Tool Mark and Impression Evidence Analysis
Domain 4 tests tool mark and impression analysis: recognizing how a tool or key left a mark, separating class characteristics from individual characteristics, and stating a comparison conclusion at the strength the data supports.
Core concepts
- Tool marks are either impressed, striated, cut, or gouged. Direction and substrate matter: brass, zinc alloy, steel, wood, and drywall record marks differently.
- Class characteristics include tool type, width, shape, and general profile. Individual characteristics include microscopic striations, chips, and defects that may be unique to a tool.
- Comparison requires a known exemplar made on a similar substrate with similar force and direction, examined under a comparison microscope or high-magnification imaging.
- Impression evidence can include keyway marks, wax or putty impressions, dust prints, and marks in paint or wood. Casts and photographs must be scaled.
- Conclusions should be graded: identification, strong association, inconclusive, or exclusion. 'Consistent with' is not identification.
Worked example
A brass strike plate has a fresh gouge beside the screw. A suspect screwdriver is submitted. You note the gouge width matches the screwdriver tip, which is a class feature, then make test marks in soft brass with the same tip at a similar angle. Under magnification, three striation groups align in sequence and spacing. You report a strong association, not a unique identification, because the substrate is soft and the mark is short.
Traps
- Declaring a unique match from class features alone.
- Comparing a mark in drywall to a test mark in brass.
- Ignoring direction of force and tool angle.
- Overlooking wear or damage that changed the tool between crime and test.
- Using 'match' without defining the comparison scale.