A Metallurgical Review of the Interpretation of Bullet Lead Compositional Analysis
Forensic Science International 127 (2002) pp. 174-191
18 Pages Posted: 9 Dec 2012
Date Written: September 20, 2001
Comparison of the minor and trace element compositions of bullet lead alloys has been used by some forensic examiners to make definitive positive associations between bullets or lead fragments at a crime scene and samples of bullets linked to a suspect(s). Such conclusions have been based on elemental analysis of isolated groups of bullets with no consideration of the metallurgical processes involved in the production and refining of the bullet lead alloys. An understanding of the metallurgy of lead refining reveals that elements quantified in the forensic analysis are carefully controlled in the refining process and that there are logical reasons why some elements are more discriminatory than others. Data for lead alloys supplied to two major ammunition manufacturers confirm that multiple indistinguishable shipments of lead alloys from secondary lead refiners to the ammunition manufacturers are made each year and over a period of many years. The data also demonstrate that distinguishable compositions can come from the same melt or "source" of lead alloy. These results clearly indicate that bullets with indistinguishable compositions could have come from different lead sources produced in the same or different years. Furthermore, the observation that two bullets have a distinguishable composition does not necessarily mean that they came from different sources.
Our results show that the forensic examiner using a method of bullet lead alloy elemental analysis, which quantifies up to six elements, is restricted to concluding only that indistinguishable bullets might have come from the same source, not that they did come from the same source. In addition, it is quite possible that multiple bullets with similar but distinguishable compositions could have come from the same source. The authors therefore conclude that there is no scientific validity to any conclusion more positive than attributing the possible association as to molten source among bullets from different samples. An understanding of the metallurgical principles operative in the melting/casting process, as well as the data acquired for this study, indicate that any forensic conclusion which associates unknown bullets with the "same source", and/or "same box", should fail most or all Daubert criteria.
Keywords: forensic, bullet lead, comparative bullet lead analysis, CBLA, bullet manufacturing, bullet lead smelting, bullet lead refining, retail bullet distribution
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