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Fingerprint powders are fine powders used in dusting for fingerprints by crime scene investigators and others in law enforcement. The process of dusting for fingerprints involves various methods intended to get the particles of the powder to adhere to residue left by friction ridge skin on the fingers, palms, or feet.
Physical development of fingerprints using powders is just one of a selection of methods used to develop fingerprints. Fingerprints often leave residues of oils in the shape of the friction ridges, but the friction ridge skin itself does not secrete oils, and so some fingerprints will only leave a residue of amino acids and other compounds which the powder does not adhere to well. For this reason, 'dusting' is used as part of an array of techniques to develop fingerprints, but is often used on larger areas in a crime scene which cannot be removed for analysis, or cannot be subject to more rigorous analysis for other reasons.
Fingerprint powders have various formulations, and the appropriate powder must be used on the appropriate surface. For example, dark coloured powders will show up a fingerprint far better on a light surface.
Powders may be applied with a fingerprint brush, a brush with extremely fine fibers designed to hold powder, and deposit it gently on the fingerprint to be revealed, without rubbing away the often delicate residue of the fingerprint itself. They may also be applied by blowing the powder across the fingerprint, or by pouring the powder onto the print, and then blowing away the excess.
Magnetic powders are also used, where a fine magnetic powder is held by a magnetic applicator, which may then be gently moved across the fingerprint. As no bristles touch the surface, this often damages the print less than other methods of developing the print.
Modern fingerprint powders have a variety of compositions, and are often a matter of personal choice by the expert using them or down to the standard procedure of the department or agency. Many agencies use proprietary powders produced by independent companies, and so the exact formulation of these powders is not revealed.
Some surfaces, such as organic ones, do not take to fingerprint powders at all and use of alternate methods is necessary. Other media, such as certain types of glue, can be "smoked" over these surfaces with fair results.
Factor influencing fingerprint powder quality
There are several factors influencing the effectiveness of fingerprint powders.
- The powder must be fine enough to show the detail of the fingerprint. Finer powders would be theoretically capable of displaying greater detail than coarser powders.
- The powder must display the right level of adhesion, so that it will adhere to the residue of the fingerprint (often oils) and not adhere to the rest of the surface where it would obscure the view of the print. When a powder coats a surface, this is known as 'painting'.
- Sensitivity is related to adhesion, and is how well the powder adheres to a surface. For example, aluminum flake is more sensitive than aluminum powder, but greater sensitivity is not always desirable.
- The fingerprint powder must be a suitable color for the surface in question.
- To a lesser extent, it is important that the powder can flow, and does not 'cake' into a solid block, which would render it useless.
As these various qualities are not normally all present in any one material, compositions of various materials are generally used. For example, lampblack is particularly black in color, absorbing around 98% of incident visible light, but other materials may have greater adhesion, or flow more effectively, producing a better overall powder than either alone.
Examples of materials
- Metal powders
Of these, one of the more common powders in current use is aluminum powder, as it will show up on a variety of surfaces.