Etched fingerprints in metal

[Printz S. AndtheP]

I have viewed cases previously processed for fingerprints worked by examiners other in the past. Some of these cases are several years old. I did not have the opportunity to see the evidentiary items immediately after their initial processing. In some cases, additional prints were found on porous items that had long before been processed with ninhydrin. This phenomenon I am familiar with. However, I have encountered something new. I reopened a case with evidence that was metal. The first examiner who looked at this case a year prior had noted no prints on the item. However, I found an etched print that belonged to one of individuals who had access to the item prior to its submission to the lab. Though there are many variables to consider with a print that may be etched into a surface....can anyone share any similar experiences or specifics as to these circumstances?

[Gerald Clough]

I don't recall if I've described this one before. I worked an arson in which the owner attempted to burn the house by piling gasoline soaked clothing in several places. Their mistake was closing the house tightly, and the fire produced heat and smoke but failed to burn much beyond the piles before it starved for lack of oxygen.

An aluminum Coke can was found left on a ledge. The owner said, in an initial statement, that she only bought drinks in plastic bottles, and the can was surely left by the arsonist. The can was somewhat heat-discolored and bore a near complete fingerprint that was etched into the metal. The ridges were perfectly visible, and you could run your fingernail across it and feel the ridges without harming them. It was, of course, the owner's print, and she was subsequently convicted on, in addition to the can, an accomplice statement.

Coke,Pepsi etc. are about ph 3.4, about like grapefruit and orange juice. Eccrine secreations are generally isotonic but the ph of final sweat can reach 5 as it passed through the duct, reaching that acidity when the sweat rate is low. Aluminum is acid-etchable, and heat would accelerate the etch. My feeling is that the Coke was the primary acid.

The heat was sufficient to cause the colors of the paint to fade slightly, about what you would expect of a can left in the sun for a while. It was not sufficient to noticably damage the painted surface of the ledge. Damage to the house, other than the carpet was primarily from smoke. Without having exprimented, I think it not unreasonable that, left undisturbed, the etch could have appeared in time without high heat, although probably not so deep.

(Aluminum soda cans are, of course, aluminum alloy, with, I presume, magnesium for corrosion resistence. Magnesium is also acid-etchable.)

[Pat A. Wertheim]

I have made three identifications of right thumb prints etched into the brass of shell casings. All three involved semiautomatic handguns with the ammunition in the magazine. It appeared the fingerprints were made when the bullets were pressed into the magazines. I believe the salt in the fingerprint residue was sufficient to lightly etch the metal.