Review: Objective Analysis of Impressed Chisel Toolmarks

Emily C. Lennert

Category: Patterned Evidence

Keywords: patterned, comparison, toolmarks, striations, impression

Article to be reviewed:
1. Spotts, R.; Chumbley, L. S. “Objective analysis of impressed chisel toolmarks.” Journal of Forensic Sciences. 2015, 60 (6), 1436–1440.

Disclaimer: The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.

Summary:

Comparative forensic practices were challenged in the 2009 National Academy of Sciences (NAS) report on forensic science.1 The report noted a lack of sufficient scientific basis for comparative examinations, such as those performed on toolmarks, and led to validation studies about several areas in comparative forensic analysis.1 The underlying theory behind toolmark evidence is that no two tools will leave the same mark. The theory states that the individual surface topography of the tool will leave a “unique” mark, and therefore, a tool may be linked to a toolmark discovered at a crime scene. Herein, the word “unique” will be used only in reference to the theory underlying toolmark examination. The authors were evaluating the theory of uniqueness and are not making any definitive statements regarding the “uniqueness” of toolmarks as a whole.

In this study, 50 identical chisels were obtained directly from a single manufacturer. The chisels were manufactured under observation by the researchers to ensure that the chisels obtained were made sequentially. The authors state that manufacturing processes influenced the final topography of the tool surface. The possibility of manufacturing variations may explain the need for sequential manufacturing, which would help to ensure that chisels were as similar as possible with no other possible variables. Each chisel was marked with an identification number on the handle and placed into a sealed plastic bag. The chisels were used to make impression toolmarks. Impression toolmarks are typically made when the tool is perpendicular to the surface, such as a mark that would be made when a chisel or similar tool is used to break into a safe or a locked desk drawer.

A jig was created that held the chisel at 90° to the striking surface, which was a lead plate. The jig held the chisel to the plate, and two 2lb weights held at 6” above the chisel were released to strike the chisel and create each impression in the lead plate. Ten impression marks were made with each chisel, for a total of 500 impressions. The striking end of each chisel created an impression with two sides, and the side edges of the chisel were disregarded, resulting in 1000 toolmarks, total. Sides were labeled for each impression as A or B. (See Fig. 1)

Figure 1. Example of an impression mark made in a soft surface, such as lead. Toolmark sides A and B of the impression are labeled, and the edges are labeled as disregarded areas.

An optical profilometer was used to obtain and digitize the topography of each plane (A and B) for each toolmark, using an algorithm previously developed for use with striated toolmarks, such as those made by a screwdriver. A topographical profile was obtained for each of the 1000 toolmarks, such as in figure 4 within the study. Statistical comparison of the topographic profile results was performed to determine the validity, with respect to this study, of the theory that every tool will leave behind unique toolmarks that are characteristic of the tool itself. Three different statistical comparisons were made to validate this theory.

The first set of comparisons was the statistical comparison of matching pairs. The matching pairs were defined as toolmarks made by the same tool and of the same side of the tool, A or B. For example, chisel one, side A, replicate 1 (denoted as C1A1) would be compared to side A of its nine replicates. Then chisel 1, side A, replicate 2 (C1A2) would be compared to side A of the nine other replicates, and so on. This comparison was made for each chisel and each side for a total of 4500 comparisons. Comparisons from this set were found to have high levels of correlation, meaning that toolmarks originating from the same chisel and same side were able to be correlated.

The second set of comparisons was the statistical comparison of the nonmatching pairs from the same chisel, meaning the opposite sides of each chisel, for all replicates. For example, C1A1 was compared to C1B1, including all of the related replicates for each. It was expected that each side of the tool would produce different toolmarks, and therefore would show a low level of correlation. Comparisons from the second set were found to have low to zero levels of correlation, supporting the theory that each side of the chisel leaves different toolmarks and is, essentially, a different tool.

The third set of comparisons was made between the nonmatching pairs from varying sides of different chisels, such as a comparison between C1A1 and C2B1, including all their replicates. This was repeated so that each chisel was compared to one another and so that each side was compared (A to A, A to B, and B to B) for a total of 7500 comparisons. As expected, the comparisons yielded little to no correlation between tools.

This study appears to provide support for the idea that toolmarks may be unique to the tool that they originated from; high correlation between toolmarks suggests a common source. Additionally, although the algorithm used was not developed for this specific purpose, is applicable to impression toolmarks.

Scientific Highlights:

• Sequentially manufactured chisels were examined to determine the validity of the claim that tools contain “unique”, distinguishable topographies that can be identified via toolmark examination.
• Impression toolmarks were made with a jig to ensure uniform toolmarks were created.
• Toolmarks created by the same chisel, from the same side of the chisel, showed high degrees ofcorrelation, suggesting a “match.”
• Toolmarks created by opposite sides of the same chisel showed little to no correlation, suggesting eachside of the same chisel may possess a “unique” topography.
• Toolmarks originating from different chisels and from different sides of the chisels showed little to nocorrelation, suggesting that each tool may possess a “unique” topography.

Relevance: Although the validity of comparative toolmark examination has been challenged, this study provides support for comparative toolmark examination and serves to help validate the discipline.

Potential conclusions:

• This study provides support for the practice of comparative toolmark examination.
• Each tool creates an identifiable mark, and the marks created by the same tool will show high degrees ofcorrelation when the marked surface records the impression well.

Legal Brief: Objective analysis of impressed chisel toolmarks

Steve Krejci

Toolmark evidence received heavy criticism in the NAS report for its lack of an underlying scientific basis. See Professor Jules Epstein, Preferring the “Wise Man” to Science: The Failure of Courts and Non-Litigation Mechanisms to Demand Validity in Forensic Matching Testimony, 20 WIDENER L. REV. 81 (2014). While Florida case law doesn’t give toolmark evidence the same blistering treatment found in secondary sources, limitations are nonetheless acknowledged. See Ramirez v. State, 810 So. 2d 836, 845 (Fla. 2001). When the mark is in a substance that can accurately record the abnormalities of the suspect tool, toolmark evidence may be used to reach the conclusion that the tool recovered was the same tool used in the crime at hand. See id. However, when the substance is softer, for example human tissue, the conclusions of the expert are more limited. See id. Even when the threshold for admissibility is met under Ramirez, the foundational questions required still serve to establish the credibility of such techniques. To echo the sentiment of the Epstein article, these studies are being conducted to establish a foundational basis for evidence that used to be generally acceptable. See Epstein.