bron suchecki said:
Perth Mint is taking this seriously, it is a bit of a puzzle as we have pretty clean minting area, air filters etc. Have sent some coins off to a university for analysis of what is in the spots.
That's a reaction worth a thumbsup. The RCM folks just continue production blind and deaf as the choice they made.
I'm very interested in a chemical and/or production process based explanation for milk spot development and redevelopment on silver.
It's clear that quite some other coins are much less prone. The sole difference I can see is that the RCM's silver is 9999 instead of 999.
How common did milk spot talk became by now? Too common. It's a problem, a problem has specific reasons, and knowing these fully is the way to a solution.
These milk spots, and their seemingly random distribution / sizes / shapes suggest a fluid based origin, and they look extremely well on the spots that (especially hard) water drops leave on the window. The substance is mostly calcium and magnesium.
So a possible explanation may be that somewhere in the blancs handling or coin stamping process, hard (in a degree) water is sprinkled.