There are several common problems that crime scene evidence technicians and detectives encounter. One such problem is that of preserving fingerprint evidence on rough surfaces, human skin, blood prints, and curved surfaces.

All fingerprints must be photographed before any type of recovery is attempted. Photography, however, should not be the only means used to preserve fingerprints. Hard to lift prints, such as those on rough surfaces, human skin, blood, and curved surfaces can be accomplished by using magnetic powder and casting silicone. Silicone lifts provide a means to preserve fingerprints from some of the most difficult surfaces. Additionally, transparent silicone will allow the technician to lift the prints and perform a direct fingerprint comparison without having to reverse the image. The silicone lifts are permanent and will not smear or become smudged when touched.

There are various types of materials used for casting, such as: AccuTrans Auto-Mix, a casting silicone applied by an extruder gun; Mikrosil, a self-mix putty with a catalyst applied by spatula; Liquid Silicone, mixed with several different agents used to release the rubber or thin the rubber; and DuroCast, a compound mixed with a catalyst (hardener) applied by spatula.

From: Casting a Wide Net: Lifting Fingerprints from Difficult Surfaces by Mill Morris