Glass Fracture Analysis and Collection

Why Glass Evidence Matters

Broken glass appears at burglary scenes, vehicle collisions, assaults, and shootings. The fracture pattern left behind is not random — physics controls how glass breaks, and those physical rules let a trained technician determine where force was applied, from which direction, and in what order multiple impacts occurred. Glass fragments transferred to a suspect's clothing or shoes can also place a person at the scene. Knowing how to read, document, and collect glass evidence without cross-contamination is a core competency for the IAI CCSI credential and most state evidence technician certifications.

How Glass Breaks: Radial and Concentric Fractures

When a concentrated force strikes a flat pane of glass, two distinct crack types form:

• Radial cracks extend outward from the point of impact like spokes on a wheel.
• Concentric cracks arc around the impact point, connecting the radial cracks like rings on a target.

Together, the pattern resembles a spider web. The impact point is usually at or very near the center of the web.

The FORCE Rule: Determining Direction of Force

The side of the glass on which force was applied can be identified by examining the stress fractures (rib marks) on the edge of a radial crack fragment. Use the FORCE rule:

> Fracture Originates on the Radially Cracked Edge

Rib marks on radial cracks form at right angles to the surface that received the force. In practice: look at the cross-section of a radial crack edge. The curved rib marks (also called conchoidal fracture lines or hackle marks) curve away from the surface that was struck first. On a radial crack, the rib marks are perpendicular to the surface where force originated. On a concentric crack, the rib marks are perpendicular to the opposite surface.

A simpler field memory aid for radial vs. concentric edges:

• Radial cracks — ribs are at 90° to the force side.
• Concentric cracks — ribs are at 90° to the side away from force.

By picking up a fragment and viewing the broken edge with a hand lens, you can determine which face of the glass received the blow.

Sequencing Multiple Impacts

When a pane of glass is struck more than once, each new impact sends out its own radial and concentric cracks. However, a crack cannot cross a crack that already exists — it terminates (stops) at the existing fracture line. This is the fracture termination principle:

> Radial cracks from a later impact will stop when they reach a crack produced by an earlier impact.

To determine which impact came first:

1. Trace a radial crack from Impact Site A outward.

2. If that crack is terminated (blocked) by a crack from Impact Site B, then Impact B occurred first.

3. If Impact A's crack runs freely past that area, then A occurred first.

By systematically tracing which cracks are stopped by which, you can reconstruct a sequence even when three or more impacts are present.

Safety First

Glass handling carries serious laceration risk. Before approaching any broken glass:

• Put on heavy-duty cut-resistant gloves rated for glass handling — standard nitrile exam gloves are not sufficient.
• Wear safety glasses or a face shield at all times; a face shield is preferred when leaning over fragments.
• During live demonstrations, remain behind the protective barrier until the instructor confirms the area is safe.
• Never reach blindly under or behind glass pieces; use forceps.
• Dispose of glass waste only in a puncture-resistant sharps or glass disposal container — never in a regular trash bag.

Collecting Glass Fragment Evidence

Glass is a trace form of physical evidence, and contamination or breakage during packaging can destroy its evidentiary value. Follow these steps:

1. Photograph in place before touching anything. Capture overall, mid-range, and close-up images.

2. Sketch and measure the fracture pattern, noting all radial and concentric cracks, the probable impact point, and any missing sections.

3. Use forceps — never bare hands — to pick up individual fragments. Handle by edges when possible to preserve any latent prints.

4. Package each significant fragment separately in a rigid, crush-proof container (a pillbox or rigid plastic vial) with cotton padding to prevent the fragment from shifting and breaking further. Paper folds are acceptable for bulk glass but rigid containers are required for key fragments.

5. Seal with evidence tape, label with case number, item number, date, time, location, and your name.

6. Complete a chain-of-custody tag attached to or written on the container before the item leaves your hands.

7. For glass dust and micro-fragments on nearby surfaces, use a trace-evidence tape lift: press the tape firmly to the surface, lift smoothly, and mount the tape sticky-side down on a clear acetate sheet or card. Package in a paper or glassine envelope — not plastic, which can cause static and fragment loss.

Documentation Standards

Every fracture analysis should be documented with:

• A scaled sketch of the fracture pattern with arrows indicating the determined direction of force.
• Written rationale citing specific rib marks or fracture termination as the basis for each conclusion.
• Photographs taken before any fragment is moved.

Sloppy documentation is a common reason evidence is challenged in court. Treat your notes as if a defense attorney will read every word — because they will.