Patina Spreading From Coin To Coin

Patina and mould are two different things... patina is good and mould is bad.

The green stuff (verdigris) can spread from coin to coin, particularly if stored in jars and such. It is known as a coin desease and left unattended could affect all the coins in the long term. Any like that should be sorted out and put aside from the healthy coins.

This kind of contagious verdigris is called "bronze disease" because it can indeed spread across the surface of a coin over time and even jump from coin to coin, "infecting" them in an almost organic fashion, even though it is caused by a catalytic chemical reaction rather than an actual biological agent like mould.

If the coins get jostled around a lot, a single "diseased" coin can "infect" most of the coins in the jar, as all it takes to start is a tiny speck of the green stuff breaking off and landing on a pristine coin. And like a mould, it requires cool damp conditions to grow; high heat (putting the coin in an oven at 100 degrees C for half an hour) and/or dry conditions can stop it from forming and spreading. But to remove it altogether, the coin needs to be chemically treated in some fashion, such as a chemical cleaner like Verdi-Care.