Happy 2026 to you, Mark! Nice picture. I've seen that phenomenon many times in SOX lamps during run up, and I've wondered about the precise cause of it. This cannot come from a spurious sodium release from a local hot spot, liquid sodium has a too high thermal conductivity to result in any significant temperature differences across such small metal blob. I think impurities must be involved, causing a local reduction of the vaporization temperature of the metal, or "dust" particles may trigger the formation of sodium vapor bubbles (or even the projection of sodium microdroplets if thete is a sudden bubble collapse following its formation) when the metal pool temperature is high enough.

Thanks, Max. Yes, indeed a Philips SOX lamp dimple. Like you I have watched this many times expecting to see a ripple on the surface of the sodium relating to something like ‘bump’ boiling, but the sodium never shows any movement on its mirrored surface. Also, interesting how it occurs as brief often fragmented flash and doesn’t hang around in a localised cloud. It seems to occur at the same time as a sodium glow starts to appear on the walls of the discharge tube not long after the liquid phase of sodium. I can see dissolved impurities or trapped argon / neon being the reason for the light show.

I'll look more in detail next time I light a SOX lamp to see if the surface of the molten sodium blob moves or not. If it doesn't, then this could be caused by impurities released from the sodium surface upon the metal's evaporation, without involving any bubbles.