These are crystals... of silver
How was that done? Silver vapour in a closed container, and they slowly lower the temperature? or, wait, that would turn into silver condensation. Also a container that hot would probably radiate in the visible spectrum, glowing.
The caption from wikipedia says "Silver crystals forming on a copper surface in a silver nitrate solution"
So they are forming in a liquid saturated with silver, similar to how crystals form in caves. To be fair, ice crystals work the same way but we often forget that air is a solution filled with water