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

The pre-melting process begins with the raw powder being heated inside a quartz or platinum crucible (depending on the exact type of glass), in a furnace at a temperature of more than 1000 degrees Celsius. The furnaces are on platforms raised several feet above the factory floor. The mixture is added to the crucibles by machines very gradually. If all the powder were dumped in at once, only the surface of the mixture would melt.

With quartz crucibles, some of the quartz inevitably melts into the mixture. This is accounted for in the formula, but since they become thinner over time as the quartz melts, the crucibles have a limited lifespan - in some cases, this can be as short as two days. We weren't allowed to show the crucibles in this article, but the ones we were shown were roughly the size of a small domestic water boiler.

Once the glass is fully melted, a hole is opened into the bottom of the crucible to allow the molten glass to escape into a large tank of water, positioned underneath the furnace at floor level. That's what you can see in the image above.