Why does Platinum evaporate if left long enough? I have been reading into research relating to the redefining the 1 kg weight as the current Platinum-Iridium is becoming smaller. In this article, here, it mentions that the original metal weight couldn't be pure Platinum as it evaporates over time. This article was referenced from 'The Scientist' which is a respected science magazine.
My questions are:


*

*why does Platinum evaporate over time? As a (relatively soft) solid, I would have assumed it wouldn't lose particles due to evaporation, as the external environment is way below even the melting point of Platinum.

*Is this not localised to just Platinum and other metals are included, indeed, will most solids evaporate over time?

 A: Say the block of Platinum is in vacuum. For such low pressures, the melting point is not the same as the one in atmosphere: it becomes very close to a zero temperature. In other words, everything would tend to become gas under zero pressure.
Indeed, even for a solid in vacuum, atoms sometimes will escape due to fluctuations. It might condense back on the solid later on. This competition between evaporation and condensation leads to an equilibrium at a pressure equal to the vapor pressure.
For a mixture, say Platinum+air, things are slightly different, but the idea is the same: atoms can escape the solid and condense later.
UPDATE:
Following Alexander's comment, I calculated that, at the maximum temperature where Pt is solid, 2000 K, there is still no significant evaporation inside a 1 m$^3$ volume: mass changing by $10^{-8}$. I used data from here and an ideal gas law. So the evaporation does not seem to affect Pt at room temperature. Maybe chemical processes? That would be a question for chemistry.SE.
A: The fun stuff is that even if a material has e.g. an average temperature of room temperature, some of its atoms have a higher temperature (because of quantum laws). Some of these atoms have such a high temperature that they can evaporate (which cools down the whole body). But when the body gets reheated (because the room is an infinite amount of heat storage), some atoms are again too fast (have too high temperature) and evaporate.
This is the reason why every material looses weight over time...
