It doesn't. But some of the early flint glass mixes used to make achromatic doublets were rather prone to surface recrystallisation.
This text is a bit mangled from automatic translation of a German patent but shows the gist of the problem of recyrstalisation of flint glass.
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Or this one from Korean which is slightly less mangled.
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Their aim was to find an alternative and affordable low lead content optical flint glass that was good and stable wrt to recrystallisation.