mould over gum
This is not a new technique, but a problem submitted me by a friend.
She told me that some of her gum bichromates have developed small white dots, and asks for something able to kill the mould and its spores but not the color of the image.
I am thinking to salicylic or ascorbic acid (or their salts) instead of the old and more poisonous mercuric chloride, or to some commercial agricultural products. Have you a better solution?