The firm skin is a clear sign of microorganism infestation, since opened ink bottles at room temperature offer ideal growth conditions for moulds. That also explains why inks without preservatives (e.g. by Visconti, Schreibmayr or OMAS) are affected, while so-called chemical inks like Waterman do not show this. Such infested ink should be disposed of, since skin remnants can clog the feed. Preservatives in mass-market inks thus have a real practical justification.