THE CLAUDE GLASS AND DR DEE
Dr John Dee's Magical Speculum or Scrying Glass was a wonder of Elizabethan London. Two museums in London hold black mirrors said to be connected with Dr Dee. The Science Museum has a "Claude glass believed to be John Dee's scrying mirror, Europe, undated" while the British Museum holds "Dr Dee's mirror 1300/1599", made of obsidian rock and of Aztec origin. I don't know if these institutions contest each other's claims in a rivalry like that between the Louvre and the National Gallery over who owns the first or best version of "The Virgin of the Rocks" but it seems clear that the British Museum's claim is more persuasive.
The Science Museum's Dee claude glass is an oval of black glass fitted in a wooden box covered in sharkskin and fastened with brass clasps of a conventional kind. The box is of the sort that was made for scientific instruments of all kinds and thus the Science Museum's Dee mirror has the look of something that could have been bought in a good instrument-maker's shop, that other people in London might have owned. An article resembling something that had been seen before in London was unlikely to attract the notoriety of Dr Dee's magic mirror and therefore I think this mirror was not Dr Dee's and that claude glasses, however named, were unknown in London in Dee's time.
Throughout history, glass has been used to simulate objects made from various stones: carved rock crystal was imitated by colourless cut crystal glass; glass imitates mineral jewels in costume jewellery; opaque lithyalin glass and malachite glassware are other examples. Perhaps the portable black glass mirrors we now call claude glasses were first made in conscious imitation of Dr Dee's famous black stone mirror. Could another origin be that the entirely oxidised, blackened silvering on old mirrors was something some imaginative people found attractive and a fashion for black mirrors arose?