Ancient Egyptian glass production: Natural glass, man-made glass, raw materials, techniques

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Glass

Pectoral of Tutankhamen; Source: Jon Bodsworth

Natural glass

Glass occurs only rarely in nature. Because of its beauty and physical properties it was prized by toolmakers and much sought after by jewellers.
The mainly dark coloured Obsidian, the result of volcanic action was not found in Egypt, but was imported in small quantities from Turkey ^[1].

Pectoral of Tutankhamen
Picture source: Jon Bodsworth

Libyan Desert Glass ^[8], probably the result of meteorite impacts, was also traded. It was treated like a precious stone and incorporated in jewellery. The scarab's body in the pectoral found in the tomb of Tutankhamen is seemingly made of it ^[9].

Man-made glass

Glass beads from Gurob, Middle Kingdom; Source: UCL

Early fabricated glass lacked the translucency of the naturally occurring glasses, but could be given form and decoration, had permanent colouring and was - unless broken - practically imperishable.

Middle Kingdom glass beads
Source: Petrie Museum, excerpt

It could thus replace to some extent precious and semi-precious stones and Egyptian glass beads were traded throughout the Levant ^[6]. Still, glass remained a material little used until Roman times and is badly documented, unlike clay pottery which, economically of great importance and used as a symbol for the act of creation, was often depicted. Glass inlay, Amarna; Source: UCL

The range of glass objects remained limited to jewellery, amulets, the occasional small receptacle and flat pieces of glass used as inlays.

Glass inlay (UC 22077)
Amarna
Source: Petrie Museum website ^[2]

Many of the 2nd millennium glass objects have few decorations and are coloured rather dully without the lustrous sheen we expect from glass.

Glass vase with decorated rim (UC 22081)
New Kingdom
Source: Petrie Museum website ^[2]

Great technological advances were made during the New Kingdom. The most accomplished 18th dynasty glasses were almost completely transparent. Glass production declined drastically during the third Intermediate Period.

During the Late Period glassworking techniques were improved and the range of colours extended. The Ptolemies saw objects made up of different brightly coloured glasses merged together with brilliant, clear surfaces.

Ptolemaic (UC 22258)
Source: Petrie Museum website ^[2]

The main practical advantage glass has over ordinary pottery is its imperviousness, but because of the difficulty of working with red-hot masses of molten glass larger water-proof vessels like plates, vases and bottles continued to be made of mostly glazed earthenware, lead-based, usually blue mezzo-maiolica until the Roman occupation and of tin-based faience thereafter.

The raw materials

Pure silica has a melting point of 1700°C. Adding a flux reduces this to less than 1000°C, a temperature achievable with the help of bellows which came into widespread use during the New Kingdom. Ancient glass was a mixture of

the major constituent part, silica, i.e. quartz desert sand. (About half of the sand near Akhetaten, for instance, was made up quartz, about a third was calcite, the rest was feldspar, pyrosenes and small amounts of illmenite ^[12].)
alkali, mostly from plant ash during the New Kingdom and often trona (natron) from Wadi Natrun or Beheira during the Graeco-Roman period ^[12] or more rarely potassium oxide, as flux,
calcium oxide from limestone as a stabilizer,
colouring agents. These were naturally occurring impurities or metal oxides added on purpose. The much coveted blue-tinted glass was made by adding cobalt. Yellow was the result of using iron and antimony, turquoise of copper or purple of manganese ^[3]. Clear, almost colourless glass could be made by adding decolouring agents such as manganese oxide (MnO) as was done by the Romans.
lead (as early as the 15th century BCE ^[12], but much more common in Roman times)

The ingredients were ground as finely as possible and mixed. Fritting required only a relatively low temperature (at least 800 °C ^[13]), but the mixture had to be stirred for hours to achieve consistency. The frit, after removing and discarding unmelted sediments which had collected at the bottom and the vesicular mass of the top, was ground up, melted at up to 1100 °C and then poured into moulds or ground up again, so that the resulting glass particles could be worked over an open fire ^[4]. The proximity to the fire and lack of protective measures could cause eye and skin problems for the worker.
The transparency of the end product was affected by the amount of bubbles included in the glass. This could be reduced by vigourous stirring during fritting and the addition of antimony trioxide (Sb2 O3 ), but there is no proof that this was done intentionally during the New Kingdom. Impurities which affected the clarity could be neutralized by additives.
Cylindrical glass ingots were made from raw materials at specialized factories such as the one discovered at Qantir-Piramesse, which was active during the second half of the 13th century BCE. They were shipped to workshops in Egypt and abroad where craftsmen made the final products.

Techniques

When cold-working glass the material was treated similarly to stone. Blocks of it could be cut and ground to give them desired shape and drilled to create a hollow space. But glass being very brittle and easily shattered, this method was only rarely employed.

The most frequently used technique was core-forming, where a core of clay-containing sand or the like was tied to a stick with bandages and covered with Glass vessel interior; Source: UCL a thin coat of molten glass. By rolling it on an even, hard stone slab the glass surface was smoothed. While it was still hot small amounts of differently coloured glass could be applied to the surface for decorative purposes ^[5]. As the melting was often incomplete, little air bubbles remained in the glass which turned opaque. By the time of Tutankhamen people had learned how to prevent this bubble formation and transparent glass could be produced.

Interior of glass vessel (UC 22099)
Source: Petrie Museum website ^[2]

After cooling the object the core was removed. The surface of the interior remained uneven and the impressions left by the core could often be seen.

Casting glass into moulds was only of minor importance during the New Kingdom, but continued to at least a small extent after core-forming had been abandoned due to the introduction of glass blowing under the Romans.

In the New Kingdom the Egyptians had all they needed to produce blown glassware: the raw materials in abundance, aerated furnaces reaching high temperatures and ceramic blow tubes. Still, glass blowing was not invented until the first century BCE in Syria. Mass production of blown glass objects was not introduced until Roman times.

Blown glass vessels were created by sticking a piece of molten glass onto one end of a blowpipe and through the other end introducing pressurised air into the pipe. This was done by blowing which could result in the glass-worker damaging his lungs and developing emphysema. The expanding bubble of glass could be

introduced into a mould and further inflated until it filled the whole matrix. After the glass had cooled down the mould could be removed.
shaped with an instrument, while turning the blowpipe slowly to prevent deformation of the object due to gravity.

Decorations were added by pinching the hot glass, adding handles or other features to it like strands of differently coloured glass which could then be pinched to change simple straight patterns into more intricate ones.

Vase fragment
Roman Period
Picture source: Petrie Museum website ^[2]

Embellishments could also be added after the object had cooled down. Writing or images might be incised, or paint or enamel applied.

[ ] The photograph of Tutankhamen's pectoral courtesy Jon Bodsworth.
[ ] Source of the photo excerpt of the glass beads: The website of the University College, London ^[2].

Footnotes:
[6] The Syrian-Canaanite region was a major centre of glass production and was in the forefront of the development of new technologies. According to some Amarna letters raw glass was imported into Egypt from Tyre, Askalon, Lakhish and Acre. ^[7]
[9] The Italian mineralogist Vincenzo de Michele who examined the scarab with a refractometer in 1998 concluded that it consisted of Libyan desert silica glass ^[10][11].
[12] Julian Henderson Science and Archaeology of Materials: A Textbook, Routledge, ISBN: 0415199344, pp.25ff.
[13] Lucas speaks of possibly less than 750°C, Rehren and Pusch of about 900 to 950°C

		About glass making at Akhetaten from Tell el Amarna by W. M. F. Petrie
		Index of topics
		Main index and search page

	Offsite links	(Opening in a new window)
	These are just suggestions for further reading. I do not assume any responsibility for the content of these sites

		[1] A Resource for Glass (Pdf file, 1.8 Mb)
		[2] Glass (University College London)
		[3] The chemical composition of glass in Ancient Egypt by Mikey Brass
		[4] Glass: Stages in the making of raw glass
		[5] Glass: the making of an object
		[7] Raw glass: imported from the Near East?
		[8] Libyan Desert Glass
		[10] Sahara-Prehistory and history of the Sahara, Volume 10
		[11] Das Tutanchamun-Pektoral
		Gemrocks: Ornamental & Curio Stones Compiled by R. V. Dietrich, Professor Emeritus
		Glas (in German)
		Glass Beads, "Fabulous Fakes" and the Birth of Costume Jewelry

	Feedback: please report broken links, mistakes - factual or otherwise, etc. to me. thanks.