It should be noted that there are very few convincing explanations for the unusual composition of cobalt-blue glass, with respect to other colours. Furthermore, there are very few attempts to explain the disappearance of glass from the archaeological record at the end of the LBA. What has been shown here is that the cobalt-blue colourant connects these two issues. Furthermore, the exhaustion of easily accessible silver-bearing ores, thereby requiring the expeditious exploitation of less accessible silver sources, potentially resulted in the development of a system where the by-product was also a valuable commodity. Cobalt frit as a transportable and valued commodity is verified to some degree in the illustration in the Annals of Thutmosis III at Karnak (Figure 1). Here, the king lists glass after gold and silver, suggesting its importance (Nolte 1968, 12-13). Some of the glass is seen as circular pieces of fairly consistent size, perhaps ingots, while other pieces are shown as irregular lumps. Sixty kilograms of the dark blue glass is represented as ingots, with an additional 55 kilograms appearing as lumps, for a total of 115 kilograms (Bianchi 2002, 20). This could suggest that two forms of dark blue glass were travelling: coloured ingots and concentrated vitreous colourants (i.e. frit). Furthermore, the fact that the Hurrian and Akkadian words ehlipakku and mekku, recorded in the Amarna letters regarding the 'stones-of-casting', were both used to denote glass (Oppenheim 1973), not only suggest that New Kingdom glass was a foreign import, but may also hint at the two distinct forms that were imported to Egypt (possibly as tribute), along with other raw materials including lapis lazuli, malachite and silver, from an unknown land after Thutmosis III's campaigns into the Near East from the middle of the 15th century BC. This unknown land may have been in modern-day Iran, where native silver ores in association with cobalt were exploited, exported and exhausted within the time frame of the New Kingdom.
The repercussions of the cobalt colourant for ancient glass deriving from the Near East will clearly have ramifications on how ancient trade networks worked. The glass found at Mycenae and on the Uluburun, for example, may have been made with base glasses from Egypt, but the cobalt required to provide value to this glass required interactions with those regions exploiting five-element ores for both silver and cobalt. As the five-element ores in Iran appear to have a compositional signature consistent with the elements found in ancient glass of both Egypt and Mesopotamia, this region must therefore re-emerge as a potential source for the concentrated cobalt frit.
The transition from native silver to argentiferous lead sources reflects the exhaustion of easily extracted silver, which in turn affected the production of cobalt-blue frit in the form of slag. The exploitation and exhaustion of these cobaltiferous silver-bearing ores would have been sandwiched between the exploitation of easily extracted silver from native silver and dry silver sources and those requiring the smelting and refining of silver from argentiferous lead. This suggests a short flowering of these sources before they were exhausted, in a similar fashion to the sudden arrival and departure of cobalt glass in the New Kingdom period in Egypt, and its limited presence in the archaeological record from Mesopotamia. In essence, the depletion of cobalt resulted in a decrease in glass production and an increase in recycling after the 18th Dynasty until both cobalt and glass almost completely disappear around 1250 BC.
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