Glass: Composition, Types, Fractures and Examination

Glass is a non-crystalline (amorphous) solid built around a three-dimensional network of silicon-oxygen tetrahedra. Pure fused silica (over 99.5 percent SiO2) is chemically inert and thermally stable but very hard to melt and shape, so the industry adds network modifiers and intermediates to lower the working temperature and tune the properties. Four families account for the large majority of glass evidence encountered in casework.

Soda-lime glass. The everyday glass: window panes, beverage bottles, drinking glasses. Approximate composition is 72 percent SiO2, 14 percent Na2O (network modifier from sodium carbonate, soda ash) and 10 percent CaO (network stabiliser from limestone), with the balance as MgO, Al2O3 and trace iron. Refractive index sits in the 1.51 to 1.52 band, which is the first number to remember.

Borosilicate glass. Replace the soda with boric oxide and the glass tolerates thermal shock. About 80 percent SiO2 and 13 percent B2O3, with smaller Na2O and Al2O3. Refractive index about 1.47. This is the Borosil and Pyrex family used in laboratory beakers and pharmaceutical vials.

Lead crystal. Soda-lime base with a high lead oxide (PbO) load, typically 18 to 30 percent. PbO raises the refractive index above 1.55 and increases density and brilliance, which is why decanters and chandeliers use it. Lead crystal is easy to identify by its very bright SEM-EDX lead peak.

Fused silica. Over 99.5 percent SiO2, used in semiconductor optics, UV cuvettes and high-end fibre optics. Rare in casework but classic distractor in MCQs as the answer to "highest silica content".

The composition matters forensically because two fragments from the same source share elemental fingerprints (trace Ba, Sr, Zr, Hf, REEs) that can be matched on LA-ICP-MS or ICP-OESeven when refractive index alone is ambiguous.

Glass type determines failure mode, which in turn shapes what evidence a broken pane leaves behind.

Annealed soda-lime sheet. Standard window panes, bottles, tableware. Breaks into long sharp shards. Used in residential windows and bottles, which is why most break-in casework involves annealed soda-lime fragments.

Tempered (toughened) glass. Heat-treated to put the surface into compression. Roughly four to five times stronger than annealed glass. When it does break, the stored surface stress shatters the whole pane into small, blunt cubic fragments (dicing). Used in car side and rear windows, shower screens, glass doors, and most public-transport glazing. The dicing pattern is the diagnostic feature in vehicle break-in cases.

Laminated glass. Two sheets of annealed glass bonded with a 0.38 to 0.76 mm polyvinyl butyral (PVB) interlayer. The glass cracks but the fragments stick to the PVB, so the pane stays in place. Standard for car windshields (under IS 2553, the Bureau of Indian Standards specification for automotive safety glazing), and for bank counters, jewellery showcases and balcony glazing. The PVB layer holds tool-mark and DNA traces and is itself examined.

Bulletproof glass. Multi-ply laminate of glass and polycarbonate, sometimes with an outer ceramic or sapphire layer for the highest threat ratings. Stops projectiles by spreading energy across the layers; the back face spalls fragments inward (the polycarbonate catches them). Tested under EN 1063 or NIJ levels.

Wired glass. Annealed glass with an embedded wire mesh, traditionally used in fire doors and stairwell windows. The wire holds fragments in place at high temperature. Easily identified by the visible mesh.

Float glass (manufacturing format). Almost all modern flat glass is manufactured by the Pilkington float process, where molten glass spreads on a bath of molten tin. The tin-contact face picks up tin and fluoresces dull blue-white under short-wave UV (254 nm). The opposite face does not fluoresce. This lets the analyst tell which side of a fragment faced inward, which matters for fracture-direction analysis.

When a blunt object strikes a glass pane, two crack families develop around the impact point.Radial cracks start at the impact point and propagate outward like spokes of a wheel; they form first because the pane bends and the back face goes into tension.Concentric cracks form a moment later, running in roughly circular arcs around the impact point on the front face, where the pane has bowed back and put the front face into tension. Together they make a spider-web pattern.

The single most important fracture rules in the syllabus are the 3R rule and the 4C rule. They tell you which side the blow came from.

• 3R rule.Radial cracks have Ridges on the Reverse side of the force. Pick up a radial crack edge and look at the conchoidal (shell-like) Wallner ridges. The side with the ridges parallel to the surface is the side opposite the impact. So if ridges are on the inside face of the fragment, the blow came from the outside.
• 4C rule.Concentric cracks have ridges on the side of the Cause of force (same side as the force). The mirror image of the 3R rule. Together, 3R and 4C cross-check each other.

Two more features appear on every fracture surface.Wallner lines are wavy, curved ridges that record the position of the crack front at successive instants; trace them backward to find the origin of the fracture.Hackle marks are tiny step-like ridges that fan out from the origin in the direction the crack is travelling.

Sequencing two impacts. A fracture from a second shot or second blow stops when its radial cracks meet the radial cracks of the first impact, because those existing cracks have already relieved the stress. So the impact whose radial cracks run uninterrupted is the first one. This is a standing favourite in MCQs and in ballistics short-answer questions on multi-shot windscreens.

Bullet hole bevel. When a high-velocity projectile passes through glass, it punches a clean hole on the entry side and chips out a cone of glass from the exit side. The hole is therefore wider on the exit side than on the entry side. This is the inward bevel rule: the cone narrows from the exit face back toward the entry face. Combined with radial-crack 3R analysis, the bevel direction tells the investigator which side the shot came from.

Heat fractures versus mechanical fractures. Glass broken by fire shows smooth, wavy fracture lines with no radial or concentric pattern and no Wallner ridges, because the cracks form slowly under thermal stress rather than as a fast brittle response to an impact. This distinguishes arson from forced entry.Tempered-glass dicing(cubic fragments) is its own pattern and does not produce radial or concentric cracks at all, because the stored compressive stress shatters the whole pane uniformly.

A glass-fragment exhibit at an Indian SFSL or at CFSL Chandigarh (the trace-evidence reference laboratory) goes through a stepped workflow that moves from cheap and fast to expensive and specific.

1. Visual and stereomicroscopic examination. Colour, thickness, curvature, surface coating, presence of mesh wire or PVB residue. A 4 mm flat pane with PVB on one face is almost certainly a windshield fragment.
2. Refractive index. The Becke line method on an immersion-oil mount under a polarising microscope gives an estimate. For casework, the GRIM3 (Glass Refractive Index Measurement)hot-stage system raises the temperature of a silicone oil to the match point where the fragment disappears against the oil; the match point gives RI to four decimal places. Soda-lime is roughly 1.51 to 1.52, borosilicate about 1.47, lead crystal above 1.55.
3. Density measurement. A density-gradient column of bromoform and bromobenzene (or modern halogen-free equivalents) ranks fragments by where they float. Two fragments from the same pane settle at the same height.
4. Elemental analysis. SEM-EDXgives the major-element fingerprint (Si, Na, Ca, Mg, Al) and flags lead crystal at a glance. LA-ICP-MS gives trace and rare-earth elements at parts-per-billion sensitivity and is the current gold standard for source matching of float glass.
5. UV fluorescence. Short-wave UV (254 nm) makes the tin-contact face of float glass fluoresce dull blue-white. Useful for orienting fragments and confirming float-process origin. The companion UV-Vis spectrophotometryworkflow on the lamp side is a related forensic casework topic.
6. Polariscope for tempered glass. A circular polariscope reveals the residual stress birefringence pattern of tempered glass, which annealed glass does not show.

Every step is documented and the exhibit is logged in the chain of custodyregister from collection through to the final report. Under the Bharatiya Sakshya Adhiniyam 2023, the analyst must be able to defend each step at trial.

Glass evidence appears in five recurring Indian casework scenarios.

Vehicle break-ins. Car side windows are tempered glass; a forced entry leaves a distinctive carpet of small dicing cubes on the driver's seat and footwell. Recovery of dicing fragments from a suspect's clothing or shoes can place them at the scene.

Hit-and-run windshield analysis. Modern windshields are laminated. Pedestrian-impact cases leave the windshield cracked but intact, with PVB-bonded fragments that hold tissue, hair and blood traces. The crack pattern around the impact point can be analysed for the height and angle of contact, and the forensic examination of firearms, bullets, shells and cartridgesworkflow on bullet-through-windshield cases applies the bevel rule on the laminated assembly.

Burglary through window panes. Annealed soda-lime windowpane fragments transfer onto a suspect's clothing, hair and shoe soles. Two fragments matching on RI, density and SEM-EDX major-element profile become strong circumstantial evidence in property-offence trials.

Assault with a glass bottle (BNS Section 116, voluntarily causing hurt by dangerous weapon). Beer-bottle fragments at the scene are matched to a suspect's broken bottle by fracture-line jigsaw fit and by RI / SEM-EDX comparison. The 3R rule is used to confirm the direction of the strike.

Arson investigation. Smooth wavy fracture lines and the absence of radial or concentric patterns distinguish fire-origin glass damage from forced entry, which is critical when the fire investigator is deciding between accidental and incendiary cause. CFSL Chandigarh's trace-evidence division maintains a reference set of float-glass RI standards used across Indian state SFSLs.