The Making of a Glass Batch
by Cambridge Glass Company
Rainbow Review Glass Journal - March/April 1974
Webmaster's Note: The following is a reprint from a 1939
Cambridge Glass Company pamphlet. It appeared in the March and April
issues of the Rainbow Review, but it has been combined here.
The Miracle of Heat and Many Diverse Materials
In one of the tales of the ancient Greeks, there is the story of
Prometheus who stole fire from Mount Olympus, the home of the gods, and
carried it to earth in a hollow staff. That deed was to make him the
founder of civilization. For it is fire that makes our modern world
possible ... fire that savors our cooking, warms our homes, moves our
machinery, makes our iron and steel. It is fire, too, that performs the
"miracle" or melting sand, alkali, and other materials into glass.
And a "miracle" it is, that such diverse materials, so opaque, so
incongruous, should, under the influence of temperatures as high as
2700 degrees Fahrenheit, become the beautiful, scintillating glassware
that graces our dining room tables.
The essential elements of glass are many, depending upon the kind
and quality of glass to be made.
- SILICA - That principal element is silica sand and
"cullet," which is composed of all those pieces of glass which have
failed to pass the eyes of the inspectors, the tops removed from
blown pieces, and all the glass that adheres to the working
tools.
While silica sand is found in practically all parts of the world,
the pure sand, required for making Cambridge glass, is not so
generously distributed. The best in the United States is found in
Pennsylvania, West Virginia and in eastern Ohio.
You can see the necessity for this purity when you consider that
only .06 of one per cent of iron in the sand inevitably produces an
off-color and unsatisfactory glass.
But it is not enough for the sand to be pure. In the Cambridge plant
it must go through an exacting preparation process in which it is
thoroughly washed and calcined so as to remove all impurities and
vegetable matter and then ground up into a fine white powder.
- LIME - Lime is used to furnish alkaline content, necessary
to give the glass strength and workability. The form mostly in use is
granulated quick lime; however either quick lime or hydrated lime may
be used. Lime glass, commonly used by makers of cheaper grades of
glass, lacks the addition of those other materials, notably lead,
which produces those qualities wanted by makers of finer glass.
- LEAD - In the better, higher priced crystal glass, such as
Cambridge makes, lead is used, usually in the form of red lead oxide.
It gives to glass many valuable properties, increasing the density,
resonance, and refractive index. Peddle, a well-known English
authority on glass, says that lead is necessary to secure color,
density, brightness, durability, easy melting, and easy working.
Because of the comparative softness of lead glass, it has a lower
melting point and is easier to plane and consequently, freer from
common defects such as "cord" and "seed." 8eautful designs can also
be cut into lead glass.
The oxygen in the lead oxide performs another function. As lead
oxide has the highest specific gravity of any of the materials used,
it has a tendency to sink to the bottom of the pot liberating bubbles
of oxygen which carry to the surface any impurities or unmelted
portions of the batch.
- LITHARGE - This material acts in much the same manner as the
red lead oxide but is not as valuable for the reason that it does not
contain as much oxygen.
- SODA ASH - Soda ash or sodium carbonate is used principally
as a flux to start the batch melting. It also furnishes part of the
alkali necessary for making glass.
- POTASH - In the making of fine glass, potash is used in
place of soda ash. It not only acts as a flux, but also imparts the
resonance or ring which is characteristic of fine glassware such as
Cambridge.
- BORAX - Another flux, sometimes used, is borax. It also
forms part of the alkaline content and gives to the glassware certain
heat-resisting qualities.
- MANGANESE - This is used as a decoloring agent to offset or
overcome the greenish tinge or "low color" caused by impurities such
as iron in the batch. If too much manganese is put into the batch,
however, the glass becomes pink and is said to have "high color."
Manganese is also used to produce a purple or pink glass, but if
allowed to get too hot, turns the glass an unsatisfactory yellow or
brown and finally green.
- ARSENIC Arsenic, which acts as a clarifying agent, is used
as arsenious oxide or arsenate tri-oxide. It is associated with
copper and is a by-product of the copper industry.
Other materials utilized in the manufacture of crystal glass are:
barium oxide. zinc oxide, alumina, feldspar, etc.
MIXING THE BATCH
This whole mixture of silica sand, cullet, potash, lead or lime,
etc., ready for charging into the pot or tank, is called "the
batch."
Preparing this batch is a most exacting process not to be achieved
by any hit-or-miss methods.
In the making of Cambridge glass. this exactness begins with the
setting up of the original specifications for a given kind of glass. It
continues in the chemical laboratory where all incoming materials are
tested and analyzed. It follows through to the mixing room where giant
weighing machines measure out the heavy materials and sensitive
balances parcel out the finer ones, often in quantities as small as a
single ounce for a batch weighing 2,000 pounds. The result is that a
given grade of Cambridge glass is always the same - today tomorrow,
five years hence.
These materials, having been most carefully proportioned, are placed
in a steel car which remains in the mixing room until ready to be
charged into the pot, when it is wheeled over to the furnace.
The Weird Chemistry of Coloring Glass
If this exactness is necessary in mixing the basic materials, it is
even more necessary when measuring the mineral salts and oxides which
give color to Cambridge colored crystal. Just a pinch of some salts and
oxides, for instance, will color an entire batch.
This chemistry of color is one of the most fascinating aspects of
the making of glassware. The choice of coloring materials is seemingly
so illogical, so contrary to what you would expect in the results each
achieves.
For instance, cadmium sulphide, a silvery white metallic element,
produces yellow or canary glass. Selenium, a lead gray crystal which is
an element of copper ore, produces a beautiful amber or a brilliant
red, depending upon the quantity used. Black oxide of copper, black as
its name implies, imparts a rich blue-green. And so it goes,
practically every mineral salt or oxide used in coloring glass produces
a color totally different from its own.
While the entire process of making colored glass is too technical
and complicated to go into, except briefly, in a booklet of this kind,
here at least are a few colors and the materials used to produce
them:
- BLUE - Blue is secured in glass by using jaffre, an impure
cobalt arsenate, made by roasting the crude ore or calcining it with
excess air. This ore is found in Canada and the Belgian Congo. Cobalt
oxide and smalt are also used to make glass blue.
- YELLOW or CANARY - Cadmium sulphide is one of the
materials used to make glass yellow. Cadmium, a metal never found
free in nature, is silvery while in color and soft at ordinary
temperatures. Uranium, an element which is found with radium in
pitchblende, used as sodium uranite, produces a canary yellow
fluorescent glass.
- GREEN - Oxide of chrome is used to secure certain shades
of green. It is made from sodium dichromate, which is produced from
chromite ore and is found in several countries, but principally in
Asia Minor. Red oxide of iron, prepared either by precipitation or
calcination, produces certain shades of yellowish green. Black oxide
of copper is used for making blue-green glass.
- AMBER - Selenium, cerium, and flowers of sulphur impart
beautiful amber colors.
- RED - Selenium, which is an element from copper ore and is
a by-product of the copper industry, is also used to produce
red.
Gold, used as "purple of Cassius" produces a beautiful ruby glass.
With different treatment it also makes the glass amber or
purple.
These are only a few of the many materials used to give color to
glass. Sometimes they are used alone, sometimes in combinations of
several, depending upon the color and tint desired.
Melting the Glass
There are two distinct methods of melting glass depending upon the
quality of the glass to be made.
The cheaper kinds of glass, known as tank glass, are made by feeding
the materials to be melted into a large rectangular tank. The batch is
fed into one end of this tank through an opening known as the "dog
house." The molten glass is drawn off at the other end by workmen or
machinery.
In the second method, used for making fine glassware like Cambridge,
pots are used in place of tanks. As many as fourteen of these pots,
each capable of holding a ton of molten glass, are grouped in a furnace
and charged separately. During the process of melting, these pots are
tightly covered so that no soot or dirt can enter and discolor the
glass. So important is cleanliness, that in the Cambridge plant, where
much of America's choicest glassware is made, even the gas that is used
in the furnace is filtered of all soot and dirt. No precaution is too
great to keep the batch clean.
The pots or crucibles are made of fire clay, thoroughly glazed on
the inside, and allowed to season for several months before they are
placed in use. Expensive to make and of problematical life, these pots
are warmed-in gradually to eliminate any tendency toward cracking. How
long they last in service depends upon this initial care and the kind
of glass made in them. When they have served their purpose, they are
broken up and reground to make other pots. It seems that the more this
fire clay is used and burnt, the better it becomes.