Weird artistic(?) effect.

When I buy anything on petty cash at work I have to write a number on the receipt. A while ago I realised that if I use a whiteboard pen, and the till ticket was thermally printed, I get an interesting effect - the paper turns dark grey around the edges of what I've written. I think that what actually happens is that the solvent in the ink is turning the paper underneath and around the edges dark, but since whiteboard inks are opaque you only see the effect at the edges. I pass this on for anyone who might have a use for it. Here's an example, not as good as some in the last batch I gave to the school office: