All About Glass

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The Mystery Slab of Beth She'arim

All About Glass

Beth She'arim was a cemetery located in Galilee. It was one of the most sacred places in the ancient Jewish world. Just adjacent to its catacombs is a natural cave that had long ago been made into a large cistern for storing water. It apparently fell into disuse at the end of the 4th century and filled up partially with four or five feet of clay-like silt.

In 1956 it was decided to convert the cave into a small museum. A bulldozer was taken in to clear the rubble and level off the surface. But, unexpectedly, the bulldozer bumped into something large—so large, that it wouldn't even budge. It turned out to be a large, rectangular slab that looked like concrete. Because of its size, it was left where it was, and the surrounding area was paved over with flat stone.

Beth She'arim slab

The slab measures 6½ x 11 ft. and is 18" thick. Its top is perfectly level.

In 1963, members of a joint expedition of The Corning Museum of Glass and The University of Missouri were surveying the region for possible remains of ancient glass factories. Someone suggested that the Beth She'arim slab might be made of glass. The suggestion was greeted with skepticism—indeed, one member of the team volunteered that if the slab was made of glass, he would eat it. A chemical analysis though, confirmed that it was, in fact, made of glass.

In the summers that followed there was a thorough examination of the slab; some of the adjacent paving stones were lifted up and excavation beneath it occurred in several places. Pottery remains found there indicated to Israeli archaeologists that the slab had been in place since about the end of the 4th century.

There was, of course, much speculation as to what this huge piece of glass was. But after the excavations began, the answer became clear. Whoever made this glass some 1600 years ago was not making a glass artifact; they were making glass as a material.

Glassmaking in ancient times was often carried out in two stages. The first was an engineering stage—the hot, hard, dirty work of heating the granular raw materials to a temperature of about 1100°C (around 2000°F) in order to bring about the chemical reactions that transformed them into a fluid melt. When the melt cooled down from this near white heat, it set up into a glass.

Having this work out of the way, the raw, unshaped chunks of glass could be transported to smaller factories where artisans would reheat the glass to soften it and fashion it into wares. The second stage could have been carried out 20 feet away or 500 miles away.

The Beth She'arim slab is a huge piece of glass meant to have been broken up and fashioned into objects somewhere else. But that never happened. Instead, the glass was abandoned right where it was made.

Beth She'arim slab Beth She'arim slab Beth She'arim slab Beth She'arim slab

There are two truly astounding things about the slab. First its sheer size: remember it measures 6½ x 11 feet. That means it weighs about 9 tons—18,000 pounds. When discovered, it was the third largest piece of man-made glass in the world and it was made centuries ago. Its size is still rivaled only by the giant telescope mirrors of the 20th century. More astonishing still are the conditions under which it was made. It is estimated that about 11 tons of raw materials had to be heated to 1100°C (around 2000°F), and held at that temperature for perhaps 5 to 10 days. This could have required as much as perhaps 20 tons of wood for fuel. Imagine what it must have been like in that cave—a veritable inferno.

How was the slab made? Excavations showed that the glass still rests on a bed of limestone block that formed the floor of the stone tank in which it was melted. A few of the stones that formed the walls of the tank are said to have been in place in 1956, before the bulldozer arrived. From evidence of heavy burning alongside the slab, we know the fire boxes were located there. A top of some sort must have arched over the tank, so that the flames would have reflected down onto the top of the batch mixture.

We know the glass batch was heated from above. During the excavation, a pavement corer was used to remove an 18" cylinder of glass near the center of the slab. It took an entire day to drill it out. After grinding and polishing the piece was removed. A scientist showed that the glass was homogeneous from the top down, until it came within a few inches of the bottom on the tank floor, where there were partially reacted ingredients that had not completely melted. At the very bottom was some of the original batch, which looks today like coffee grounds. Coming from above, the heat had not penetrated all the way through.

The glass itself has blobs and bubbles and other evidence of its molten history, but the material does not look like glass--it is not transparent. We know why that is too. Chemical analyses have shown that whereas most of the glasses of that time contain about 8% lime—that is, calcium oxide—this glass contains twice that much.

Apparently, some of the limestone, or the plastered arch over the tank, had become disintegrated by the heat, and sifted down into the molten glass as a chalk-like powder. We know that when glasses with excess lime cool slowly—and this glass must have cooled very slowly—the glass becomes partially crystallized. The myriad tiny crystals, visible through a microscope, turn the glass opaque. That is probably why the glass was never broken up to be used for making vessels.

If the Beth She'arim slab had actually been put to its intended use, probably 50 to 60 thousand small vessels could have been blown from its glass.

While the mysteries of the Beth She'arim slab thus appear to be solved, its extraordinary lesson remains. Without it, who would ever have imagined that, at the beginning of the 5th century, glass technologists had the ability—and even more so, the boldness—to undertake such an astonishing adventure in pyrotechnology (watch: Beth She'arim Slab).

UPDATE MARCH 2019: In 1999, Ian Freestone and Yael Gorin-Rosen posited that the great glass slab at Beth She’arim dates to the 9th century A.D. Therefore, rather than being an example of typical Roman period glassmaking, the slab represents a transitional phase at the beginning of early Islamic glass production, when glassmakers began to replace mineral natron with plant ash as a fluxing agent. The early Islamic date proposed by Freestone and Gorin-Rosen has been generally accepted by other scholars. To learn more, see The Great Glass Slab at Bet She’arim Israel: An Early Islamic Glassmaking Experiment?.
Katherine Larson, Assistant Curator of Ancient and Islamic Glass

Published on December 8, 2011