Abstract
The chemistry of the magic ingredient in this ancient glass is no longer a mystery. The red colour of gold-ruby glass is caused by small particles of metallic gold that form when the gold-containing colourless glass is annealed. But the chemical state of the gold before striking red has been a mystery — is it dissolved in the glass as individual neutral gold atoms, like a frozen-in metal vapour that precipitates on annealing1, or as gold cations that must be reduced before the metallic clusters can grow and impart the lustrous colour2,3? Here we use Mossbauer spectroscopy to show that this gold is monovalent in the colourless glass, forming linear bonds to two neighbouring oxygen atoms.
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