Researchers in the United Kingdom inadvertently found that using a chemical compound on inkjet printer ink makes it possible to read the contents of a letter without removing it from an envelope, which could help with forensics.

Inkjet ink on paper usually transfers on contact to, for instance, an envelope in which a printed letter is placed. When exposed to the chemical compound disulfur dinitride, an envelope showed the words that had been transferred, making it possible to read what the letter said without opening the envelope, said Paul Kelly, a researcher at Loughborough University in England.

The words were visible long after the letter was removed from the envelope, Kelly said.

"If you received a letter printed in inkjet, opened it and threw the envelope away, we could use the discarded envelope to image ... the letter content," he said.

The compound, applied to the envelope in gas form, interacted with one or more components of normal printer ink, which crystallized the ink and made the print more visible. The discovery came about by accident when researchers were exposing different materials to the compound, Kelly said.

During an experiment, a student left a letter in a sealed envelope overnight and words from the letter were visible on the envelope after it was exposed to the compound, Kelly said.

In addition to highlighting inkjet ink on envelopes, the compound also exposes fingerprints. This could be a useful investigative tool to help determine a letter's sender, Kelly said.

"We'd know who you were and what someone had written to you. Unless they had used gloves, we'd have the sender's prints from the envelope as well. There are obvious security possibilities and implications for that," Kelly said.

However, the researchers are trying to overcome challenges, including the time it takes the ink to display after being exposed to the compound -- up to many hours in some cases -- so they are trying to speed that process.

"We are trying to get funding in place to carry on with this. Although there are challenges to be met, it shouldn't take too long to optimize, and then we take it from there," Kelly said.