A study titled “Supercritical Fluid Cleaning of Banknotes” has been published in the Industrial and Engineering Chemistry Research journal, where scientists claim they have created a method to “remove human grease, microbes and motor oil” from banknotes using supercritical fluid (“substances that maintain liquid and gas properties through manipulation of their temperature and pressure”).
The report states that the cost of replacing unfit banknotes is near US$10 billion each year, in addition to the costs of destroying and disposing of 150,000 tonnes of unfit banknotes. It also states that 60-80% of banknotes are classed as unfit due to soiling (a build-up of dirt such as dust and human oils) which are subsequently destroyed.
Scientists from Rhode Island, USA, have tested using supercritical carbon dioxide to clean soiled banknotes. They claim that when CO2 is heated to 140 degrees Fahrenheit (60 degrees Celsius) and pressurised to 2,000 pounds per square inch, it permeates the pores of a banknote like gas whilst dissolving the soiling particles like a liquid.
Importantly, the scientists found that banknote security features such as watermarks, security threads, holograms and ultra-violet (UV) ink were not affected by the cleaning process.
The tests also showed that banknotes stored in 100 note bundles could be cleaned together, rather than cleaning the banknotes one at a time, and can be used on both cotton and polymer banknotes.