Date: Jan 18, 2014 Source: Economist Magazine (
click here to go to the source)
EVERY year the world's central banks withdraw and replace a huge amount of currency deemed unfit for further circulation. Some is damaged, but a lot is just dirty. The upshot is the printing of nearly 150 billion replacement notes at a cost of nearly $10 billion. There is also the challenge of destroying the withdrawn notes—which, collectively, weigh about 150,000 tonnes—in a way that is both environmentally friendly and secure. (One of the most famous crimes in British history, the Great Train Robbery, was of banknotes on their way to be destroyed.) It is all a monumental nuisance. But a new technique for cleaning money could soon make it a lot less so.
The main reason banknotes get dirty is that they pick up an oily substance called sebum from human skin. This is unpleasant in itself, and also turns brown as it oxidises, making it harder for machines that handle and sort notes to recognise them. In principle, sebum can be removed by solvents, but most solvents also damage security features such as holograms and special magnetic and fluorescent inks. So it is easier simply to withdraw notes that are heavily encrusted with sebum from circulation.
But Nabil Lawandy and Andrei Smuk, of Spectra Systems, a company in Rhode Island, think that by picking the right solvent they can get around this. The solvent in question, as they explain in Industrial and Engineering Chemistry Research, is carbon dioxide.
CO2 is most familiar either as a gas or as a solid ("dry ice") used to keep things cool. Compress it enough at the right temperature, though, and it will also turn into an unusual state of matter known as a supercritical fluid. This has properties halfway between those of liquids and those of gases.
Most cleaners are either oxidising agents or polar molecules (chemicals such as detergents that bind both to oil and to water, and so can dissolve the one in the other). It is these harsh chemical properties which damage banknotes' security features. But supercritical CO2 is neither oxidising nor polar. And it is already widely used to remove grease from machinery. The researchers speculated that it might be ideal for stripping sebum from banknotes.
They therefore soiled some notes deliberately with sebum and left them for eight days at a relative humidity of 65% and a temperature of 90°C, in order to oxidise the soiling. They then soaked them in CO2 heated to 60°C and at a pressure of 340 atmospheres, conditions in which it becomes supercritical.
And it worked. After being put through the cleaning procedure, the once-soiled banknotes reflected light almost as well as new ones do, with none of their security features compromised. Dr Lawandy and Dr Smuk thus seem to have found a way to keep banknotes circulating longer. Train robbers everywhere will have to think again.
