Fingers on the hot money

A system using heat-sensitive ink has been launched by a UK company to deter counterfeiters.

So in future, as well as trying to copy the usual watermark and metallic thread, forgers will have to contend with an image embedded in the paper that disappears when the note is slightly warmed — simply by placing a finger over the image.

The colour inks rely on reversible heat-sensitive chemical systems, developed by Portals, which has been supplying high-security paper since the early 18th century.

When the ink colour changes, the image or text beneath is revealed for a few seconds, and then as it cools down the image or text is hidden from view. This is because the top colour ink becomes colourless at higher temperatures — making it easier to distinguish the hidden image beneath.

The ink designed for banknote use is purple at 25°C, pink up to 35°C, then transparent above 35°C, with pink text revealed underneath. This occurs as the crystalline state of the material changes when heated from a coloured solid to a colourless liquid.

The idea has already won Portals the Royal Society of Chemistry Industrial Team Award, plus a cheque for £3,500, which the company picked up last month.

Tracey Wells Royal Society of ChemistryTel: + 44 (0)20 7440 3317Fax: + 44 (0)20 7437 8883E-mail: