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-
