The lump of blue glass found by H. R. Hall at Eridu in 1919 is well known to students of early glass. From the circumstances of the excavation Hall deduced that the piece was made at least as early as 2000 B.C. and this view is generally accepted.
The piece has been fully discussed, with other pieces of glass made before 1500 B.C., by H. C. Beck. He considers that the piece was a manufacturer's piece of material made very near to where it was found, and so is evidence of a glass factory in Mesopotamia much earlier than any previously known, the earliest factory in Egypt being about 1000 years later. The Eridu glass supports the view, held by some authorities, that glass making was introduced into Egypt from Mesopotamia.
The cause of the blue colour of early Egyptian glass was the subject of controversy for many years and it was not until a systematic series of tests were made by Farnsworth and Ritchie in 1937 that the question was finally cleared up. They examined 58 pieces of blue glass of the XVIII Dynasty and showed that the pieces of bluish green colour were invariably coloured by copper and those of a pure blue or violet blue were invariably coloured by cobalt. The amount of cobalt required to give a deep colour is very small, less than 0.2 per cent.
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