Asperity
Definition
A microscopic surface protrusion on a tool's cutting edge; each asperity produces a groove in the substrate when the tool slides, contributing to the striated-mark pattern.
- Scale
- Microscopic
- Location
- Tool cutting edge
- Effect
- Produces a groove when the tool slides across a substrate
- Forensic role
- Source of individual striation pattern
Common questions
Why do asperities make each tool mark potentially individualising?+
The arrangement of asperities on a cutting edge results from random manufacturing and wear processes unique to that tool, so the resulting striation pattern is treated as a candidate for individualisation, similar in logic to a fingerprint ridge pattern.
Can asperity patterns change over a tool's working life?+
Yes, continued use, sharpening, or damage alters or removes existing asperities and can create new ones, which is why tool mark comparisons ideally use a reference tool in a condition close to when the questioned mark was made.
Related terms
- AFTE Theory of Identification
- The Association of Firearm and Tool Mark Examiners' standard (1992, revised 2011) for reporting tool-mark comparison conclusions as Identification, Inconclusive, or Elimination...
- Black-Box Validation
- A study design where examiners are given unknown questioned-vs-known pairs and asked to match or exclude without feedback, allowing error rates to...
- Class Characteristics
- Features deliberately or systematically imparted to a group of items by design or manufacture, such as the calibre of a bullet, the...
- False-Positive Rate
- The proportion of authentic media files that a detection tool incorrectly classifies as manipulated or synthetic. Deepfake detectors and ELA tools both...
- Impressed Mark
- Also called indented or compression mark. Tool is pressed perpendicularly into the softer surface and reproduces the contour of the tool's working...
- Individual Characteristics
- Surface features on a fired bullet or cartridge case that arise from random or unique events (tool wear, corrosion, service damage) and...
- OSAC (NIST)
- Organization of Scientific Area Committees for Forensic Science under the US National Institute of Standards and Technology; the successor body to SWGDOC,...
- PCAST 2016
- President's Council of Advisors on Science and Technology 2016 report 'Forensic Science in Criminal Courts', which assessed foundational validity of forensic disciplines...
- Shear Zone
- The region of a bolt-cutter or wire-cutter mark showing striations from the blade edge sliding across the cut face; the highest individual-characteristic-content...
- Striated Mark
- Also called scratch, abrasion or scrape mark. Tool slides laterally across the surface and leaves parallel striations from microscopic imperfections on the...
- Subclass Characteristics
- Features common to a small batch of weapons made with the same tool before the tool wore out. Sit between class and...