This article was originally published in Forensic Magazine's August/September 2005 issue and was written by Mill Morris.
Casting materials are not just for bite and tool mark impressions, but can be used to obtain fingerprint evidence on rough surfaces, human skin, blood prints, and other types of evidence.
Ask any evidence technician or crime scene detective, “What is the best way to collect or preserve bite and tool mark evidence?” The answer would most likely be: use a casting material. Casting materials are great for recording gouges, scrapes, and scratches in metals and some types of wood products.
Traditionally, firearm and tool mark examiners use casting materials to record the scratches and striations on recovered projectiles, firing pin marks on bullet casings, and tool mark impressions from tools used to break in or pry open doors and windows. Tools often have nicks from wear and tear that leave individual characteristics. There are various types of materials used to record impression marks, 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.
New Technology for an Old Problem
As a Crime Scene Detective, I often look for new techniques that will aid me in the collection and preservation of crime scene evidence. When problems arise, old and new methods must be obtained or adapted to meet these problems. 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.
Photography is the current choice to record fingerprints on rough surfaces, human skin, blood prints, and curved surfaces; however, poor lighting conditions, type of surface, and a technician’s knowledge of photography all play a crucial role in the quality and usefulness of the photos. Photographs should always be taken but other methods should also be used to preserve fingerprint evidence, especially when photographical means fail to record the image properly. This is the reason why I began to experiment with using casting materials for applications other than what they were intended.
Casting materials usually come in white or brown. Different colors give the evidence technicians a better choice of selecting a contrasting color when used with fingerprint powders. When casting materials are used to lift fingerprints, the technician lifts a reverse image of the print. This image must be reversed for comparison. Reversing the image is accomplished by photography. The image is photographed and the negative is simply reversed for printing. Technicians with access to an AFIS (Automated Fingerprint Identification System) simply scan the lift into the AFIS. The image can then be directly checked in the database.
Rough surface prints do not lift with ordinary hinge lifters. The textured surfaces tend to break up ridge formations. Casting materials fill in the textured areas allowing the whole print to be lifted. I set out to determine if casting materials could be used for lifting prints from surfaces that are traditionally difficult.
Current casting studies talk about lifting prints from uneven surfaces and tool marks. Few studies have been done involving the use of casting materials to recover latent fingerprints on other types of surfaces or evidence.
Choice of Casting Silicone
For my experiments, I chose to use AccuTrans® Auto Mix from Ultronics, Inc., a casting silicone applied by extruder gun. The material, also used in the dental industry for making impression molds, is flexible and does not distort the image. Once the impression is lifted, it cannot be smeared or smudged; it is permanent on the lift. The extruder gun allows the material, and not the tip of the gun, to come in contact with the fingerprint, preserving the integrity of the print.
The silicone comes in both white and brown and is also available as a transparent material. This allows for instant comparison of the print without reversing the image. The transparent silicone allows the technician to place the lift on any color of background. This works well for photographic purposes. The silicone can be used on curved surfaces, horizontal, and vertical planes. When used on vertical planes, only a small amount of the silicone is needed. The material will smooth itself over the area. If too much of the material is applied, the silicone may run down past the fingerprint. To avoid this, the technician can simply place a piece of tape a few inches below the print. The tape will allow for the excess to gather in this area. This casting silicone can be used on rough surfaces, human skin, blood evidence, and curved surfaces.
It should be noted that before treatment with silicone, rough or grooved surfaces should first be dusted with magnetic fingerprint powders. Magnetic powders come in a variety of colors, which is beneficial for surface contrast and for photographic purposes. Magnetic powders contain ferromagnetic particles. The powder is applied using a magnetic applicator or wand. The powder sticks to the wand. The advantage here is that the applicator does not come into direct contact with the print, just the powder. Excess powder is easily removed by moving the applicator back over the print. Magnetic powders cannot be used on other ferrous surfaces.