In the late 1960’s, while designing tape drive interfaces, I used a material known as “Tape Developer” to reveal the magnetic patterns recorded on tape to help debug the controller. The developer consisted of iron or iron oxide powder mixed in a fast-drying solvent, likely Freon. When applied to tape it was quite easy to see the recorded data visually. Of course the densities then were in the few-hundred of bits-per-inch range.
For my Christmas card one year I wrote a program that spelled “Merry Christmas” on the tape. I developed it, lifted the image with a piece of scotch tape, and stuck it to an IBM card (i.e. punched card). Now that was a really nerdy Christmas.
Use rust particles to reveal the data on your credit-card’s magstripe - Boing Boing
