// TECHNICAL FAILURE ANALYSIS // GLASS ENGINEERING MARCH 2026

NICKEL SULFIDE (NiS)

SPONTANEOUS FAILURE IN TEMPERED GLASS

Causes • Heat Soak Testing • ASTM Code Guidance • Manufacturing Impurities

290°C ~8 hrs ~4% ~95%

Heat Soak Temperature Max Soak Duration NiS Volume Expansion HST Risk Reduction

// SECTION 01

WHAT IS NiS FAILURE IN GLASS?

Nickel Sulfide (NiS) failure — also called spontaneous breakage — is the unpredictable, self-initiated

fracture of tempered (toughened) glass caused by microscopic inclusions of nickel sulfide crystals trapped

within the glass matrix during manufacturing. These inclusions are typically 0.05 mm to 0.5 mm in

diameter, yet store sufficient strain energy to shatter an entire glass panel without any external force —

days, months, or years after installation.

Unlike stress-related or impact breakage, NiS failures occur spontaneously as the inclusion slowly

transforms phase at ambient temperature, expanding volumetrically and initiating cracks in the tensile

zone of the tempered glass.

KEY DANGER PROFILE

Delayed failure: NiS breakage can occur months to decades after installation.

Unpredictable: No reliable external sign or precursor before fracture.

High-altitude risk: Facade glazing failures can drop glass from height — a serious hazard.

ORIGIN Nickel

contamination during

glass melting reacts with

sulfur to form NiS

crystals embedded in

the molten glass.

WHY TEMPERED

ONLY Tempering locks

the glass in a stressed

state. NiS expansion in

the tension zone triggers

catastrophic fracture.

PHASE

TRANSFORMATION

NiS undergoes slow

alpha-to-beta phase

transition at room

temperature, expanding

~4% in volume.

FRACTURE PATTERN

Characteristic "butterfly"

or figure-8 fracture origin

at the inclusion site

confirms NiS as cause.

// SECTION 02

THE FAILURE MECHANISM

// CHEMISTRY OF NiS FORMATION

Ni + S → NiS (during melt, ~1550°C)

NiS (alpha, high-temp hexagonal) → NiS (beta, low-temp monoclinic)

Phase transition below 379°C — volume expansion ~4% on alpha→beta reversion

Nickel Contamination in Melt

Trace nickel enters the glass melt from stainless steel equipment, raw material impurities (silica

sand, soda ash, dolomite), or contaminated cullet. At ~1550°C, nickel and sulfur combine to form

NiS inclusions in the alpha (high-temperature) phase, trapped as the glass forms.

Rapid Quench During Tempering

During tempering, glass is heated to 620–680°C then rapidly quenched with cold air. This rapid

cooling freezes the NiS inclusion in its alpha-phase before it can complete natural transformation to

the stable beta-phase (transition temperature: ~379°C).

Slow Phase Reversion at Ambient Temperature

After installation, NiS slowly reverts to its stable beta-phase. This thermally activated,

time-dependent process may take months or years. Elevated temperatures (sun-heated glass)

accelerate the transformation rate.

Volume Expansion and Crack Initiation

The ~4% volumetric expansion generates intense localized tensile stress in the surrounding glass

— critical when the inclusion lies within the tensile stress zone (central ~60% of glass thickness).

Once local stress exceeds fracture strength, a crack initiates.

Catastrophic Propagation

Stored elastic energy in the tempered panel releases through rapid crack bifurcation, shattering the

pane into characteristic small fragments. The fracture origin — "butterfly" or figure-8 mirror zone —

marks the NiS inclusion location.

CRITICAL INCLUSION ZONE — TEMPERED GLASS STRESS PROFILE

Zone Stress Type NiS Fracture

Risk

Notes

Surface layers (outer

~20%)

Compression LOW Compressive zone resists crack opening

Mid-thickness (central

~60%)

Tension HIGH Tensile zone amplifies crack-opening stress

Transition zones Neutral MODERATE Risk depends on proximity to stress reversal

// SECTION 03

HEAT SOAK TESTING: DOES IT HELP?

YES — Heat Soaking Significantly Reduces NiS Failure Risk

Heat soak testing (HST) is the only commercially proven and standardized method to provoke NiS-related

breakage before glass is installed. By artificially accelerating the alpha→beta phase transformation at 290°C,

HST triggers fractures in susceptible panes during controlled testing — rather than after installation.

Estimated to reduce post-installation failure risk by approximately 95%.

THE HEAT SOAK PROCESS

RAMP UP ~2–3 hrs

SOAK AT 290°C ± 10°C Duration: ≥ 2 hrs (EN 14179) /

up to 8 hrs

CONTROLLED COOL

DOWN

INSPECT & SORT

Why 290°C? This temperature is scientifically chosen because it lies below the NiS transition temperature

(379°C) — ensuring the beta-phase remains thermodynamically stable — while being hot enough to

dramatically accelerate the alpha→beta transformation kinetics. At 290°C, transformation that might take

years at ambient temperature occurs within hours.

COMPARATIVE RISK REDUCTION

Glass Type / Treatment NiS Failure Risk

Tempered Glass — No HST

HIGH

Tempered Glass — With HST (2 hr, EN 14179-1)

LOW

Tempered Glass — With Extended HST (8 hr)

VERY LOW

Laminated Tempered + HST

VERY LOW

Annealed Float Glass

NONE

HST COMPARISON: EN 14179-1 vs. ASTM GUIDANCE

Aspect EN 14179-1 (Europe) ASTM / US Guidance

Soak Temperature 290°C ± 10°C 290°C (GANA referenced)

Min. Soak Duration 2 hours at temperature Not mandated; 8 hrs often specified

Ramp Rate Controlled, ≤50°C/hr recommended Not specified

Mandatory / Optional Mandatory for certain uses Voluntary — project-specific

Documentation Time-temperature records required Per contract requirements

Important Limitations of Heat Soaking:

1. Not 100% effective — inclusions in compressive zones may survive HST and still cause post-installation

failure.

2. No US legal mandate — ASTM does not currently mandate HST; specification is the responsibility of

architects and engineers.

3. Cost and time — HST adds ~15–25% to tempered glass cost and several days to delivery.

4. Complementary measures needed — HST is most effective when combined with laminated glass.

// SECTION 04

ASTM CODE GUIDANCE ON NiS &

TEMPERED GLASS

ASTM International provides key standards for tempered glass in the United States. While no dedicated

ASTM NiS failure standard exists (unlike Europe's EN 14179), several core standards contain relevant

provisions and guidance.

AST

M C1

048

Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass

The primary ASTM standard for tempered flat glass. Defines Kind HS and Kind FT. Specifies

surface compression requirements (≥69 MPa for FT) and fragmentation patterns. Explicitly

acknowledges that the standard does not guarantee freedom from NiS-induced spontaneous

breakage. Recommends heat-soak testing consideration when NiS risk must be minimized,

particularly for overhead glazing and accessible public areas.

TEMPERED GLASS | FRAGMENTATION | SURFACE COMPRESSION

AST

M C1

036

Standard Specification for Flat Glass

Governs quality requirements for flat glass including allowable inclusions (seeds, bubbles,

knots, foreign inclusions). NiS inclusions are typically too small (<0.5mm) to be detected by

standard visual inspection — reinforcing that NiS is a latent defect invisible to standard QC.

The standard's inspection criteria are not sufficient to identify NiS inclusions.

FLAT GLASS | INCLUSIONS | QUALITY CONTROL

AST

M C1

172

Standard Specification for Laminated Architectural Flat Glass

Covers laminated glass products. While not addressing NiS directly, laminated tempered glass

is the de facto solution for managing residual NiS failure risk post-HST. When a tempered ply

fails from NiS, the laminate interlayer retains broken fragments, preventing fallout. Specifies

interlayer adhesion and post-breakage performance requirements critical for overhead

applications.

LAMINATED GLASS | INTERLAYER | SAFETY CONTAINMENT

AST

M E2

431

Practice for Determining Resistance of Single-Glazed Annealed Glass to Thermal

Loadings

While focused on thermal stress, this standard reinforces that glass selection must account for

multiple failure modes. Specifiers are directed to consider heat soak testing as additional risk

mitigation, particularly in high solar exposure applications where elevated temperatures

accelerate NiS transformation kinetics.

THERMAL STRESS | GLASS SELECTION | SOLAR LOADING

AST

M C1

281

Standard Specification for Preassembled Interior Glazing Systems

Addresses interior glazing applications using tempered and heat-strengthened glass types.

References the need for project specifications to address spontaneous breakage risk,

especially overhead uses, skylights, and high-foot-traffic locations. Recommends explicitly

specifying heat-soak tested or laminated tempered glass where NiS failure creates

unacceptable safety risk.

INTERIOR GLAZING | SAFETY | SPECIFICATION

ASTM vs. European Standards — The HST Gap: In contrast to EN 14179-1:2005, which provides a fully

prescriptive HST protocol, ASTM currently has no equivalent dedicated heat-soak standard. US projects

requiring HST must typically reference EN 14179-1 as the testing protocol, or rely on GANA technical

bulletins. This gap must be actively addressed in project glass specifications.

// SECTION 05

ARE IMPURITIES THE ROOT CAUSE OF NiS

FAILURE?

YES — Impurities Are the Fundamental Root Cause

NiS inclusions do not arise from inherent glass chemistry — they are entirely a consequence of

nickel-containing impurities entering the glass melt and reacting with sulfur compounds present in raw

materials or combustion processes. Controlling these contamination pathways is the primary prevention

strategy at the manufacturing level.

PRIMARY IMPURITY SOURCES IN THE GLASS MELT

Impurity Source Description & NiS Role

Stainless Steel Equipment

Furnace hardware, mixing equipment, and raw material handling from stainless steel

introduce nickel via wear particles and corrosion. Considered the single largest source

of nickel contamination in float glass plants.

Raw Silica Sand

The primary glass component can carry trace nickel as mineral impurities. Sand quality

varies by geological source; premium "glass sand" has strict Ni content limits (<1 ppm

Ni).

Soda Ash (Na2CO3)

Industrial soda ash can contain trace heavy metals including nickel from its production

process. Low-grade soda ash proportionally increases NiS formation risk.

Dolomite / Limestone

Calcium and magnesium carbonate flux materials can introduce nickel from deposits

with elevated heavy metal content. Geographic origin of dolomite significantly affects

impurity levels.

Glass Cullet (Recycled)

Recycled glass added for energy efficiency can introduce nickel from metal fittings and

hardware contamination in the recycling stream — particularly post-consumer and

industrial cullet.

Sulfur Compounds

(Na2SO4)

Sulfates used as fining agents and combustion gases (SO2/SO3) provide the sulfur that

reacts with nickel to form NiS. Without sulfur, nickel dissolves as NiO rather than

precipitating.

Furnace Combustion

Impurities in furnace fuel introduce sulfur oxides that combine with any nickel at melt

temperatures. Both nickel AND sulfur must be simultaneously present for NiS crystal

formation.

Nickel-Containing

Colorants

Certain glass colorants historically contained nickel compounds. Modern formulations

have largely eliminated these, but impure batch chemicals remain a documented risk

factor.

Critical Insight — NiS is a Threshold Phenomenon:

At very low nickel concentrations (<1 ppm total in batch), NiS inclusion frequency is dramatically reduced.

Modern high-quality glass manufacturers target nickel content below 0.5–1 ppm through strict raw material

specifications and equipment material controls. The simultaneous presence of BOTH Ni and S in the melt is

the essential condition for NiS crystalline inclusion formation.

MANUFACTURING CONTROLS TO MINIMIZE NiS FORMATION

Control Measure Target Effectiveness

Raw material Ni content limits Silica sand <1 ppm Ni; Soda ash <2 ppm Ni HIGH

Cullet quality control Reject metallic contamination; limit industrial

cullet

HIGH

Stainless steel equipment replacement Use Ni-free alloys or ceramic-lined equipment MODERATE-HIGH

Sulfate fining agent reduction Minimize Na2SO4 dosage; use alternatives MODERATE

Furnace atmosphere control Monitor SO2; control combustion stoichiometry MODERATE

Incoming material testing ICP-MS analysis of raw materials for Ni content PREVENTIVE

// SECTION 06

PREVENTION STRATEGY & APPLICATION

GUIDE

LAYER 1: SOURCE

CONTROL

Strict raw material specifications for nickel content. Equipment material controls —

eliminate stainless steel contact with the glass melt. Cullet sourcing standards.

Regular ICP-MS testing of incoming batch materials. Most cost-effective

intervention.

LAYER 2: PROCESS

DETECTION

Heat Soak Testing per EN 14179-1. Laser-based inclusion detection systems

(detects inclusions >0.3 mm). Quality control breakage monitoring as a process

health indicator. Statistical process control on spontaneous breakage data.

LAYER 3: DESIGN

MITIGATION

Specify laminated tempered glass (HST ply + interlayer) for overhead, accessible,

and high-consequence applications. Structural interlayer systems for

post-breakage load retention. Fall-arrest framing to contain fragments from failed

panels.

APPLICATION-BASED SPECIFICATION GUIDE

Application Recommended Glass Type HST Required? Standard

Overhead glazing / skylights Laminated HST Tempered YES — mandatory IBC, ASTM C1172

Structural glass floors Laminated HST Tempered YES ASTM C1281

Exterior facade (accessible) HST Tempered or Laminated Strongly recommended ASTM C1048 + GANA

Curtain wall (upper floors) HST Tempered Recommended ASTM C1048

Balustrades / balconies Laminated HST Tempered YES Local codes + ASTM

Interior partitions Tempered (HST optional) Optional — risk-based ASTM C1048

Non-accessible spandrel Tempered (HST recommended) Recommended ASTM C1048

// SECTION 07

TECHNICAL SUMMARY

NiS FAILURE

Spontaneous, time-delayed fracture of tempered glass caused by the

alpha→beta phase transformation of NiS inclusions in the tensile zone. ~4%

volume expansion generates fracture-initiating tensile stress without any

external force.

ROOT CAUSE

Nickel and sulfur impurities in the glass batch — from stainless steel equipment,

raw materials, and cullet — form NiS crystals during the melt, quenched into a

metastable alpha-state by the rapid cooling of the tempering process.

HEAT SOAK

Heating tempered glass to 290°C for ≥2 hours accelerates phase transformation,

causing susceptible panels to break safely in the furnace before installation. Per

EN 14179-1. Reduces post-installation failure risk by ~95%.

ASTM CODES

ASTM C1048 acknowledges NiS and recommends HST. No dedicated US NiS

test standard exists — EN 14179-1 is the reference. ASTM C1172 laminated

glass provides post-failure containment. Project specifications must address

HST explicitly.

KEY TAKEAWAY FOR SPECIFIERS:

For any application where tempered glass failure could injure people or cause significant damage, the

best-practice specification is: Heat-Soaked Tempered Glass (per EN 14179-1) + Laminated with

structural-grade interlayer (per ASTM C1172). This two-layer approach addresses both the failure

trigger (via HST) and the consequences of failure (via laminated fragment containment).

REFERENCES & STANDARDS

• ASTM C1048 — Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass

• ASTM C1036 — Standard Specification for Flat Glass

• ASTM C1172 — Standard Specification for Laminated Architectural Flat Glass

• ASTM C1281 — Standard Specification for Preassembled Interior Glazing Systems

• ASTM E2431 — Standard Practice for Determining Resistance of Single-Glazed Glass to Thermal Loadings

• EN 14179-1:2005 — Glass in Building — Heat Soaked Thermally Toughened Safety Glass

• GANA Technical Bulletins — Glass Association of North America

• Karlsson, S. (2017) — Spontaneous Cracking of Thermally Tempered Glass, Review and Outlook

• IBC Chapter 24 — Glass and Glazing Requirements (International Building Code)

This document is for technical reference purposes. For project-specific glass specifications, consult a licensed structural engineer and certified

glazing specialist.