The ancient glass industry was divided into two categories: glass making, and glass working (1). Glass making workshops were usually situated near places where the raw materials were available. The materials needed to make glass include sand, nitrate, and lots of heat. To fuse the ingredients together, a kiln must be heated up to a temperature of 1100 degrees fahrenheit. This was accomplished with large amounts of wood for burning, and a strong breeze to provide enough oxygen for the fire (2).
The color and fabric of the glass depended upon the levels of natural metals in the sand. A small amount of iron caused glass to turn green. Manganese could have created yellowish or purple glass. It is uncertain what was used to produce blue glass, but Romans probably utilized cobalt or copper. If a glassmaker desired colorless glass, he could add a neutralizing agent (3).
Once the ingredients were melted together, they created molten glass. This substance, too liquid to work with, would be cooled until it hardened into a solid (4). The raw, unshaped glass chunks would be given to glass workers in separate shops. The craftsmen would need to remelt the glass in order to make it pliable enough for shaping. A kiln only needed to reach 750 degrees fahrenheit to become workable, and that temperature could be accomplished in a simple shielded hearth or average Roman bread oven (5).
Once the glass was plastic enough for working, there were a few methods available for shaping glass vessels. These were core-forming, sagging, free-blowing, and mold-blowing.
1. "It is important to realize that there is a distinct difference between the knowledge of how to make glass and how to work it." Quote from Robert J. Charleston, Masterpieces of Glass: A world history from the Corning Museum of Glass (New York: Harry N. Abrams, Inc. 1980):12.
2. Stuart J. Fleming, Roman Glass: Reflections of Everyday Life (Pennsylvania: University of Pennsylvania Museum, 1997):10-11.
3. Donald B. Harden, Roman Glass from Karanis (Ann Arbor: University of Michigan Press, 1936):6-9.
4. E. Marianne Stern, Roman Mold-Blown Glass (Toledo, Ohio: Toledo Museum of Art, 1995):42.
5. Fleming (1997):10-11.