Which instrumental techniques do Indian SFSLs use to match two glass fragments?

The standard workflow is refractive index first (Becke line and then GRIM3 hot-stage to four decimal places), density-gradient column second, then SEM-EDX for major-element composition (Si, Na, Ca, Mg, Al, and lead for crystal), and finally LA-ICP-MS for trace and rare-earth elements at parts-per-billion sensitivity. Short-wave UV (254 nm) is used to identify the tin-contact face of float glass, and a circular polariscope reveals the residual stress pattern of tempered glass. CFSL Chandigarh maintains reference RI standards used across the state SFSL network.

Glass: Composition, Types, Fractures and Examination

Q: Why does tempered glass break into cubes instead of shards?

Tempered (toughened) glass is made by heating annealed glass to about 620 degrees Celsius and then rapidly cooling the surfaces. The surfaces solidify and contract while the core is still hot, locking the surfaces into compression and the core into tension. The stored stress is roughly four to five times the strength of annealed glass, but once a crack penetrates the compressive layer the whole pane releases its stress almost simultaneously and shatters into small, blunt cubic fragments called dicing. The pattern is diagnostic of tempered glass, which is why a carpet of glass cubes on a car footwell tells the investigator the side window was broken.

UGC-NET Paper 2 Unit VII notes on glass: soda-lime, borosilicate, tempered, laminated, refractive index, 3R and 4C rules, bullet-hole bevel, SEM-EDX and CFSL practice.

Last updated: 18 May 2026

Glass is one of the most informative trace materials NTA tests in Unit VII, because every sub-part has a clean one-line answer: composition (soda-lime ~72 percent SiO2 + 14 percent Na2O + 10 percent CaO), the fracture rules (3R radial reverse, 4C concentric same side as force), refractive index by Becke line or GRIM3, and the bullet-hole inward bevel that points the way the projectile travelled. The syllabus bullet bundles four sub-questions into one line, so candidates who memorise the composition table, the fracture diagram and the RI workflow pick up most of the available marks without reading further.

Treat this as a memorisation-heavy bullet anchored on a single visual: a sheet of glass with radial and concentric cracks around an impact point. Learn the four common glass families, the four fracture features (radial, concentric, Wallner lines, hackle marks), the two crack rules (3R and 4C), the bullet-hole bevel direction, and the SFSL examination chain from refractive index to SEM-EDX. The book chapter on glass evidence, fracture and refractive index covers the spatter side and the GRIM3 hot-stage workflow in much more depth.

Key terms

Soda-lime glass: The commonest glass family. About 72 percent SiO2, 14 percent Na2O (from soda ash) and 10 percent CaO (from limestone), with small amounts of MgO and Al2O3. Used in windows, bottles and tableware. Refractive index roughly 1.51 to 1.52.
Borosilicate glass: Heat-resistant glass with about 80 percent SiO2 and 13 percent B2O3 (boric oxide). Low thermal expansion. Used in cookware (Borosil, Pyrex) and laboratory glassware. Refractive index about 1.47.
Tempered glass: Thermally or chemically toughened glass that breaks into small, blunt, dice-like cubes (called dicing) instead of shards. Used in car side windows, shower screens and mobile-phone screen protectors.
Laminated glass: Two glass sheets bonded with a polyvinyl butyral (PVB) interlayer. Cracks but does not shatter. Standard for windshields and bank counters. The PVB layer is itself a forensic substrate.
Float (Pilkington) glass: Sheet glass manufactured on a bath of molten tin. The tin-contact face fluoresces under short-wave UV, which lets the analyst orient a fragment and confirm float origin.
Radial crack: A crack that runs outward from the point of impact, like spokes of a wheel. The 3R rule applies: Radial cracks have Ridges on the Reverse side of the force.
Concentric crack: A crack that runs in roughly circular arcs around the point of impact, joining radial cracks. The 4C rule applies: Concentric cracks have ridges on the same side as the Cause of force.
Wallner lines and hackle marks: Wavy curved lines (Wallner) and step-like ridges (hackle) on a fracture surface that record the direction the crack front was moving. Used to trace cracks back to the origin.

Composition: the amorphous silica network and its modifiers

Silica plus modifiers. Change the modifiers, change the glass.

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. For NET MCQs, remember four families.

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 Indian kitchens, 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-OES even when refractive index alone is ambiguous.

Types: windows, bottles, tempered, laminated, bulletproof, wired

Six families, six failure modes, six investigation handles.

NTA's "types" sub-question is asking you to map a use case to a glass family and a failure mode.

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 AIS-001, 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.

Fracture analysis: radial, concentric, 3R and 4C, Wallner lines

Two crack families, two rules, one diagram you must memorise.

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 UGC-NET 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.

Examination workflow in Indian SFSL and CFSL practice

Refractive index first, density second, then SEM-EDX, then LA-ICP-MS.

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.

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.
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.
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.
Elemental analysis. SEM-EDX gives 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.
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 spectrophotometry workflow on the lamp side is a related Paper 2 topic.
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

Indian casework context

Vehicle break-ins, windshields, burglary, BNS Section 116 assault with a bottle.

Glass evidence shows up in five recurring Indian casework scenarios that NTA likes to weave into short-answer questions.

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 cartridges workflow 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.

MCQ takeaways

Soda-lime glass composition: about 72 percent SiO2, 14 percent Na2O, 10 percent CaO. RI 1.51 to 1.52.Borosilicate glass has about 13 percent B2O3 and RI about 1.47.3R rule: Radial cracks have Ridges on the Reverse side of the force.4C rule: Concentric cracks have ridges on the side of the Cause of force.Bullet hole is wider on the exit side, with a cone-shaped inward bevel.First-shot radial cracks stop subsequent-shot cracks. Sequence multi-shot windows by which cracks run uninterrupted.Tempered glass breaks into small cubic dicing, not shards. Standard for car side windows.Laminated glass has a PVB interlayer and is standard for windshields.Float-glass tin face fluoresces under short-wave UV (254 nm).GRIM3 hot-stage system gives RI to four decimal places by oil-immersion match point.Heat-fractured glass shows smooth wavy lines, no radial or concentric pattern.

What is the 3R rule in glass fracture analysis for UGC-NET?

The 3R rule says that Radial cracks have Ridges on the Reverse side of the force. Pick a radial crack edge and look at the curved Wallner ridges on the fracture face; the face that carries the ridges is the side opposite the impact. So if the ridges are on the inside of a window pane, the blow came from outside. The complementary 4C rule says Concentric cracks have ridges on the same side as the Cause (the side that was struck). Together the two rules cross-check each other and let the analyst tell which side a blow or shot came from.

What is the composition of ordinary soda-lime glass?

Soda-lime glass, the everyday window and bottle glass, is roughly 72 percent silicon dioxide (SiO2), 14 percent sodium oxide (Na2O, from soda ash), and 10 percent calcium oxide (CaO, from limestone), with the remaining few percent as magnesium oxide, aluminium oxide and trace iron. The refractive index sits in the 1.51 to 1.52 band. Borosilicate is about 80 percent SiO2 with 13 percent B2O3 and RI about 1.47, and lead crystal carries 18 to 30 percent PbO with RI above 1.55. These three numbers cover most NET composition MCQs.

How do you tell the direction of a bullet that has passed through a window pane?

A bullet punches a clean hole on the entry face and chips out a cone of glass from the exit face, so the hole is wider on the exit side. This inward-bevel rule means the cone narrows from the exit face back toward the entry face. Combine it with the 3R rule on the radial cracks around the hole: the side carrying the Wallner ridges is the reverse side of the force, that is, the exit side. Both rules together fix the direction of fire unambiguously.

Why does tempered glass break into cubes instead of shards?

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