The Mechanics of Glass Manufacture

by Anita Wood
Volume 18 No. 10 - June 1992

Machinery for the manufacture of glassware was developed in the early part of the 1900s, and many innovative uses for the machines were put to the test. Heat-resistant glass such as Pyrex was one of those innovations. The properties of heat_resistant glass were discovered in 1915.

Ten years later, most major glass manufacturers had purchased the machinery and began to produce large quantities of inexpensive tableware.

The least expensive combinations of materials comprised the formulas of the various colors. Lime glass, a mixture of silica sand, soda ash and limestone had been used in the manufacture of bottles, jars and window glass prior to the development of the machinery, and this formula was continued.

Soda ash was used to lower the temperature of the heated sand to make it easier to work with and the limestone makes the glass hard. The materials are mixed and heated in a tank, then made of ceramic, holding an enormous quantity. From the tank, the molten glass flows through a system of pipes into the molds. Colors are determined by the addition of various minerals to the basic formula. For instance, iron oxide is used to make green, chromium or copper to make blue. The addition of iron slag makes black amethyst.

Molds for mold-etched glass, which is how most of the depression glass patterns were made, are made by covering a steel plate with acid-resistant wax, Patterns are then cut in the wax and the plate is washed with acid to remove the pieces cut out, leaving the acid-resistant wax in place. Then the design was cut into the steel plate. From this plate, the pattern was again waxed and transferred to tissue paper and put inside the mold. After heating to melt the wax, the pattern was transferred to the mold in a mirror image. The mold was then given an acid bath, which left the pattern in the mold. This type was efficient, in that only one design had to be made in the original steel plate, and the same design could be put into the mold all the way around, thus assuring more than one distribution of the pattern. This process can be visualized by looking at a Dogwood plate. The sameness of the design shows more than one use of the original pattern.

Press molds were made by taking a model of the piece desired and designing a mirror image in metal. A plunger, used to force the glass into the mold,was made to fit the inside of the piece. At this point the mold became a part of the mechanization process, and was handled by a machine.

Cut-mold and paste-mold are two other types of molds used in the production of glass, but most depression glass was made by the etched-mold process.

After annealing and cooling, the glass was packed in barrels, with plain straw used as packing material. In the bustle of all these operations, occasionally a piece of straw might fall into a mold, and produce what today we call straw marks. Obviously, quality control wasn't high on the list of priorities of the glass companies.

Not much has changed in the manufacturing process. The molds still must be designed and made by hand. Technology has much improved the machinery, electronic controls have been added, and the formulas have changed, but the basic ingredients remain the same.

BIBLIOGRAPHY

  • Klampkin, Marian. The Collectors Guide to Depression Glass. New York. Hawthorne Books, 1973.
  • Weatherman, Hazel Marie. Colored Glassware of the Depression Era. Springfield. Missouri. Weatherman. 1970.