Squeeze sodium hard enough and it turns clear

Near two million atmospheres, sodium ceases to be a shiny metal and becomes transparent, and an insulator. Release the pressure and the metal returns.

Sodium at room pressure is a soft, bright metal. Compress it in a diamond anvil toward 200 gigapascals — about two million times atmospheric pressure — and the atoms are forced into new structures. The conduction electrons that made it reflective are driven into a paired, insulating state. Light that used to bounce now passes. A 2009 experiment reported the transition: dense sodium, optically transparent. It is not a coating and not a film. It is the element itself, bullied into behaving like a crystal of a different kind.

The change reverses when the pressure comes off, which is why there is no transparent sodium to pass around. The fact is a phase, the way ice is a phase. It says that ‘metal’ is not a permanent label on an atom. It is a relationship among the electrons, and a relationship can be renegotiated by force. Gold’s colour, elsewhere in this library, is the mild version of the same lesson. This is the violent one.