With Latent Prints, the Age of the Evidence Can Be a Factor
When you’re working with latent prints, the age of the evidence can also be a factor in processing. For example, if you have fresh prints on documents, you can use standard fingerprinting powder or magnetic power. But because paper is porous, the prints will eventually seep into the surface and you won’t be able to lift them with powder. If you’re not sure of the age of the evidence, you can follow the PINS processing technique, which stands for powder, iodine, ninhydrin, silver nitrate. The chemical methods are best done back at your office or at the lab in a controlled environment. Begin with powder. If you don’t get a print, move on to iodine, then ninhydrin, and finally silver nitrate if necessary. Once you use one of the chemicals you can’t go backward, though.
From: Decision Time: Processing Prints by Dick Warrington