A single crime scene will offer a variety of surfaces that potentially harbor latent fingerprints—some present little difficulty in developing and lifting these prints while others offer significant challenges. One surface that comes to mind are textured surfaces such as leather furniture, handbags, wallets, certain counter and desk tops, and automobile interiors.
A second type of difficult surface involves materials such as plastic baggies, cellophane packaging, shrink-wrap, “Saran” wrap and aluminum foil. These materials are favorites of the drug trade and they provide convenient, easy to handle, economical storage and transfer packaging. But their extreme flexibility often presents problems.
No major difficulty is encountered when developing any latents present on textured surfaces. But preserving the latent is often a challenge. Many CSIs report that magnetic latent powder provides the most consistent results when processing non-ferrous, rough surfaces. The biggest advantage with using a magnetic powder is that the powder itself forms the brush so there is no need to use an animal hair or fiberglass brush on delicate surfaces like crumpled cellophane or aluminum foil.
The following steps are used by many skilled CSIs who report excellent results:
1. Carefully flatten the material to be dusted onto a flat, smooth surface. It may be necessary to use a small length of tape to hold one end stationary.
2. Measure out a small amount of the magnetic powder to be used onto a clean sheet of paper (this is to avoid contaminating the powder supply with foreign material like dust, dirt, grease, etc. from the surface being processed). Use a powder that offers good photographic contrast against the substrate.
3. Pickup the powder with the magnetic wand and apply the powder to the surface.
Once latents appear they should be photographed before proceeding. Be Certain to Include a Scale in All Photos.
Lifting prints from the above mentioned surfaces will be the most difficult part of this process, since lifting tapes, hinge lifters, etc., will most likely stick to the surface and removal will most certainly damage some materials. Another reason to avoid this type of lifting procedure is consideration of the textured or roughness of the material.
The preferred method, one that will not damage the material holding the latent, is to employ any of several silicone materials. Among the most popular methods are Mikrosil and Liquid Silicone Rubber.
Mikrosil is silicone putty available in several different colors. It has the consistency of toothpaste. If a black or gray powder produced the latents, use white Mikrosil. This material is also available in gray, brown and black.
Mikrosil is a two-part formulation: One tube contains the base material and a much smaller tube contains a blue-colored catalyst. Mikrosil, like most silicone products, requires a catalyst to make it harden or “set up.”.
Follow the manufacturer’s instructions when mixing the base and catalyst. Mixing time must be kept to a minimum as the material begins to setup in 20-30 seconds. Stir vigorously until the color of the blue catalyst is no longer visible in the base material, and then apply it to the latent print to be lifted. Setup time is about 10-15 minutes.
Download a copy of the Mikrosil Technical Information Bulletin Here
Liquid Silicone Rubber is a viscous liquid supplied in jars. It has the consistency of thick maple syrup. It too requires the use of a catalyst. To use this substance, stir the contents of the jar well as some settling may have occurred. Pour the quantity needed to cover the area with latent prints into a suitable mixing container and add liquid catalyst according to instructions. After mixing for at least one minute, pour the mixture onto the surface, and allow 20-30 minutes for setup.
Once either casting material is cured, merely lift the cast from the surface, Photograph the lift with a scale.
Download a copy of the Technical Information Bulletin for No. 634C Liquid Silicone Rubber Here
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