Any piece of silver bullion has the potential to have defects, which appear as white (milk) spots on the surface. For decades, this has been a common source of annoyance for collectors and silver stackers. It is absolutely impossible to foresee when the infuriating milk spots will show up because they are utterly random. It could occur either immediately after the rounds are minted or at a later time; spontaneous and unpredictable.
Although milk spots can appear on both coins and bullion rounds, they are far more common on silver bullion rounds with a purity of.999 or higher. They are not commonly found on older coins that contain alloys with 90% or 40% silver that were produced in the U.S. before 1964. The occurrence of milk spots is speculation at this point. There is a widespread belief that the milk spots are the result of cleanser residue on the bullion that is not cleaned off before entering the furnace.
Good storage practices, such as placing bullion rounds in good capsules and maintaining them in an environment that is isolated from chemicals, humidity, or oxidation, perhaps can decrease the likelihood that milk spots would appear on the bullion. There’s really no scientific explanation for these, and maybe the only way to prevent milk spots is to pray they never happen. If the minting process is somehow flawed or compromised, a bullion round will get white spots later on. This shifts the responsibility back to the mint and why choosing a good mint is crucial.
There’s no way to verify this, but those bullion rounds struck at the Mexican and Chinese mints, as well as the Perth Mint, have apparently been shown to have a lower incidence of milk spots. According to silver forums and people who have acquired silver bullion rounds from more than one mint have noted that Royal Canadian Mints are the most likely to get milk spots, followed by American Silver Eagles as the next most likely to have spots.
The good news is that having milk spots does not decrease the value of a bullion round. However, numismatic rounds or coins will be judged if they have milk spots which might lower their grade and value. At the moment, the milk spot is almost unavoidable as it will develop on the bullion sooner or later. It’s indeed infuriating
