Inside Nikon's Hikari Glass factory
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Inside Nikon's Hikari Glass factory

And here is the frit - they look like little flakes of snow, but that's where the similarity ends. In the pre-melting process, the frit aren't meant to have exactly the exact refractive qualities of the finished glass - it's still basically a raw material. And there's some variation in the flakes of frit, too. Depending on where the glass was positioned inside the crucible, the makeup of each frit might be slightly different (i.e., it might contain more or less quartz, thanks to the melting of the crucibles during the process).