The Great Glass Slab at Beth Shearim

In a cave adjacent to an ancient cemetery near Beit She'arim, an old Jewish town in northern Israel, there lies a huge slab of glass approximately 6.5 feet wide, 11 feet long and 18 inches thick. It weighs 9 tons. Although chemical analysis confirm it’s glass, the slab doesn’t look anything like the delicate, translucent material. Rather, it looks like a large block of limestone, for it is completely opaque and bluish-gray.

When the slab was discovered in the 1960s during an exploration of the cave, workers thought it was concrete. An attempt was made to dislodge the slab, but the slab was too heavy for the bulldozer. So the slab was left where it was and the surrounding area was paved, for they were building a museum inside the cave.

The Great Glass Slab at Beth She’arim.

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, when someone suggested that the Beth She'arim slab might be made of glass. A piece of the slab was broken off and sent to the labs for testing. The results stunned everyone, including the researchers themselves. The slab was indeed made of glass, and it was 1,600 years old.

Humans have been making glass for 6,000 years, much longer than they have been making iron tools. The earliest glass objects, mostly beads, were made from glass that was accidentally created as by-products of metalworking, or during the production of faience, a ceramic-like material. These early glasses were rarely transparent and often contained impurities and imperfections. True glass didn’t appear until 15th century BC in the region around modern day Lebanon, the coastal north Syria, Mesopotamia, and Egypt.

During Roman times, this region in the Mediterranean became the center of glass making because of the easy availability of raw materials, such as sand and soda ash. Glass was manufactured in the workshops in huge blocks, which were then transported to different glass-working sites across the Roman, and later, the Byzantium Empire, where artisans reheated the glass blocks to soften them and blew them into all kinds of glassware.

Making glass was a difficult process because the ingredients used to make glass often contained impurities, which were not easily measured, but their presence changed the nature of the glass. For example, the main component of glass is silica or sand, which also contains some alumina and about 8 percent lime. Lime is desirable because it is that magic ingredient that prevents molten sand from crystalizing, which results in an amorphous, clear and nearly colorless solid which we call glass. However, add too much lime and it loses its magic property, causing the molten sand to crystalize into millions of microscopic structures and the resulting glass loses its transparency. The glass slab at Beth She‘arim is an example of such a failure.

Why did the glass slab at Beit She'arim fail?

Polished core section of Beth She'arim slab, 7th-9th century, Israel. Section is approximately 11cm in height. Photograph by Robert Brill/Corning Museum of Glass

At Beit She'arim, glass was manufactured in a tank made of limestone blocks. The interior dimension of this tank was 6.5 feet by 11 feet and its height was slightly larger than 18 inches. The tank was filled with around 11 tons of raw material and heated to 1100°C. This temperature was held nearly constant for 5 to 10 days to allow all the materials in the tank to melt. This must have required as much as 20 tons of fuel.

Because the tank was incredibly heavy and made of limestone, heat was applied not from below but from above. There is evidence of heavy burning alongside the slab, from which archeologists know where the fire boxes were located. The tank was covered by a plastered arch so that the flames were reflected down onto the top of the batch mixture. A core drilled out from the slab 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 coffee grounds. Apparently, the heat had not penetrated all the way through, so the slab was essentially half baked. Additionally, some of the limestone, or the plastered arch over the tank, had become disintegrated by the heat, and had fell down into the molten mixture, raising the lime content of the glass to twice the recommended figure of 8 percent. The result was a solid block of solidified silica.

Consequently, the glass was never broken up to be used for making vessels. Had the glass turned out perfect, it could have produced 50 to 60 thousand small vessels.

The chunk of glass is still there, where it was discovered, on the floor of the cave which now functions as the visitor center of the museum.

Photomicrograph of crystals formed by devitrification of the glass of the Beth She'arim slab. Photograph by Robert Brill/ Corning Museum of Glass

References:
# The Mystery Slab Of Beth She'arim, Corning Museum of Glass
# Freestone, Hughes, Stapleton, The Composition and Production of Anglo-Saxon Glass, Research Gate