Little is known about the first attempts to make glass. However, it is generally believed that glassmaking was discovered 4,000 years ago, or more, in Mesopotamia. The Roman historian Pliny attributed the origin of glassmaking to Phoenician sailors. He recounted how they landed on a beach near Ptolemais (in modern-day Israel), propped a cooking pot on some blocks of natron (a naturally-occurring alkali substance) they were carrying as cargo, and made a fire over which to cook a meal. To their surprise, the beach sand beneath the fire melted and ran in a liquid stream that later cooled and hardened into glass. Though this is an interesting explanation, this scenario is not possible since a cooking fire cannot reach the melting temperature of glass, and the story most likely involved Ptolemais because its beach sand was historically known to be heavily used for glassmaking.
Scholars believe that the ability to make glass developed over a long period of time from experiments with a mixture of silica-sand or ground quartz pebbles - and an alkali. Other high heat industries, including ceramics and metalworking, could have inspired early glassmakers. Perhaps the development of glass began with potters %%firing%% their wares. Could the first glass have been colorful, hard, shiny decoration fused to a clay %%pot%%'s surface in the heat of the furnace? No one knows.
Eventually someone developed glass as a unique substance. A workshop would %%melt%% together the ingredients for glass and cool them to make ingots, or raw chunks, of usable glass (watch: Ancient Egyptian Furnace). Glassmakers knew how to color the ingots by mixing metallic oxides into the ingredients (watch: Coloring Glass). Popular colors for glass included royal blue and turquoise blue, colored by the addition of cobalt oxide and copper oxide, respectively, colorants which are still used today. Glassmakers were trying to imitate semi-precious stones, such as lapis lazuli and turquoise, which were valued by early cultures.
The cooled ingots would be shipped to completely separate workshops, sometimes very far away, where the chunks would be remelted and worked into different forms. This second workshop would not have to heat its furnace to as high a temperature as the origination furnace because less heat is required to remelt glass after it has already been formed from its raw, powdery materials.
Once a workshop remelted its ingots of glass, the glassworkers were limited to the technology of the day: workers could cast vessels and beads in molds (watch: Chunk casting), fuse small pieces of glass together and then slump the mass into, or over, a mold (watch: Fusing and slumping), grind and polish glass with stones or sand and water (watch: Grinding), or make hollow vessels by core-forming (watch: Core Forming). Before the first century B.C., 95% of all vessels were made by core-forming.
Ritual Glassmaking in Mesopotamia
As early as 3,300 years ago, ritual instructions for glassmaking in Mesopotamia were written on clay tablets in a cuneiform script. These instructions were copied and recopied over the centuries.
One group of clay tablets detailing glassmaking is from the library of King Assurbanipal (668-627 B.C.) and is currently housed in the British Museum. Part of the translation of the tablets (from Glass and Glassmaking in Ancient Mesopotamia by Leo Oppenheim) is below.
“When you set up the foundation of kiln to make glass, you first search in a favorable month for a propitious day, and only then you set up the foundation of the kiln. As soon as you have completely finished in the building of the kiln, and you go and place Kubu-images there, no outsider or stranger should enter the building thereafter; an unclean person must not even pass in front of the images. You regularly perform libation offerings before the Kubu-images. On the day when you plan to place the glass in the kiln, you make a sheep sacrifice before the Kubu-images, you place juniper incense on the censer, you pour out a libation of honey and liquid butter, and then only, you make a fire in the hearth of the kiln and place the glass in the kiln…
If you want to produce zagindurû-colored [greenish type of lapis lazuli] glass, you finely grind, separately, ten minas [about one pound] of immanakku-stone [sand], fifteen minas of naga-plant ashes, and 1 2/3 minas of 'white plant.' You mix these together. You put them into a cold kiln which has four fire openings, and arrange the mixture between the four openings… You keep a good and smokeless fire burning until the glass glows golden yellow. You pour it on a kiln-fired brick and this is called zukû-glass.
You place ten minas of “slow” copper-compound in a clean dabtu-pan. You put it into a hot chamber kiln… You crush and grind finely ten minas of zukû -glass. You open the door of the kiln and throw the ground glass upon the copper compound…When the glass assumes the color of ripe grapes, you keep it boiling for a time…After it has become yellow [hot], you observe some drops forming at the tip of the rake. If the glass is homogeneous, you pour it inside the kiln in a new dabtu-pan, and out of the cooled-off kiln emerges zagindurû-colored glass.”
What are the ingredients in this recipe?
Although this ancient recipe for glassmaking is shrouded in mystical rituals, the same basic ingredients are still used today.
- immanakku-stone (sand) - the main component of glass
- naga-plant ashes (soda ash) - the alkali flux that helps to lower the melting temperature of the sand
- 'white plant’ - unknown ingredient
- “slow” copper compound - can be used as a colorant for glass