Chapter 24 — Key Takeaways

A scannable one-page card. For the full argument and the worked examples, see index.md.

The core claims

• Four materials, one lesson. Gunshot residue, paint, glass, and soil are all transfer evidence (Locard, Chapter 3) and almost always class evidence: each associates an item with a category or environment, not with one source to the exclusion of all others. They exclude well, associate honestly, and prove almost nothing by themselves.
• GSR establishes presence, not action. A primer particle (lead + barium + antimony fused in a characteristic shape, identified by SEM-EDX) on a person means GSR is present, not that the person fired a weapon. The contamination/transfer problem — secondary transfer, occupational/environmental background, patrol-car and booking-area contamination — fills the gap. (The FBI Lab discontinued routine GSR exams in 2006.) A negative GSR proves little (particles are lost within hours).
• Paint's weight is in the SEQUENCE. One common layer proves little; a long, unusual stack of layers (factory system + an aftermarket repaint, same order, same chemistry) makes coincidence implausible. A physical (fracture) fit is the rare exception that approaches individualization.
• Glass: refractive index excludes; it associates weakly. A difference in refractive index excludes a common source cleanly. A match is "consistent with a common source" only — float glass is mass-produced. Fracture analysis (radial vs. concentric cracks, edge stress marks) reveals which side the force came from and the sequence of breaks — mechanics, not the identity of the breaker.
• Soil is the richest associative material — and still class evidence. A composite of color, mineralogy, texture, pollen (Chapter 13), and chemistry; its strength scales with distinctiveness and with the quality of control samples (the distinctive spot and the background). It can place an object/person in an environment; it cannot fix a time or prove guilt.
• Strength is probabilistic: rarity = distinctiveness × number of independent features. The question is always: what is the probability this association arose by coincidence? Distinctive + many independent matching features → strong ("strongly supports"); common or single-feature → weak ("consistent with").
• Overstatement is the failure mode — and it is a sentence, not a measurement. The instruments rarely lie; the characterization does. Retire the bare word "match" (the jury hears "same object"); never invent a frequency without a validated database.

The method-validity verdict (NAS 2009 / PCAST 2016)

Where they sit: above the discredited pattern methods (bite marks) because the core measurements are objective and instrument-grounded; below single-source DNA because the associations are class-level and rarely come with a validated frequency. A respectable middle — kept there by refusing to overstate.

What you can honestly say on the stand

• GSR (presence): "Particles consistent with primer residue were present on the sample. Their presence is consistent with the person having discharged, handled, or been near a fired firearm, or with contact transfer from a contaminated person, surface, or environment. I cannot, from the particles alone, distinguish among these."
• Paint: "The questioned and known paint are indistinguishable across [N] layers, including [an unusual buried repaint]. This strongly supports a common origin, but I cannot exclude another vehicle finished the same way."
• Glass: "The fragment is indistinguishable from the known glass in refractive index and density — consistent with a common source — but glass of this type is mass-produced, so I cannot say it came from this window to the exclusion of others." (Or, for a mismatch: "...differs in refractive index; it did not come from that window.")
• Soil (the cold case): "The soil on the boots is consistent with, and — given that the controls show the matched mineral and pollen combination to be uncommon in the region — strongly supports, the boots having been in the cabin's distinctive environment. It does not establish when the soil was acquired, does not prove a crime, and associates the boots with a place, not a person with the killing."
• What you must NOT say: the bare word "match" with no limit; any invented frequency ("one in a million"); "GSR proves he fired the gun"; "the paint/glass came from this object" (without "cannot exclude"); "the soil proves he was there when it happened" or "proves he did it."

The cold-case line

The soil on Roy Keller's boots strongly supports placing his boots — and so, reasonably, Keller — in the cabin's distinctive environment (the controls show the match is to an uncommon local soil). That is a class/probabilistic association with a PLACE: not a time, not guilt. Keller is physically tied to the scene's environment — one honest brick (Chapter 39). The glass is a class-level supporting thread; the GSR is negative, consistent with the no-gunshot homicide (Chapter 11) and the red-herring cartridge case (Chapter 15). No one is excluded here; Keller is associated with the environment — which is not the same as named the killer. Resist the overstatement (§24.6).

The themes this chapter advanced

• Exclusion over proof (Theme 1) — every material is surest when it says no (different layer, different refractive index, absent mineral, no residue); associations are stated as probabilities, never "proves."
• The validity spectrum (Theme 2) — objective measurements supporting class associations: above the discredited pattern methods (sound measurements), below DNA (class-level, rarely a validated frequency).
• (Also touched: cognitive bias — the GSR examiner can be biased by case context, and the contamination happens upstream where the analyst can't see it, §24.1; and the CSI effect — the television "GSR test" that proves the shooter, §24.1.)