
and 1t falls back in a wavy sheet A
block OF wood on the end of an iron
rod rubbed over and over IL while it is
soft makes 1T perfectly fiat and smooth.
French plate glass is made by cast
ing. There is a casting table having
two fiat bars of iron at the sides, and
on top of these bars runs a heavy
roller OF iron or steel A narrow space
is left between the roller and the top
of the table and the depth OF this
space is the thickness of the plate of
glass; for thick plates the bars are
set higher, for thinner ones they are
lowered. When all is ready a pot
filled with melted glass is run out
from the furnace, lifted by crane
suspended over the casting table and
poured out; then the heavy roller fiat
tens II and rolls ii to an even thick
ness. They must work very fast, so
that the plate, while as hot as possl-
BILE and only partly solid. may be laid
in an oven to cool slowly. If cooled
too rapidly, the outside surface would
become hard before the inner part was
solid. and this would make 1t brittle.
This cooling is called annealing and
requires from four to five days The
sheet of glass is then rough. and must
be ground smooth, and polished.
Our table ware is nodded. Two
molds are made. one like the outside
OF the dish to be made. and the other
like the inside, and between these the
melted glass is poured and pressed
into shape, This sounds like very
simple thing to do but IL the mold
is too hot. the glass viii stick to II;
II too cold, the surface will not be