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.

Other materials utilized in the manufacture of crystal glass are: barium oxide. zinc oxide, alumina, feldspar, etc.


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:

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.