Desert-amethyst transmission

This is an old glass insulator taken from a collapsed telegraph pole in the Australian desert. Being subjected to intense UV light from the Sun over many years, 'colour centres' (defects) are created in the glass (containing iron and manganese) to produce the purple (amethyst-like) colour. For me, this is a highly-valued object and it was very hard to find one!
Manganese used to be added to the glass to counteract (decolourise) the green tint from the iron impurities. The colour centres can be destroyed by heating the glass to several hundred degrees C. In self-darkening spectacles however, the colour centres created by exposure to sunlight will remove themselves in the dark at room temperature.

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