Inspired by our visit to the special exhibition “Ancient Glass” at the Allard Pierson Museum in Amsterdam, we decided that for our 11th episode, we wanted to learn more about glass.
Naturally occurring glass, such as volcanic glass or obsidian, was used as early as the Stone Age. But what about man-made glass?
One question that came up was: What is glass, and how is it made?
Making Glass: from Sand to Object
To make glass, three main things are needed: the vitrier, the flux and the stabiliser.
The vitrier provides the body and is usually silica (sand). The flux is needed to lower the melting temperature of the silica, which means temperatures need to be high, but not as high as inside a volcano. Ancient glassmakers used sodic plant ash or mineral natron. The stabiliser is in the name. It makes the glass durable so it does not dissolve in water. The classic one is lime.
Origin of glassmaking
Glassmaking, following the ingredients mentioned above, goes back to Mesopotamia more than 3600 years ago. There is evidence that glass technology started in India as early as 1730 BC. Most likely, Phoenician traders brought perfume vessels that were core-formed during the Iron Age to the rest of the Mediterranean. During the Hellenistic period, mosaic glass became popular, where small pieces are put into creative designs and patterns and are then fused together.
There are different ways of making glass: 1. Casting: placing the liquid glass in an open mould; 2. Core forming: a core of mud is covered with glass, and after the glass has dried, the core is removed; 3. Glassblowing.
Glassmaking recipe
There are several cuneiform tablets which provide us with recipes on how to make glass. One such example is a tablet which is now in the British Museum. It came to the museum in 1929.
The clay tablet with Akkadian writing is perfectly preserved. It’s 8.25cm x 5.23cm. According to an article from 1936 (see below), the tablet was found in Tell Umar /Tall ‘Umar, which was confirmed by a “trustworthy vendor”. The site is located in today’s Iraq.
In 1936, the tablet contained the “earliest record known of the actual formulae for the making of glazes.” This specific recipe refers to making red glass. Allegedly, written in some kind of code, his specific recipe refers to making red glass. “Written in a slightly obscure style so as to be understood only by skilled craftsmen” (British Museum website) and “the writer, guarding his secrets with true professional jealousy [...] has purposely disguised his meaning by artifices of writing which amount to a form of cryptography” (Gadd & Campbell Thompson, 87). How accurate this assumption is is doubtful, as we modern readers are able to decipher it.
A short excerpt of the translation (taken from Gadd & Campbell Thompson):
(1) To a mina of zukû-glass (thou shalt add) 10 shekels of lead (2) 15 shekels of copper, half (a shekel) of saltpetre, half (a shekel) of lime: (3) thou shalt put (it) down into the kiln, (and) shalt take out ‘copper of lead’
(4) To a mina of zukû-glass (thou shalt add) 1/6th (mina = 10 shekels) of lead (5) 14 (shekels) of copper, 2 shekels of lime, a shekel of saltpetre: (6) thou shalt put (it) down into the kiln, (and) shalt take out “Akkadian copper”
Mina and shekels are measurements; one mina is roughly 500g, and 1 shekel is ca. 8.3g.
While the last lines of the tablet seem to provide a date, referring to the reign of Gulkishar, the Sixth Sealand Dynasty King who lived in the 16th century BC, it seems more likely that the recipe was written down in clay between the 14th and 16th century BC (Oppenheim and Thavapalan).
Glassblowing
The consensus seems to be that glassblowing originated in Syria in the areas of “Sidon, Aleppo, Hama, and Palmyra in the 1st century BC” (Britannica), from where the vessels for everyday and luxury use were exported into the known world.
The process has not changed much over time: “the molten glass is gathered on the end of a hollow pipe, inflated to a bubble and formed into a vessel by blowing, swinging, or rolling on a smooth stone or iron surface.”
In 2023, UNESCO added the “knowledge, craft and skills of handmade glass production on the Representative List of the Intangible Cultural Heritage of Humanity.” Adding glassblowing to this list will help to preserve it for future generations. Here, experimental archaeology plays an important role: The Borg Furnace Project (Glasofenprojekt Römische Villa Borg) is one example.
Types of Glass
Medieval Glass Myths and Crown Glass
A longstanding, popular story during the Middle Ages was that medieval window glass is thicker at the bottom because glass slowly flows like a liquid under gravity. According to the tale, the panes were originally uniform, but over the centuries, the glass supposedly ‘poured downward’, creating the thicker lower edge that can still be observed in many old buildings and cathedrals (see middle image).
However, the variation in thickness has a different explanation. First, the glassblower created a hollow sphere of molten glass. The sphere was then reheated and spun rapidly, causing centrifugal forces to flatten it into a circular disk and thereby creating the “bull’s eye” or so-called crown glass. The production process was first described in detail by Johannes Mathesius in 1562 in his “sermon of glassmaking.”
When glaziers then installed the panes in a window frame, they typically placed the thicker edge downward. This made the pane more stable and reduced the risk of breakage. At first, the plates were placed in even rows (bottom image) as it can be seen in the painting by Jan van Eyck, Annunciation from 1436/37. Later, the plates were staggered (see upper image).
This type of glass window was used in Europe since the 14th century, with the centre being in Normandy “where a few glassblowers monopolized the trade and enjoyed a kind of aristocratic status.” The earliest document mentioning this type of glass is from 1330: Philipp V of France grants permission to Philippe de Cacqueray, Sieur de Saint Immes, to erect a building for making this type of glass.
Crown glass from Masada
Crown glass was already known in antiquity, as the example from Masada will show.
Masada was located on a flat-topped hill (450m) in the Judean Desert, west of the Dead Sea and 62km south of Jericho. The city was first occupied by the Hasmonean kings of Judea, who ruled from 140 BC to 37 BC. However, the city was rebuilt by King Herod the Great from 37 BC until Herod’s death in 4 BC, using material and craftsmanship from Rome.
During the excavations in 1963 and 1965 by the Hebrew University of Jerusalem and the Israel Exploration Society, 7 partly restorable round glass windows, blown with the crown glass technique, were uncovered in Masada. Four come from the large bath house, and the three from the entrance rooms. They vary in diameter (37cm to 43cm) and thickness (1.8mm to 3.7mm). The exact function of the crown glass windows is unknown because the ceilings in all the contexts did not survive. Most likely, there was an oculus somewhere in the caldarium, but it is not confirmed.
Interestingly, it is not clear whether the glass was ordered to fit the windows or whether the window was built to fit the glass panel. Likely, the glass windows were not made in Masada because the “fabrication of such a quantity of melt requires special facilities”, which did not exist. But Herod was a client king of the Romans, and would therefore have had access to craftsmen and material from Rome. The glass shards found during the excavations are dated to the reign of Herod because there were no other major reconstructions in later periods of the city. This type of glass is very fragile, and the only reason it survived in Masada was because of an earthquake that buried them.
(Hollow) Glass Vessels
These are probably the objects we think of when thinking of glass. Glass objects can be anything from drinking glasses, vases, salad bowls, glass bottles or even bananas. Most of the objects on display in the Allard Pierson Glass exhibition are glass. We both brought one example of a glass object, but of course, there are many more to discover.
Oldest dateable glass object?
For a long time, the glass chalice was considered the oldest dateable glass object. However, with research continuing, there may be other objects that carry this title. Beads mentioning Queen Hatshepsut (1473-1458 BC) were discovered (Nicholson, p.11), which could be interpreted as the oldest dateable objects.
Nevertheless, the chalice is a fascinating artefact. The chalice has the shape of a lotus. It is light blue with dark blue and yellow thread like decoration. It’s ca. 8cm tall and has a diameter of 6cm. The coloured rods are embedded in the body of the glass (are not a mosaic); the same goes for the cartouche with the pharaoh’s name.
It was bought in 1825 by the English archaeologist and collector Edward Dodwell who lived in Rome at that point and had decided to expand his collection. Dodwell died in 1832 and the chalice “passed into the possession of Munich” (Newberry 154). But there are conflicting information when the item came to Munich. The Bavarikon website (see below) states that the object was bought in 1830 by Ludwig I and Nicholson (15) states that the chalice was part of the “Dodwell collection and was purchased in 1832, probably at Thebes.” The artefact is now in the Staatliches Museum Ägyptischer Kunst in Munich.
The date of the object comes from the name of the pharaoh embedded on the surface. It refers to Thutmose III who lived frim 1481 to 1425 BC. This led egyptologists to the conclusion that the artefact is from around 1450 BC.
Glass cicada
During the Han Dynasty (206 BC - 220 AD) in China, jade was used in funerary practices, especially in creating suits (see upper image above). The burial suits were primarily used for emperors, princes, and high-ranking aristocrats due to jade being an expensive material. They are made of thousand of small plaques, shaped to follow the human body. The plaques are commonly made from bephrite jade and they can be square, rectangular, triangular, trapezoid or rhomboid plaques. Tiny holes are then drilled into the corners, allowing them to be stitched together. Jade was used because it was believed that it would protect the body and prevent decay. The suit formed part of a ritual system aimed at achieving immoritatlity or transformation after death because it was thought to absorb natural energy and repel evil spirits.
Jade was exremely valuable and scarce, so glass was sometimes used to imitate jade in funerary objects. One example are glass cicadas. They are called ‘imitation jade’ (仿玉) and could replicate the colour or appearance of jade. These substitutions indicate technological experimentation with glass during the Han Dynasty and attempts to replicate the symbolic value of jade when real jade was too expensive or not available.
It is important to note that most complete burial suits that were discovered in archaeological excavations were made from jade, not glass. Glass imitations usually appear in smaller ritual objects.
Recycling and Re-use
Broken glass or glass cullet (broken scrap glass) was seen as valuable as the raw material and not just considered trash. In 1331 in Marseille, it was actually illegal to export broken glass because local workshops needed it to make the melting process easier and cheaper. There is evidence of re-melting, mixed recipes, and composite chemical signatures. Glass was widely traded for re-melting and mixing.
One such evidence of trade can be seen in the image above. It shows raw blue glass indigos from a shipwreck. The ship sank during the Late Bronze Age (1330-1300 BC) near Uluburun (Kaş, Antalya Province, Turkey). It was excavated between 1984-1994 by the Institute of Nautical Archaeology under George F. Bass and later Cemal Pulak.
Approximately 175 raw glass ingots were recovered from the ship with a total weight of 350kg. The most common colour was cobalt blue but there are also turquoise, lavender/purple and amber ingots. They are flat and circular “bun-shaped” or truncated-cone ingots; they are between 10-15cm in diametre. The rough surface is due to cooling in moulds or shallow containers.The semi-finished raw material would then be used to produce objects such as beads, inlays, small vessels and jewellery.
The finds from the shipwreck are important because they confirm an important and extensive international trade network. The primary production of the raw glass found was most likely in Egypt (chemical compositions of trace-element signatures suggest Egypt) or the Levant with secondary workships across the Mediterranean.
Breaking and Throwing Away
After centuries in the ground, glass is not always clear. It can suffer from ‘pests’ or ‘decay’. While glass is chemically stable, it is physically fragile and therefore, can survice ‘perfectly’ as a fragment, but ‘badly’ as a whole object.
There are five diseases.
* “Rainbow Sickness” (Iridescence)
At first glance, it does not look like a disease but like a shimmering rainbow, almost like mother-of-pearl. The effect comes from ‘de-alkalization’ or ‘leaching’. This means that the water in the soil where the object was found acts like a sponge, pulling out the soluble alkalis (e.g. sodium and potassium) to leave behind a brittle, silica-rich skeleton. The process creates microscopic, flaky layers that reflect light and create that famous iridescent shine.
* The ‘Pox’ (Pitting and Craters)
The surface of the glass loks like the surface of the moon: full of dull, milky holes and deep craters. This happens when the glass suffers from network dissolution, often driven by highly acidic or alkaline groundwater. Uneven surfaces are more likely to be affected as they decay faster. If the ancient glassblower left ridges or valleys on the vessel, the corrosion factors attack those areas logarithmically faster than sleek, even surfaces
* The “Bug” Attack (Biocorrosion)
Glass beads show weird, alien-looking patterns like concentric circles, dark bands, and tiny, perfectly circular holes. Microorganisms in the burial environment actually interact with and deteriorate the glass surface over time. Bugs can ‘eat’ yourglass windows if left buried long enough.
* “The Dark Stains” (Magnanese Browning)
The glass develops dark, brownish-black spots that look almost like a bad skin rash or mold. Ancient glassmakers often added manganese to the glass batch to act as a decolorizer (to make it clear). But after centuries in wet soil, that manganese oxidizes and precipitates into dark inclusions right inside the leached layers of the glass.
* The “Total” Collapse” (Enemal Weathering)
The glass turns completely opaque white or yellow, looking more like a piece of chalk or thick enamel. This is end-stage glass disease which means that the glass has lost all cohesion and its entire internal structure is gone. It is just a fragile ghost of an object held together by the dirt around it.
Image references
-Cuneiform tablet with red glass recipe: https://www.britishmuseum.org/collection/object/W_1929-0715-1
-Transcription of cuneiform tablet: C.J.Gadd & R. Campbell Thompson, A Middle-Babylonian Chemical Text. Iraq 3.1. 1936. 87-96. https://www.jstor.org/stable/4241587
-Chalice with Thumoses III’s name in a cartouche: https://www.bavarikon.de/object/bav:SMA-DDD-0000000000000002?lang=en
-Han Dynasty Burial Suit: https://upload.wikimedia.org/wikipedia/commons/2/2f/Han_jade_burial_suit.JPG
-Glass cicada: https://glasscollection.cmog.org/objects/5231/funerary-object
-Hans Burgkmair the Elder, The Weisskunig in the Painter’s workshop: https://www.mfab.hu/artworks/13696/
-Jan van Eyck, Annunciation: https://www.nga.gov/artworks/46-annunciation
-Medieval crown glass: https://images.openai.com/static-rsc-4/55XR7LhBxF188D96TVzfNOxwfhg1q_5oUr5dQZVDVD-A4QJbQVd2tM5UetoRpSP9M4WB_XuYWoIG7lKXzG4NMfvVDeLbNbalVghcDXDvK3FtFF-mpcqt18ZVCePe2sxrzq7qsA9IxWpSMEberiiaXT5HrafoLJ8uJbJhs-ewH-NPYcT4NyyLoICqqcLLaXwF?purpose=inline
-Crown glass window from Caldarium, YY. Max, Crown Glass Windows from Masada, Journal of Glass Studies 62. 2020. 23-40. https://www.jstor.org/stable/26951071 , p.31
-Masada, aerial view: https://www.wanderlustmagazine.com/inspiration/masada-israel/
-Rainbow sickness fragments: https://bpb-eu-w2.wpmucdn.com/blogs.ucl.ac.uk/dist/9/186/files/2019/05/Capture1.jpg
-Raw glass indigos: https://www.livius.org/site/assets/files/66550/uluburun_wreck_glass_ingots_mus_bodrum.jpg
-Glass cicada: https://glasscollection.cmog.org/objects/5231/funerary-object
-Medieval Crown Glass: https://images.openai.com/static-rsc-4/55XR7LhBxF188D96TVzfNOxwfhg1q_5oUr5dQZVDVD-A4QJbQVd2tM5UetoRpSP9M4WB_XuYWoIG7lKXzG4NMfvVDeLbNbalVghcDXDvK3FtFF-mpcqt18ZVCePe2sxrzq7qsA9IxWpSMEberiiaXT5HrafoLJ8uJbJhs-ewH-NPYcT4NyyLoICqqcLLaXwF?purpose=inline
Bibliography
UNESCO https://ich.unesco.org/en/RL/knowledge-craft-and-skills-of-handmade-glass-production-01961
Britannica - Crown glass https://www.britannica.com/technology/crown-glass
Britannica - Glassblowing https://www.britannica.com/technology/glassblowing
RDK Labor - Butzenscheibe https://www.rdklabor.de/wiki/Butzenscheibe
History of Glass https://en.wikipedia.org/wiki/History_of_glass
Glass and Procedural Knowledge in Cuneiform Cultures https://historyofknowledge.hypotheses.org/10014
Roman Glass Makers https://www.romanglassmakers.de/p_katalog_frei.htm
Archeoglas - Glasofenexperiment https://archeoglas.glasofenexperiment.de/index-en.html
Corning Museum of Glass https://whatson.cmog.org/exhibitions-galleries/origins-glassmaking
Frank Wiesenberg, Das römische Glasofenprojekt im Archäologiepark Römische Villa Borg (“Borg Furnace Project”) -Rekonstruktion und erste Betriebsphasen https://archeoglas.glasofenexperiment.de/downloads/pdfs/books/Wiesenberg_2015_EXAR-Bilanz-14_-_Borg_Furnace_Project.pdf
C.J.Gadd & R. Campbell Thompson, A Middle-Babylonian Chemical Text. Iraq 3.1. 1936. 87-96. https://www.jstor.org/stable/4241587
S. Thavapalan, Keeping Alive Dead Knowledge: Middle Assyrian Glass recipes in the Yale Babylonian Collection, Journal of Cuneiform Studies 73. 2021. 135-178. https://www.jstor.org/stable/48743398?read-now=1&seq=1#page_scan_tab_contents
P. E. Newberry. A Glass Chalice of Tuthmosis III. Journal of Egyptian Archaeology 6.3. 1920. 155-160. https://www.jstor.org/stable/3853912
P. T. Nicholson, Glass Vessels from the Reign of Thutmose III and a Hitherto Unknown Glass Chalice. Journal of Glass Studies. 48. 2006. 11-21. https://www.jstor.org/stable/24191145
Y. Max, Crown Glass Windows from Masada, Journal of Glass Studies 62. 2020. 23-40. https://www.jstor.org/stable/26951071
Europeana - Iridescence - From archaeological glass to Art Nouveau https://www.europeana.eu/en/stories/iridescence-from-archaeological-glass-to-art-nouveau
K. H. A. Janssns (ed.), Modern Methods for Analysing Archaeological and Historical Glass, Band 1. 2013.
P. Bellendorf, H. Roemich, S. Gerlach, P. Mottner, E. López and K. Wittstadt, Archaeological Glass: The Surface and Beyond. 2010.
Credits
* Intro and outro music: “Meeting for Two – Background Music for Video Vlog (Hip Hop version, 43s)” via Pixabay Music by White_Records
* Editing and post-production: Jona Schlegel
* Cover art: Stefanie Ulrich
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