How to recognise a stained banknote
When a banknote is stained inside the intelligent banknote neutralisation system (IBNS), the security ink soaks into the banknote and leaves traces which are normally more pronounced on the edges of the banknote. The original colours of the most commonly used security inks are violet, green, blue, red or black. The ink usually flows from the edges towards the inner part of the banknotes and leaves a characteristic pattern (wave effect).
Sometimes, criminals try to wash the banknotes, but without success, and this may change the colour of the security ink owing to a reaction to the chemicals used in the attempted washing. The banknote’s original colours may also be altered, and some security features may be damaged, or may even disappear.
If there are one or two small marks on the banknote and the edges are intact, then the marks are most likely accidental, or not from an IBNS.
Here are some photos of IBNS ink-stained banknotes (some of which have been washed).