Check washing is a form of fraud. The criminal uses various solvents to remove data from a signed check—the “pay to” name, the amount—and replace it with data more beneficial to the criminal: his own name, a larger amount.
This webpage—I know nothing about who these people are, but they seem a bit amateurish—talks about check fraud, and then gives this advice to check writers:
WHAT TYPE OF PEN TO USE WHEN WRITING A CHECK:
If you are a ballpoint pen lover, switch to black ink when security is important. Among water-based inks, remember that gels are the most impervious. But when you’re writing checks to pay the monthly bills, only one type of ink, the kind in gel pens, has been found to be counterfeit proof to acetone or any other chemical used in “check washing.” Most ballpoint and marker inks are dye based, meaning that the pigments are dissolved in the ink.
Based on recent ink security studies, we highly recommend that you use a gel pen, like the Uniball 207 that uses gel ink that contains tiny particles of color that are trapped into the paper, making check washing a lot more difficult. The pen sells for about $2. Personally I sign all my checks and important documents with one. But if you don’t want to switch, do not hesitate to to use your favorite fountain pen. Just fill it with ink in one of the more durable colors and enjoy!
I just wish they footnoted this statistic, obviously designed to scare people:
Check washing takes place to the tune of $815 million every year in the U.S. And it is increasing at an alarming rate.