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THE APPLICATION AND CHEMICAL COMPOSITION OF SODA LIME GLASS IN
ANCIENT EGYPT
Jack G. Morris
DO NOT CITE IN ANY CONTEXT WITHOUT PERMISSION OF THE AUTHOR(S)
Jack Galen Morris, Department of Anthropology, University of Arizona, Tucson, AZ 85719
Abstract
Glass is a material that has been widely used for cultural and utilitarian purposes in Egypt
since the turn of the 5th and 4th centuries BCE. The appearance of glass in ancient Egypt marked
a new cultural art form, as well as the development of innovative, practical applications and
advancements in chemistry. Glass manufacturing in ancient Egypt was crafted by liquefying an
amalgamation of an alkali (soda) and silica and was employed in the manufacture of amulets,
figures and vessels. Oftentimes it was used as a replacement for semi-precious stones such as
lapis lazuli, jasper and feldspar. The methods of glass production in the Pharaoic period can be
classified into two broad categories: core forming and glass casting. The vessels fashioned by
core forming were generally small, usually only a few centimeters tall, and were primarily
utilized for valuable substances such as unguents (topical healing ointment). Cast glass included
sculptures in addition to small amulets and other decorations. Development of glass
manufacturing for cultural purposes exhibits large advancements in chemical technology and
permitted the creation of antiquities that are well preserved through time.
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Archeology of Glass
The first items of glass appear in the archeological record of Egypt during the reign of
Thutmose III (Lilyquist and Brill 1993). The birthplace of glass manufacturing is still debated by
many, but evidence shows that glass originated in either Egypt or Mesopotamia. The earliest
datable glass possibly originated in Mesopotamia in the last half of the 16th century BCE, thus it
is expected that glass production originated there as well. Although glass has origins in
Mesopotamia, glass making and design arguably reached its apex in creativity and chemical
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innovation in Egypt. It is not entirely clear how glassmaking became active in Egypt, but
glassmakers brought to the country as captives from the Near East and Mesopotamia may have
initiated its creation (Petrie 1925). These workers perhaps would have been incorporated among
Egyptian workers. Existing Egyptian faience makers could be taught to work with glass
relatively easily, since faience shares similar properties and technological material with glass
(Oppenheim 1973). It is clear that faience and glassworking endeavors occurred in close
proximity to each other from data and samples collected at Amarna (Petrie 1894).
The manufacture of glass items in Egypt can be separated into two distinct stages:
glassmaking and glassworking. Glassmaking is the manufacture of glass from raw materials,
while glassworking is the process in which raw glass is heated and shaped into finished objects.
These two processes essentially could have been accomplished in the same location, but from
archeological excavations it seems likely that they were split into two different sites (Freestone et
al. 2002). No glass factories have been discovered in Mesopotamia, however there are a few that
existed in Egypt. Initial glass production and/or glass working industrial units have been
recognized at the sites of Malqata, Amarna, El-Lisht, and Qantir (Shortland 2009). These sites
play a pivotal role in determining the use of glass and its production. These sites also pose
interesting issues regarding the stratification of laborers in the manufacture of raw glass and the
assembly of glass objects. Although these jobs can essentially be done in parallel at these sites,
we see a distinction between the locations of these processes from artifacts uncovered at the sites
(Keller 1983). It is possible that these craftsmen had different statuses in society and did not
work together, separating the chemists from the artists (Meskell 1999). Specialization plays an
important role in Egyptian society and this may be the case here. The expertise of chemicals used
in glassmaking may be beneficial for the artist to better understand his medium, but with one
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group focused on the manufacture of raw materials, and the other focused on the creation of the
form and function of the actual glass piece, a more streamlined production of glass items was
accomplished. Interpretations of glass on a large scale of production are also supported by the
fourteenth century BCE Uluburun shipwreck. The ship’s cargo included more than 175 glass
ingots and judging from their chemical composition, many of them were crafted in Egypt
(Rehren 2005). The large number of glass objects being exported alludes to glass production on a
large scale in Egypt, to a point where glass was no longer created only for ritualistic purposes
and for leaders, but also for exportation and trade. These remains of glass manufacture exhibit
huge advancements in chemistry and art, as well as pose an interesting subject as to why these
glass objects were created.
Raw Glass
Glass in the ancient world was created by melting a mixture of an alkali (potash or soda)
and silica that consists of raw materials such as quartz cobbles and sand. The chemical reaction
of the heated soda and the sand forms a translucent flowing liquid that the ancients permitted to
cool to form glass (Freestone 2001). Glass is a non-crystalline substance that is essentially a
super-cooled liquid and not a solid. Glass is distinguished as such because of its capacity to
liquefy at a much lower temperature than that required in its initial manufacture (Saitowitz 1996.
Alkalis were employed in ancient times as flux (Freestone 2001). The key chemical elements of
glass are: silica or sand (SiO2), sodium oxide (Na2O) or potassium oxide (K2O) to be utilized as
a fluxing or alkali agent in order to reduce the melting temperature of silica, and calcium oxide
(CaO) from lime. Adulterations within the silica include: alumina (A12O3), copper (Cu), iron
(Fe2O3) and magnesium (MgO). The fluxing agents listed above may have also contained traces
of chlorite, phosphoric oxide and sulphate (Saitowitz 1996). The predominant types of early
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glasses were soda-lime-silica and potash-lime-silica glasses. The raw materials utilized by
Ancient Egyptian glasssmakers would have gone through elaborate but necessary cleansing
processes before they were put to use. These procedures would have probably incorporated
practices of screening, washing and burning the raw glass to remove coarse particulates, organic
matter and other impurities (Saitowitz 1996). The key chemical material in the creation of raw
glass is silica, which can be accounted for in ancient Egypt by the record of quartzite pebbles
thought to be used in ancient glass manufacturing (Shortland 2000). With a melting temperature
of around 1700°C, these pebbles have a very high melting point that is difficult to accomplish
with ancient furnaces. A plant ash flux is added to the silica to begin the process of making glass.
This plant ash flux decreases the temperature for the production of glass to around 1100°C,
which was attainable in an ancient furnace (Turner 1954). In addition to this raw, clear glass, the
use of minerals and chemical elements in glass production are exemplified by the wide variation
in color achieved by the Egyptians.
The dark amethyst-colored glass of the 18th (1550 - 1069 BC) and 20th dynasties (1186 -
1069 BC) achieves this color with a manganese compound, of which 0.5 - 0.7% has been
analyzed as manganese oxide (Lucas and Harris 1962). Manganese compounds within the glass
experience a chemical alteration, brought about by sunlight, and not caused by heat or radio-
activity, although the latter generates a similar coloration also. The coloring of the Ancient
Egyptian black glass was produced by three elements varying in proportion to each other,
creating different visual effects: copper, manganese, and iron (Lucas and Harris 1962). While
opacified black glass was surely contrived in Egypt at a later date, the earlier black glass was due
to the use of materials that contained a large proportion of impurities such as iron compounds
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(Lucas & Harris 1962). Nearly all of the glass in Egypt was a color commonly opacified with
other chemicals and minerals.
In addition to the black colored glass, light and medium shades of blue glass were created
from copper in the form of bronze or copper scale, which was supplementary to the silica slag.
Lead isotopic examination suggests that the copper used in the color augmentation has the same
source as the copper used in Egyptian tools and weapons (Lilyquist and Brill 1993; Shortland et
al. 2000). Darker shades of blue were achieved with a cobalt colorant. Cobalt is an element that
contains a specific pattern of trace element impurities (high alumina, manganese, nickel, and
zinc), which has facilitated the colorant to be traced to cobalt bearing alums of the Western
Oases of Egypt (Kaczmarczyk 1986). Shades of blue are the most frequently used colors in the
color palette of Egyptian glass and often form the body of the core formed vessels.
On the other hand, white opacificiation can be achieved with calcium antimonite that is
used in the production of white colored glass. Adding antimony to the glass melt and allowing it
to cool serve as the basis of formation of this glass. The origin of the antimony is unknown, but it
is an uncommon element there is evidence suggesting that the source could be as far away as the
Caucasus (Shortland 2002). A kind of lead antimonate opacifier has been recognized in yellow
glasses (Lilyquist and Brill 1993). Again, the Caucasus may be the source of the antimony, but it
is also probable that the lead comes from neighboring Egyptian mines, particularly Gebel Zeit on
the Red Sea coast (Shortland et al. 2000). Combining blue glasses and these opacifiers gives
opaque blue and green colors. Other colors in glass are pink, purple, and black (all colored with
manganese of an unknown source) and red, which also use copper as a coloring resource
(Shortland and Eremin 2006).
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Glass Use
The artifacts recovered from sites in Egypt suggest that the production of glass was largely
used for ritualistic purposes and favored by royalty. Others suggest that the actual making of raw
glass might have been under state control at least until the end of the 18th Dynasty (Nicholson
2007). Glass in ancient Egypt appeared in the New Kingdom. Ancient glass was a novel and
highly prized material, which quickly found favor with the elite. The first known glass sculpture
depicted the Egyptian ruler Amenhotep (Nicholson 2011). Individuals of lower status gradually
gained increasing access to glass, which should not be surprising. As the material became more
common, it was used for a greater range of items and spread beyond the upper echelons of
society.
Glass appears to have been regarded as an addition of faience and possibly as an extension
and replacement of work with semiprecious stones such as turquoise, lapis lazuli, and green
feldspar. Processed glass may be in the form of ingots, newly made from the raw materials
(possibly from recycled materials) or in the form of scrap glass, known as cullet. However,
because vessel glass was frequently polychrome, it would be difficult to recycle, unless vessels
and other products were first divided by color. Further support for the idea that glass followed
the traditions established by faience makers may come from the way in which glass first arrived
in Egypt. Petrie (1925) and Oppenheim (1973) consider that glassmakers brought to the country
as prisoners from the Near East may have established its manufacture. If they were brought to
Egypt to establish a new industry, they would be incorporated amongst specialists who worked
on material that shared some of the properties and technology of glass such as the producer of
the artificial precious stones of faience. Glass shares similar properties to faience and it would
have been easy for the faience makers to learn to work with glass (Oppenheim 1973).
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A team of archeologists has discovered other evidence for the use of glass among the elite.
Glass use is depicted in the paintings in the tombs of Rekhmire and Menna, which both contain
glass vessels (Shortland 2000). Objects similar to these vessels have not often been identified in
Egyptian art. Two glass vessels are also depicted in tribute scenes, being offered by chiefs from
Syria. Its date is correlated with the archeological evidence of glass arriving in Egypt and could
be telling of the arrival of the first glass in Egypt (Shortland 2000). Two different fragments are
the remains of glass vessels found in the disturbed remains of the tomb of Tuthmosis I, however
it is possible that these vessels could have been supplementary to a reburial of this king during
the supremacy of Hatshepsut or Tuthmosis III. Three glass vessels were also found in Tuthmosis
III's tomb and two more vessels, along with a large amount of glass beads and inlays, were
discovered in the tomb of his foreign wives in the Wadi Qirud (Shortland 2000). Other unaltered
vessels are inscribed with Tuthmosis III’s prenomen Mn-hpr-r’ and hence dated to his reign; still
others are dated on stylistic grounds, making a total of twelve vessels that have been more or less
securely dated to his reign (Shortland 2000). These artifacts all give significant insight to when
glass was first introduced to Egypt. Similar styles of glass have been found in these tombs but
from the amount of vessels found it is likely that they all are from relatively the same time
period. Although they appear in earlier leaders’ tombs, the evidence supporting reburial is a
strong indication that glass was introduced at a later time.
Amarna
A glass site dating back to Middle Egypt in the late 19th century has been found by a team
of archeologists at Amarna (Nicholson 1995). Glass processing facilities have been found at the
southern end of the city, in the midst of the poorer quality housing. Two kilns 2 meters in length
were discovered there which display a more technologically advanced and more efficient glass
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site. The kilns were described as thick walled and highly vitrified, with a disposable, and
frequently replaced lining. These kilns are also related to a large amount of “khorfush”, the local
word for black ‘slag’ (Nicholson 1995). Associated with the site were frit, melted glass, glass
rods, and remains of cylindrical vessels. All of this strongly suggests that this site was used for
the manufacture of vitreous materials although not necessarily glassmaking (Nicholson 1995).
From the frit and melted glass we can assume that this site was similar to a glassmaking site, but
it seems more likely that this was a glassworking site. The rods and remains of vessels seem like
clear indications of a site being used for glassworking and not the production of raw materials.
The vessels would probably not be present at a glassmaking site because they would be the result
of an artisan’s work after having received the raw materials from elsewhere and serve no
utilitarian purpose to a glassmaker.
El-Lisht
In addition to Amarna, a team of archeologists has uncovered glass crucibles and slags at
El-Lisht. They have also discovered glassworking fragments in the form of rods, drips, and
wasters, and a single large glass “ingot.” The factory looks as if it was producing glass beads,
rings, pendants, and inlays, which also accounts for a glassworking site (Keller 1983). The ingot
here is enough evidence to rule out the possibility of this site being used solely for glassmaking.
There is also little evidence of raw chemicals that were used in glass production. Unfortunately,
many of these findings were not retained, making it difficult to interpret the function of the site,
but the initial findings do suggest a workplace for accessories and jewelry rather than chemical
practices (Keller 1983).
Malqata
The site of Malqata on the west bank of the Nile at Thebes was explored by the
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Metropolitan Museum of Art’s Egyptian Expedition between 1910 and 1921 (Keller 1983).
Inside the workmen’s quarter of a large palace compound built by Amenhotep III, the earliest
substantiation for a glassmaking or glassworking site in the world was discovered. The
excavators verify findings of crucible and glass slag but the objects themselves were not
preserved by the museum and are consequently not accessible for modern study. Glassworking
debris, such as rods, drips, and trails were also found at the site (Keller 1983).
Qantir
A string of glass workshops have been hypothesized at Qantir-Pi-Ramesse in the
Eastern Nile Delta dating to 1250 - 1200 BCE (Rehren and Pusch 1997, 2005). This site is
dissimilar to the previous sites described above in that it has moderately little glassworking
debris. Rather, at this site there are many cylindrical vessels and glass-coloring crucibles. There
has been no equivalent found in size or complexity. Thus far, about 1100 fragments have been
recovered, representing a minimum of 250 to 300 vessels (Rehren and Pusch 2005). One of these
crucibles, was packed with a deeply corroded block of raw glass, which seems to characterize a
glassmaking charge that was forsaken before the batch items had fused entirely, conserving
much of the original raw glass inside (Rehren and Pusch 2005). The site appears to have focused
in the manufacture of red glass, a color that is very unusual at the other glass sites mentioned.
This site is very good evidence that there were multiple sites being used to produce raw glass
with the use of crucibles and furnaces. From the evidence of the red glass, we can speculate that
there were similar sites that specialized in other glass colorations, further supporting the theory
of a division of labor. Not only is there a specialization in the production of raw glass versus
glassworking, but now we see evidence of specialization of colored glass as well. This may have
been due to the availability of chemicals and other minerals used in the production of glass,
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where one area could more easily produce one color than another. This is also substantiated by
lead being available in the Egyptian mines, as previously noted. There is a distinction to be
drawn between glassmaking factories and glassworking areas. Malqata, Amarna, and el-Lisht all
contain considerable amounts of glassworking debris, so this is what was obviously going on
here.
At Amarna, it is clear that faience and glassmaking/glassworking activities went on in
close proximity to one another, a finding confirmed by the recent work by the Egypt Exploration
Society (Nicholson 2007). Part of this technological link is probably the use of heat in the final
stage of production. In order to make useful objects, raw glass would have been reheated and
cast, probably into blocks with the estimated shape of the preferred object. The glass would then
have been annealed, an indispensable procedure in glassworking. Annealing consists of the slow
cooling of the glass object. This cooling procedure allows the stresses formed in the hot glass to
be slowly relieved so that the cooled object will not shatter. A piece of glass, which is put aside
to cool in the workshop, would quickly crack or explode. Annealing may take place in a chamber
to the side of, or above, the main furnace or might be carried out in a separate structure. It may
take several days to anneal large pieces (Nicholson 2011).
Glassworking Practices
In early glass production, once the glass was annealed it was worked cold. The center of a
vessel might have been drilled out using a bow drill, most likely with a copper cylinder
(Nicholson 2011). This process would have to be carried out very carefully because glass, like
other siliceous materials such as flint, will fracture conchoidally. Simple glassworking practices
may have included gathering a small blob of glass from the furnace and then piercing it to form a
bead. This could also be achieved more easily by gathering the glass around a rod or mandrel so
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that it would solidify with a hole in its center. The shape of the bead could be manipulated while
molten with tools and by rolling it on a flat surface known as a marver. Beads of spherical,
cylindrical, or faceted shapes could be produced in this way (Nicholson 2011).
Hot threads of molten glass were drawn out from the furnace, allowed to solidify, and
then gently reheated to be shaped (Nicholson 2011). In this way, the simple jewelry of the New
Kingdom might have been produced, along with items such as applicator rods for kohl vessels
and similar straightforward/plain pieces. Like all glass objects, once shaped and, in the case of
beads, removed from their rod, they would need to be annealed (Nicolson 2011). A process
known as core forming was the most widely used method for producing glass vessels (Fig. 1)
from the New Kingdom. It was formed around a handling rod, which allowed the piece to be
manipulated. This core was then dried and coated with glass. The exact means of coating has
been subject to much debate, with some researchers suggesting that the core was rolled in
powdered glass (Schlick-Nolte 2002).
Figure 1. Ancient Egyptian Kohl Vessel. It is believed to have been made by core-forming.
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The ancient Egyptians were extremely skilled craftsmen and chemists. This is
exemplified in the broad variety of colorant usage and chemical experimentation. Evidence
found for chemicals with distant origins displays the Egyptians desire to create this glass and the
lengths they would go to in order to secure its continuation. The process to make glass involves
very precise measurements of chemicals and temperature control for production. The amount of
innovation and advancement in chemistry is astounding and whether or not the Egyptians
discovered glass, they surely perfected its production. The artifacts discovered in the sites in
Egypt contribute to our understanding of glass used in ancient types, as well as how these
chemical advancements helped to establish a new industry that has succeeded for generations.
The purpose of glass objects has already been mentioned, as objects of individual decoration
such as inlays, and containers. For small items of adornment the use of glass was basically
indistinguishable from faience work and many types of stone, used as beads or amulets whose
color and profile had specific symbolic or ornamental significance. With glass’ first appearance,
it was likely that it was regarded highly being particularly attractive and sharing very few
properties with its chemical ingredients making it unique. The characteristics of this rare new
material may have given early glass a rank above that of faience, making it a reputable product
destined for use by the upper echelons of society and the social elite. Glass appears to have been
considered with royal approval as an innovative and mesmerizing product.
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