Enhancement of Latent Prints on Metal Surfaces

There are several ways to enhance latent prints on metal surfaces by oxidation, a 1% Gun Blue solution being the most common. These methods depend on the surface, the environment where the print will be photographed, the financial situation of the police department or lab, the substance that made the print and the training of the individual who is enhancing the print. The different methods and problems of enhancing latent fingerprints on metals can be reduced by using a commercial brass blackening and distilled water solution, or a commercial aluminum blackening and distilled water solution. These two solutions indicate better results than the 1% Gun Blue solution.

Birchwood Casey Brass Black is an oxidizing agent that is used to blacken brass, bronze and copper surfaces. Birchwood Casey Aluminum Black is also an oxidizing agent but is used to blacken aluminum surfaces. The active ingredient in these two solutions that causes the oxidization of metals is selenium dioxide, or selenious acid. When a metal surface is submerged in a solution of Brass Black, or Aluminum Black, and distilled water, contaminants on the surface, such as the sebaceous deposits and oils from friction ridge skin, protect the surface and prevent oxidation from occurring beneath. This causes the ridges of the print to remain light while the rest of the surface becomes darkened.

Although these solutions were designed to oxidize specific metals, they work on a variety of metals, including brass, copper, galvanized iron, galvanized steel and hardened steel. Due to their ability to enhance prints on a variety of surfaces, the problem of determining what method to use on different surfaces is eliminated.

These solutions are inexpensive to prepare. A 90 rnL bottle of the Brass Black or Aluminum Black costs approximately $5.95 while distilled water can be found for less than a $1.00. Furthermore, these solutions remain effective over a long period of time.

Their shelf life seems to be infinite because the Brass Black and Aluminum Black do not fall out of suspension nor do layers form when mixed with distilled water. Since these solutions are inexpensive and have long shelf lives, they can be a great asset to a police department or lab facility with restricted funds.

In addition, they can be used in a lab or at a crime scene. These solutions are easy to use and mix and require little training. Both solutions require a container that can hold a solution of 1 part Brass Black/Aluminum Black and 4 parts distilled water or a solution of 20% Brass BlacWAluminum Black and 80% distilled water. The print must be completely submerged for the enhancement to occur, so enough solution should be mixed to cover the entire print . Once the print has been enhanced, the surface should be rinsed with tap or distilled water and allowed to dry. After the print has dried, it is difficult to smudge. If the print is overdeveloped and the material becomes too oxidized, it is possible to remove the excess by submerging the surface in ethanol or methanol.

The Brass Black solution works on a variety of metal surfaces and the time that the print is immersed in the solution does vary depending on the metal. When brass is submerged, the time for enhancement is approximately 1:45 to 2:30 minutes to obtain good clarity (figure 1). To darken the surface and lighten the friction ridges, brass may be placed in bleach.

Figure 1

Figure 2

When copper is placed in the solution, the time required is approximately 30 to 45 seconds to produce good contrast between the latent and metal surface (figure 2). When galvanized steel and iron are placed in the solution, a print is enhanced in approximately 20 seconds, but the friction ridge detail is not as clear as that developed on copper or brass (figure 3). (This method of enhancing prints on galvanized metal is less destructive than hot flame and yields better results than cyanoacrylate.)

Figure 3

Figure 4

Prints placed on a hardened steel file were developed in about 12 seconds with good clarity and contrast (figure 4). Aluminum reacts with the solution to reveal a fingerprint in approximately 1:30 to 1:45 minutes. The aluminum surface does reveal visible ridge detail but the contrast between the latent and surface is less than that of copper, brass or hardened steel (figure 5).

Figure 5

Stainless steel, immersed in the solution for 2 hours, was allowed to air dry without rinsing. Ridge detail was not discernible for 72 hours. Prints developed were extremely fragile and faint by comparison to other metals. This solution cannot be recommended for stainless steels and more traditional methods should be employed.

When the Brass Black solution is compared to a solution of Aluminum Black and distilled water of same concentration, the Brass Black enhances prints with more clarity overall. The Aluminum Black solution makes the brass and latent impression darker, thereby decreasing the contrast between the ridges and surface. When copper is placed in the Aluminum Black solution, there is no significant difference in the clarity of the print when compared to its placement in the Brass Black solution. Galvanized steel and iron obtain a slightly more even background when immersed in the Aluminum Black solution compared to the Brass Black solution. (The difference between the two solutions is visible but not significant). The stainless steel is not oxidized when placed in the Aluminum Black solution. The Aluminum Black solution does, however, allow for greater contrast between the friction ridges and the oxidized surface of aluminum. Consequently, the Brass Black has a better result overall, with a wider variety of surfaces and clearer latent prints than the Aluminum Black.

The Aluminum Black solution leaves a dry, charcoal-like residue on the surface of metals. This decreases clarity in photographs and the print is harder to work with without disturbing the even background of the surface. The Aluminum Black solution does, however, permit darker lifts than the Brass Black, but it is harder to work with because the charcoal-like coating on the surface of the metal is messy.

The Brass Black can be lifted with a considerable amount of distinction between the ridges and has less residue on the surface of the metal than the Aluminum Black. Furthermore, due to this chalky, dark gray residue, the clarity of the photographs does not compare to that of surfaces immersed in the Brass Black solution. The Brass Black places a cleaner and darker residue on the surface of metals which enhances friction ridges for clearer photographs. Also, the Aluminum Brass solution has an increased amount of black particles in its solution over time due to its chalky residue. (This will decrease the number of times that the Aluminum Black solution can be used.) In contrast, the Brass Black solution remains cleaner after several uses because the residue is not as sooty.

The variety of problems and methods of enhancing prints on copper, brass, galvanized steel and iron, hardened steel and aluminum can be reduced by using a solution of Brass Black and distilled water or a solution of Aluminum Black and distilled water. The Brass Black solution and the Aluminum Black solution work on a variety of surfaces, can be used in environments outside of a lab, are inexpensive and require minimal training. The Brass Black solution exceeds the results of the 1% Gun Blue and Aluminum Black solutions due to the increased contrast and clarity of developed prints among a variety of metals. Safety glasses, protective gloves and a ventilated area should be used when performing this procedure.

For further information, please contact:


Chris Kauffman
Bettendorf Police Department
1609 State Street
Bettendorf, IA 52722-4937
(3 19) 344-4000

*From the Journal of Forensic Identification Vol. 51, No. 1, january/February 2001
The Official Publication of the International Association for Identification
"Reproduction of the Journal of Forensic Identification, in whole or in part, for noncommercial, educational use is permitted provided proper citation of the source is noted. Reproduction for any other use is prohibited without prior written permission. Requests for permission may be addressed to the editor (of the Journal of Forensic Identification -- jfieditor@theiai.org)."

Article posted: November 20, 2007