Part 1 of this series we offered a description of what a latent print is, and an overview of latent print processing methods. This article will explain the various types of powders used to develop latent prints and their specific uses. One of the first known methods for developing latent prints used black powder made from lamp black or soot (carbon black) and white talc. The powder clings to the moisture content of the latent print as described in Part 1. Dark powders are used on light-colored surfaces and light colored powders are used on dark surfaces in order to give visual and photographic contrast. Listed below are the very basic powders that have been in use throughout the world for decades. Latent print powders are divided into the following five categories: Author’s Note: All of the powders listed below are applied with a soft bristle brushes such as squirrel hair, goat hair, fiberglass, makeup style brushes, etc. More on this will be offered in a moment.
- Oxide powders: This formulation is usable on many non-porous (STICK) surfaces such as most metals, painted wood, plastics and glass. Some examples are: Black, White, Red and Gray. These powders are not recommended for use on highly polished (SLIP) surfaces such as chrome, silver, etc. Oxide powders are known as “SLIP” powders, and are recommended for use on “STICK,” non-porous surfaces.
Special oxide formulations are necessary for specific “problem” surfaces like zinc-plated (galvanized) steel used to make vending machine coin boxes.
- Metallic powders, as the name implies, are formulated using various metals like aluminum, brass and copper. Metallic powders are recommended for use on highly polished (SLIP) surfaces. Latent prints on chrome-plated metal or silver are generally destroyed (wiped away) if oxide powders are used. Metallic powders are referred to as STICK powders as they stick to latents on SLIP (slippery) surfaces, and they should not be used on STICK surfaces as the powder tends to stick to the entire surface and not just the ridge structures present.
- Combination powders: Combinations are created by a specific formulation of oxide and metallic powders. An example is the very popular Silver/Black latent powder. This is a mixture of aluminum (metallic) and black (oxide) powders. The advantage is that combination powders may be used on virtually any non-porous surface. Another unique feature offered by this type of powder is that on dark surfaces the developed latent prints appear in a light color and on light surfaces the prints will be dark. If your budget is severely limited, and you require a versatile, extremely useful and effective powder, use a combination-formulated powder such as Silver/Black, Silver/ Gray or Silver/Red. This will eliminate having to carry several different oxide and metallic powder formulations.
- Fluorescent powders: This category of powders employ a base substance that is fluorescent in nature when exposed to ultra violet (black) light or alternate light sources like blue light. Fluorescent powders are considered to be oxide in their properties and are best used on non-polished surfaces. The particular advantage of a fluorescent powder is the fact that it may be used on multi-colored, patterened backgrounds. When the area under scrutiny is darkened, and an alternate light source is used, the fluorescent material emits a brilliant glow that reveals the fingerprint ridges, and the background all but disappears-making photography easier to accomplish.
- Magnetic powders: Magnetic powders first appeared on the market during the early 1970s. Magnetic formulations include iron or iron oxide as the basic component. Various pigments are added to provide background contrast. White, black, silver and red are commonly available. Some manufacturers also offer combination formulas as well as fluorescent magnetic powders. The principle advantage of a magnetic powder is that it is applied by a magnetic applicator wand and not a brush. The wand is equipped with a strong magnet and when held about a -inch or so above the powder, the powder is drawn to the magnet and forms a powder brush. Thus-only the powder touches the surface being dusted, and this will eliminate any chance of damaging the fragile latent print on the surface. Magnetic powders are not recommended for use on surfaces containing iron or steel.
There are a several kinds of feather dusters available to be used in place of a brush. Many CSIs, prefer feather dusters when applying fluorescent powders. They are also a valuable tool in “cleaning up” or removing excess powder from latents developed using a brush. Application Procedures: As noted above, only magnetic powder is applied without a brush. The remaining powders require use of a soft bristle brush. Brushes employing fiberglass or carbon-fiber strands are thought to be the softest. Applying powder or dusting the surface is accomplished using these recommended steps:
1. After deciding which formulation to use, measure out a small quantity of powder onto a clean sheet of paper. Do not dip the brush directly into the powder jar as this tends to compact the powder, and it will also add contamination from previously dusted surfaces into the powder.
2. The best rule to follow when it comes to using powder is “Less Is Best.” Pickup the powder with the tip of the bristles, and lightly tap off any excess powder.
3. Apply the powder to the surface in gentle sweeping strokes. Some CSIs use a back and forth, side to side motion-some users twirl the brush between the thumb and index finger. Practice applying powder to various surfaces before attempting to use these methods at an actual crime scene. Learn what works best for you.
4. As developed latents appear, expand the search area by applying more powder. Remember, you are processing areas that probably have been touched.
5. After prints are developed they must be photographed before any attempt is made to “lift” them. Be certain to include a scale (ruler) in each photo. This is a necessity especially if digital cameras are used at the scene. If the latents recovered are to be submitted to an online search such as the Automated Fingerprint Identification System (AFIS), the entries must be 1:1 in size. A scaled photo makes this possible. After photos are taken, the developed latents may be lifted from the surface. Numerous methods are available for this purpose.
Included in this arsenal of lifting methods are: latent print lifting tapes (available in 1-inch to 4-inch widths, Hinge Lifters, EZ-Lifters and Rubber/Gel Lifters. And don’t forget that you will need backing cards when making lifts with lifting tapes.An exact description of the lifting process is discussed at the website listed below. So there you have it. But I’ll close this article with a few warnings:
- Be certain to obtain a Material Safety Data Sheet (MSDS) from the powder manufacturer. Some powders contain hazardous substances that you must be made aware of. The MSDS will explain what safety precautions you should take to protect yourself such as wearing a dust respirator, eye protection, latex gloves and a disposable jumpsuit or lab apron.
- Another precaution is to consider the mess you may leave behind. Most latent powder manufacturers get frequent phone calls from irate home and business owners who find it next to impossible to clean up the powder left behind. Black latent powders are especially difficult, if not impossible, to remove from carpets, furniture and other materials.
Source by Don Penven