Force a door, cut a padlock or lever a window and the tool leaves its signature behind. Tool-mark examination reads those marks — and, like firearms work, it operates on two levels: the type of tool, and the individual tool.
Two kinds of tool mark
- Impressed marks — made when a tool is pressed into a softer surface without moving sideways (a stamped or punched mark). These record the tool's shape.
- Striated marks — made when a tool slides across a surface (a pry bar levering, a blade scraping), dragging fine parallel striations.
Most break-in marks are a mix of the two.
Class versus individual detail
- Class characteristics — the tool's type, width and shape can narrow the field: a 12 mm flat-blade screwdriver, say, rather than pliers.
- Individual characteristics — the microscopic nicks and imperfections on a tool's working edge, acquired in manufacture and use, scratch a distinctive pattern of striations unique to that one tool.
How the comparison is done
The examiner makes test marks with the suspect tool — in a soft material, at various angles and pressures, because the mark changes with how the tool is held — then compares them against the crime-scene mark (or a cast of it) under a comparison microscope, looking for striations that align.
The cautions
Tool marks are trickier than they sound. A tool's edge can change over time, the mark depends heavily on angle and force, and soft surfaces deform. As with other pattern disciplines, examiners increasingly express conclusions with defined, honest confidence rather than absolute certainty — and a good one will say when a mark simply isn't suitable for comparison.