You've seen it in movies, and it happens in real life. The criminal scrawls a ransom note, then tears away the note and doesn't give a second thought to the page underneath. Later, police collect the pad of paper and submit it to the document examiner, who exposes the writing, thus proving that that particular pad was the source of the ransom note. The owner of the pad then has a bit of explaining to do.
The movement of a pen over a page indents the second page along the path of the pen, creating indented writing. Unlike what you've seen on TV, forensic technicians don't use the old pencil method (rubbing a pencil over the paper to expose the indentions) to uncover indented writing, because the pencil's markings may damage or destroy the evidence.
Sometimes, a simple angled light reveals indented writing. When it does, the technician photographs the page. A more sensitive method is the use of an electrostatic detection apparatus (ESDA), which can sometimes uncover indented writing several pages below the original page.
In ESDA, the technician places a Mylar sheet over the page in question to protect it and then places both on a porous metal plate. A vacuum pulls the Mylar tightly against the page. The examiner then passes an electric wand over the sheet and the page, producing static electricity. The charge is greatest in the indentions, so when the technician pours or sprays black toner, similar to that used in copy machines, over the Mylar, that toner attaches to the surface in proportion to the degree of charge and reveals the indented writing.
