Latent Fingerprint development has challenged crime scene investigators for decades. Collecting forensic science fingerprints has evolved from the early use of lampblack (soot) to complex formulations that incorporate some sophisticated chemistry. This article will examine the use of latent fingerprint powders-included will be the whys and wherefores of latent powder formulations.
Latent print powders are divided among several different categories-each category having very specific uses. The first consideration for the crime scene investigator to examine is the type of surface expected to yield latent prints:
Porous Surfaces: This includes paper, cardboard and raw wood. Unless latent prints are suspected to be reasonably fresh, latent print powders are not of much use on non-porous surfaces, since the moisture content disperses into the substrate and ridge detail is lost.
Non-Porous Surfaces: This is where latent powders most often come into play. The problem is, however, that all non-porous surfaces have different characteristics that require a specific formula of latent powder.
Latent print powders are divided among several different categories-each category having very specific uses. The first consideration for the crime scene investigator to examine is the type of surface expected to yield latent prints:
Porous Surfaces: This includes paper, cardboard and raw wood. Unless latent prints are suspected to be reasonably fresh, latent print powders are not of much use on non-porous surfaces, since the moisture content disperses into the substrate and ridge detail is lost.
Non-Porous Surfaces: This is where latent powders most often come into play. The problem is, however, that all non-porous surfaces have different characteristics that require a specific formula of latent powder.
Latent Powder Formulations: Latent powders fall into the following categories:
1. Standard-Oxide Powders: This category covers formulations that have been in use for nearly a century. Included are Black, White, Gray and Red
2. Metallic Powders: These powders are formulated using actual, finely ground metal particles like aluminum, copper and bronze.
3. Magnetic Powders: All magnetic powders have, as their main ingredient, iron or iron oxide-a material that is easily magnetized.
4. Fluorescent Powders: Fluorescent powders have a special use-they are ideal when applied to multicolored backgrounds. Once it is apparent that latents are present after development, the CSI dims the light available and uses an ultraviolet lamp or alternate light sources (blue, green or red) to produce fluorescence. These developed latents actually "glow" in the dark.
5. Combination Powders: A problem often encountered in latent print development is how to view a latent print on a dark surface when using a light colored powder or vice versa. Another problem is the difference between smooth surfaces and surfaces made of highly polished metals such as silver plated or chrome plated surfaces. This one powder combines the properties of an oxide and a metallic powder, and one formula may be used on wither dark or light backgrounds.
As mentioned above-all non-porous surfaces are not alike when it comes to choosing a latent print powder. For example: An Automobile Body-Standard Oxide Powders may be used on the painted surfaces, but it is necessary to use a metallic powder on chrome-plated areas like bumpers and trim.
Magnetic powders were invented back in the early 1970s and they enjoy a prominent place in the crime scene kit. Magnetic powders may be used on practically any surface EXCEPT those surfaces containing iron or steel. All of the other powders are applied using a brush-either animal hair (camel or squirrel) or synthetic materials like fiberglass or carbon fiber. Feather dusters are also used mostly as a cleanup tool after a regular brush is used to develop the print. The feather duster will remove excess powder in the case of over development. Magnetic powers are applied using a magnetic wand, and magnetic powders cling to the magnet actually forming a "brush" that touches the surface-and not bristles from a brush.
Fluorescent powders have also been available for several decades. They all contain a pigment that emits light at other than white light frequencies. This fluorescence is especially useful on multi-colored backgrounds like beverage cans, potato chip bags and candy wrappers. Under low light conditions these powders become fluorescent when exposed to longwave UV light (black light). Many fluorescent powders are also visible under alternate light sources, but the fluorescence isn't visible since the light from the source overpowers the weak glow of fluorescence. To compensate for this, the crime scene investigator uses a filter-depending on the color of the alternate light source (ALS)--which is mostly blue, green or red. An orange or yellow filter blocks blue and green visible light, and a red filter is used for red ALS.