Breaking glass showers tiny fragments backward — often onto the clothing, hair and shoes of the person who broke it. Because the fragments are easy to miss and hard to fake, glass is a classic form of trace evidence.
Comparing the fragments
The question is usually whether fragments recovered from a suspect could have come from the glass at the scene. Analysts compare physical and chemical properties:
- Refractive index (RI) — how much the glass bends light, measured precisely; a key discriminating property.
- Density.
- Elemental composition — trace elements measured by techniques such as LA-ICP-MS, which can strongly distinguish glass from different sources.
If the properties differ, the fragments are excluded. If they match, the glass is consistent with a common origin — glass is generally class evidence, so it supports rather than proves a link.
A rare exception: physical fit
Occasionally a recovered fragment can be physically fitted back into the broken pane like a jigsaw piece — a "fracture match". That is far more powerful, effectively individualising the fragment to that specific broken object.
Transfer and persistence
As with all trace evidence, interpretation depends on how readily glass transfers and how long it persists. Fragments can shed from clothing within hours, so their presence — and quantity — is weighed alongside the account of events rather than read in isolation.