Chemical Analysis of Glass

One of the least well-known methods for analyzing glass beads is through the chemical composition of the glass.  Methods for glass chemical analysis have existed for almost a century, but until recently any beads used would be destroyed in the process.  As a result, very little analysis has been conducted on archaeological material in general.  With LA-ICP-MS analysis, however, the beads (and any other glass objects) need not be destroyed upon analysis.  Consequently, the popularity of glass analysis has greatly increased, but it is still a relatively new development in bead studies.

Components of Glass

There are six specific types of elements found in glass; formers, alkalis, secondary elements, colorants, opacifiers, and trace elements.  Each occurs in varying degrees, with the formers occurring the most and the trace elements the least.

Glass Formers

Glass formers are the largest component of glass, often ranging between 55 and 75% of the total weight.  The most common glass former is silica (Si), which comes from the sand or quartz pebbles used to make the glass.  Almost all the glass found in South and Southeast Asia is silica-based.  Another important glass former is lead (Pb), which, due to its high specific gravity, can account for up to 90% of the weight of ingredients.  The focus here is on silica-based glasses, but this does not mean that lead glasses should be ignored.

Alkalis

Silica melts at too high a temperature for ancient ovens to achieve, so an alkali was added to lower the melting point.  There are two dominant alkalis used in glass-making: sodium (Na) and potassium (K).  Lead (Pb) can also function as an alkali, but is much less common.   Most alkalis were a mixture of sodium and potassium, especially if the alkali was obtained from plant ashes.  Alkalis often compose 15 to 20% of the total weight of the ingredients.

Secondary Ingredients

Secondary ingredients, as far as the literature suggests, are most likely impurities in the sand and are introduced accidentally.  They comprise between 1 and 10% each of the total weight of ingredients (2009:587).  Secondary ingredients include calcium (Ca), aluminum (Al), iron (Fe), manganese (Mn), and magnesium (Mg).  Iron and manganese are often used as colorants, but it was not made clear in the literature how one might distinguish between uses based on the weight percentages.

Colorants

Metals are often added to the glass to create a wide range of colors.  These metals include iron (Fe), copper (Cu), cobalt (Co), and manganese (Mn).  Cobalt and manganese are powerful colorants and often impart hues of blue or a deep purple.  The use of iron or copper can produce almost every color.  The literature does not explain how the weights of iron and manganese as colorants would differ from their weights as secondary ingredients.

Opacifiers and Trace Elements

Opacifiers were often added to glass to change the diaphaneity .  These included arsenic (As), antimony (Sb), salt (NaCl), bone, and various fluorides.  Trace elements are any elements which are not purposefully added, or which constitute less than 1.0% of the total weight of ingredients.  The most common are titanium (Ti), gold (Au), silver (Ag), vanadium (V), and strontium (Sr).

Types of Glass

Chart of the various silica-based glass types discussed for South and Southeast Asia.

glass chem

There are nine general categories of glass discussed in the literature, some of which subdivide into further groups.    All of the groups discussed here are silica-based glass, which means that the differences lie in the alkalis, secondary ingredients, and trace elements, not the glass former itself.  These differences can be seen in the chart above.  Here, I will discuss each type, first by explaining the general alkali category, then each of the secondary ingredient categories, and finally the trace element categories if need be.

Soda Glass

The largest alkali group of silica-based glasses uses sodium (Na) as an alkali in the form of soda (Na2O).  There are two general types of soda that can be used in glass-making: mineral soda and plant-ash soda.  Mineral soda comes in the form of natron, which is a mixture of sodium salts in mineral form and is best known from Wadi Natrun in Egypt.  Plant-ash soda, however, comes from the ash of desert or coastal plants, and is often said to be the earliest used alkalis in glass-making (2006:135).  I will discuss each of these types in turn as well as their respective subdivisions.  The primary method of distinguishing between these types in the chemical analysis is that mineral soda glasses (m-Na) have less than 2.5% MgO while plant-ash soda glasses (v-Na) have more than 2.5% MgO.  Some argue for a lower percentage, such as 1.5%, but this study will use the 2.5% cut-off, since it is the most recent.

Mineral Soda Glass

There are two predominant types of mineral soda glasses: mineral soda lime glass (m-Na-Ca) and mineral soda glass with high aluminum content (m-Na-Al).  Both will be discussed here.

m-Na-Ca:  This glass is often associated with the Mediterranean, but recent studies have shown that the “Mediterranean” glass found in Asia differed in uranium (U) count from that actually found in the Mediterranean.  The Asian m-Na-Ca glasses also differed in that they were strongly colored.  The most common bead was a dark blue, which had been colored with cobalt.  These Asian m-Na-Ca beads were found in India and Southeast Asia and their use ends sometime after the 7th century AD.  Low numbers of such glass in South and Southeast Asia have cause many to believe that it was probably imported, but from somewhere other than the Mediterranean.

m-Na-Al: Mineral soda alumina glass is the most common glass type in South and Southeast Asia between the 4th century BC and the 10th century AD.  This glass is characterized by a high alumina content (> 6%).  Recently, chemists have distinguished between five subtypes of m-Na-Al glass, which are numbered 1 though 5.

m-Na-Al 1 glass is characterized by low uranium (U) and high barium (Ba) weight contents.  Colors include opaque red, green, yellow, black, white, orange, and turquoise blue.  No cobalt blue glass has been identified as m-Na-Al 1 glass so far, even if such glass was in demand in the area where m-Na-Al 1 glass was made.  Orange is a color only available in the m-Na-Al 1 glass type.  This type is associated with South (4th cen. BC – 5th cen. AD) and Southeast Asia (4th cen. BC – 10th cen. AD) and was used to make beads and bracelets.

m-Na-Al 2 glass is characterized by high uranium (U) and low barium (Ba).  Colors include opaque red, green, yellow, black, white, and translucent turquoise and cobalt blue.  There is a higher iron (Fe) content in red beads of this type than in other colors.  This glass is associates with Africa and the west coast of India from the 9th to 19th century AD and was used to make beads.

m-Na-Al 3 glass is characterized by a high uranium (U) and low barium (Ba), like m-Na-Al 2, but m-Na-Al 3 also has a high cesium (Cs) content.  Colors are limited to red, black, and also a transparent emerald green.  Red is made through the addition of copper (Cu) and black through the use of iron (Fe).  The green seems to be a natural color, as most glasses are naturally green, made darker by the presence of iron in the sand.  This glass is associated with Southeast Asia from the 4th to 3rd centuries BC and was used to make beads and bracelets.

m-Na-Al 4 glass is characterized by low calcium (Ca) concentrations, which would have been added through the use of lime.  This glass also has lower strontium (Sr) concentrations.  Colors include green, brown, dark blue, black, and yellow.  Coloring agents include copper (Cu, used in green, turquoise blue, and red), lead (Pb, used in green, yellow, white, and red), iron (Fe, used in black, red, and green), cobalt (Co, used in dark blue), and tin (Sn, used in yellow and white, serves as a white opacifier).  This glass is associated with Southeast Asia and Kenya and was used to make beads, bracelets, and vessels, though it seems that vessels of this type were more common than beads in Southeast Asia.  This glass was used between the 14th and 19th centuries AD.

m-Na-Al 5 glass is characterized by low barium (Ba), uranium (U), strontium (Sr) and zirconium (Zr).  Colors include opaque red or brownish, dark blue, and purple.  The red and brownish glasses contain copper (Cu), purple glass contains manganese (Mn), and the blue glass contains cobalt (Co), copper (Cu), zinc (Zn), and lead (Pb).  This glass is associates with Sardis, Turkey from the 12th to 14th centuries AD and was used to make bracelets, tesserae, crown glass, and raw glass.

Plant-ash Soda Glass

There are also two predominant types of plant-ash soda glasses: plant-ash soda lime glass (v-Na-Ca) and plant-ash soda lime glass with high aluminum content (v-Na-Ca-Al).

v-Na-Ca:  This glass is claimed as the earliest form of glass, which was first made in Mesopotamia and Egypt.  As such, they are now often called Islamic glasses and are found in Southeast Asia.  Most of these glasses are cobalt blue, with high cobalt (Co) and low manganese (Mn) content.  Trace element patterns suggest an Iranian source for the cobalt.  Both beads and raw glass have been found at many Asian sites. Trade may have been in raw glass, though, as many of the beads are drawn, showing South Asian technology.

v-Na-Ca-Al:  This glass is characterized by both high calcium (Ca) and high aluminum (Al) contents.  So far, this glass has only been found at Bara, Pakistan, but others in Pakistan and North India seem to have a similar composition.

Potash Glass

Potash glass is made with potash as an alkali rather than soda.  The exact source of potash is unknown, though a mineral source seems likely due to a very low Mg content in the observed groups.  There are three main types, all differing in their secondary ingredients: calcium (Ca) and aluminum (Al).  The first has a high calcium content, the second a high aluminum content, and the third has equal amounts of both.

m-K-Ca:  This glass is characterized by higher levels of calcium (Ca) and lower levels of aluminum (Al).  This group has been called “mKC-low A” in previous studies, but for our purposes, we will use similar notation to the other glass types and call it m-K-Ca.  Primarily found at Ban Don Ta Phet (Thailand) and Giong Ca Vo and Lang Vac (Vietnam), this glass dates to the last few centuries BC and is associated with Southeast Asia.  Many of the glasses are aqua or colorless, but others are various shades of green and blue.  No opaque red glass classifies as m-K-Ca type.  This glass has only one sample in India, at Hastinapur, but several examples come from contemporary sites Korea.

m-K-Al:  This glass is characterized by low calcium (Ca) and high aluminum (Al) contents.  This group has been called “mKA-low C” in previous studies, but again, for our purposes, we will use similar notation to the other types and call it m-K-Al.  Colors include green, blue, and blue-green.  Green was made with the addition of iron (Fe) and blue and blue-green with copper (Cu), though some blue specimens found in China were colored with cobalt (Co).  There are no opaque red samples in this glass type.  Glass of this type is associated with Vietnam, China, Korea, and Japan.  There is no evidence of this glass type at Ban Don Ta Phet, suggesting that it postdates the 3rd century BC, but few samples in Korea are found after the 3rd century AD.

m-K-Ca-Al: This group is distinguished by having both high calcium (Ca) and high aluminum (Al) contents, both averaging about 2%.  Most of these glasses are darker shades of blue and colored with cobalt (Co), though some opaque red has been found (2006:135).  Distribution and chronology for this glass were not given, but it seems present in South Asia, Southeast Asia, and Korea (130).  A subtype of this group is called m-K-Ca-Al-Na (mKCA-N in other studies) and is a potash glass with high calcium (Ca) and aluminum (Al) as well as a significantly higher weight percentage of soda (Na2O).  This subtype is not well-studied, but scholars suggest that the high soda content was caused by a mixing of the m-K-Ca-Al and the m-Na-Al glasses.  More research is needed on this subtype before a definite answer can be given.

Mixed Alkali Glass

The final type of glass discussed in this study is a mixed-alkali glass, or a glass with both soda (Na2O) and potash (K2O) above 5%.  This type also has higher levels of aluminum (Al).  Mixed-alkali glass is associated with southern India and Sri Lanka.  Colors are limited to opaque reds and oranges, which are colored with high levels of copper (Cu).  This glass overlaps largely with the m-Na-Al 1 type of glass save for the high weight percentage of potash (K2O).  For this reason, some scholars view this glass as a local variant of the m-Na-Al 1 glass.

Many other types of glass exist, both past and present, but these are the primary types discussed in regards to South and Southeast Asia.  One final type that is often mentioned is lead glasses, but to my knowledge these glasses have not been studied at length.  There are also various African and Middle Eastern glasses which may well be present in South and Southeast Asia, but again, these types have not been specified in the literature.

References:

Dussubieux, L., Kusimba, C., Gogte, V., Kusimba, S., Gratuze, B., and Oka, R. 2008. The Trading of Ancient Glass Beads: New Analytical Data from South ASian and East African Soda-Alumina Glass Beads. Archaeometry, 50, 797-821.

Dussubieux, L. and Gratuze, B. 2003. Non Destructive Characterization of Glass Beads: Application to the Study of Glass Trade between India and Southeast Asia. 9th International Conference of the European Association of Southeast Asian Archaeologists, Sigtuna, May 27th – June 2nd 2002, 135-148.

Francis, P. 2002. Asia’s Maritime Bead Trade: 300 BC to the Present. Honolulu: University of Hawai’i Press.

Kanungo, A. 2004. Glass Beads in Ancient India: An Ethnoarchaeological Approach. Oxford: BAR International Series 1242.

Lankton, J. and Dussubieux, L. 2006. Early Glass in Asian Maritime Trade: A Review and an Interpretation of Compositional Analyses. Journal of Glass Studies 48, 121-144.

Ramli, Z., Shuhaimi, N., and Rahman, N. 2009. Beads Trade in Peninsula Malaysia: Based on Archaeological Evidences. European Journal of Social Sciences 10, 585 – 593.

Robertshaw, P., Wood, M., Melchiorre, E., Popelka-Filcoff, R., and Glascock, M. 2010. Southern African Glass BEads: Chemistry, Glass Sources, and Patterns of Trade. Journal of Archaeological Science xxx, 1 – 15.

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About Heather Christie

Heather is an archaeologist, photographer, and writer whose research focuses on beads and bead trade, particularly in a maritime sense. She's currently working working on a PhD in Digital Design (focusing on heritage visualisation) at the Glasgow School of Art.
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5 Responses to Chemical Analysis of Glass

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