HEATSOAK GLASSHEATSOAK GLASS – Is it worth it! – Is it worth it!

Nice Term – What Does it Mean?Nice Term – What Does it Mean?

• Used when glass breaks, and there is no Used when glass breaks, and there is no apparent cause.apparent cause. Could be due to glass damageCould be due to glass damage Or, building/framing system movementOr, building/framing system movement Or, vandalismOr, vandalism

Or, inclusionsOr, inclusions

INCLUSIONSINCLUSIONS IN FLOAT GLASS IN FLOAT GLASS

• There are as many as 50 possible inclusion types There are as many as 50 possible inclusion types in float glass. They include, among others:in float glass. They include, among others: ZirconiaZirconia AluminaAlumina SilicaSilica And, of course, Nickel SulfideAnd, of course, Nickel Sulfide

INCLUSIONSINCLUSIONS IN FLOAT GLASS IN FLOAT GLASS

• While there are a variety of inclusion types, the While there are a variety of inclusion types, the most widely discussed is nickel sulfide.most widely discussed is nickel sulfide. Can occur in float glass when “tramp” nickel rich particles Can occur in float glass when “tramp” nickel rich particles

combine with sulphur in the melting process.combine with sulphur in the melting process.

Relatively rare, practically invisible, and not practical to detect Relatively rare, practically invisible, and not practical to detect in a production environment.in a production environment.

Frequency is random within the float ribbon and, therefore, in Frequency is random within the float ribbon and, therefore, in individual glass lites.individual glass lites.

The frequency and/or location of individual inclusions is The frequency and/or location of individual inclusions is practically impossible to quantify or predict.practically impossible to quantify or predict.

A Nickle Sulfide StoneA Nickle Sulfide Stone

A 0.1mm (~0.004”) StoneA 0.1mm (~0.004”) Stone

Source – “The Achille Heel of a Wonderful Material – Toughened Glass by Dr. John Barry

There is NiS and there is NiS …There is NiS and there is NiS …

• Nickel Sulfide stones exist in a number of formsNickel Sulfide stones exist in a number of forms

NiNi33SS22, Ni, Ni77SS6, 6, NiNi99SS88 – typically not a problem – typically not a problem

NiNi1-x1-xS, where x = 0 to 0.07 – can cause spontaneous S, where x = 0 to 0.07 – can cause spontaneous breakage of tempered glass breakage of tempered glass

Stone with Multiple StoichiometryStone with Multiple Stoichiometry

Source – “The Achille Heel of a Wonderful Material – Toughened Glass by Dr. John Barry

What Happens?What Happens?

• NiS undergoes a phase change – from the NiS undergoes a phase change – from the alpha-phase at higher temperature to the alpha-phase at higher temperature to the beta-phase at lower temperature, beta-phase at lower temperature, with a volume increase of approximately 4%with a volume increase of approximately 4%

• If the stone is in the tension zone of tempered If the stone is in the tension zone of tempered glass and undergoes the phase change, the stress glass and undergoes the phase change, the stress caused by the increased volume may lead to caused by the increased volume may lead to breakage.breakage.

From Alpha to Beta …From Alpha to Beta …

Stone in the alpha phase

Stone in the beta phase

The RED line represents the size change that results in an approximate 4% increase in volume.

In Proportion, the Glass would be 4.5 Meters Thick

To Further Put it in Perspective…To Further Put it in Perspective…

Glass

The inclusion

If we magnify it by a factor of 10

When are They a Problem?When are They a Problem?

• Annealed Glass – NeverAnnealed Glass – Never Any stones undergo the phase change and become stable Any stones undergo the phase change and become stable

during the slow re-annealing of the glass in the float glass during the slow re-annealing of the glass in the float glass manufacturing process.manufacturing process.

• Heat Strengthened Glass – not when done properlyHeat Strengthened Glass – not when done properly Slower cooling allows any stones to undergo the phase Slower cooling allows any stones to undergo the phase

change and become stable.change and become stable.

• Tempered Glass – yes, even though rareTempered Glass – yes, even though rare Relatively rapid cooling may arrest the phase change of any Relatively rapid cooling may arrest the phase change of any

stones, leading to possible continuation in the field stones, leading to possible continuation in the field resulting in glass breakage.resulting in glass breakage.

Why a Problem in Tempered Glass?Why a Problem in Tempered Glass?

• In tempering, the more rapid cool down arrests the In tempering, the more rapid cool down arrests the transition from alpha (smaller) to beta (larger).transition from alpha (smaller) to beta (larger).

• In service, the “alpha to beta” phase change may In service, the “alpha to beta” phase change may continue with an accompanying volume growth of continue with an accompanying volume growth of approximately 4%.approximately 4%. The phase change is time/temperature dependentThe phase change is time/temperature dependent

• This volume growth, if it occurs in the tension zone, This volume growth, if it occurs in the tension zone, can create a stress of 500,000psi and breakage can can create a stress of 500,000psi and breakage can occur.occur.

Stone Location is CriticalStone Location is Critical

Compression

Compression

Tension

Seldom, if ever, will the glass break if the stone is in the compression zone

Breakage occurs when the stone is in the tension zone, chance of breakage increases with closer proximity to the center.

What can be Done?What can be Done?• Inspect them outInspect them out• Heat SoakHeat Soak• Specify heat strengthened, rather than tempered, Specify heat strengthened, rather than tempered,

when possiblewhen possible Heat strengthened is adequate to meet virtually all Heat strengthened is adequate to meet virtually all

thermal and wind/snow load requirementsthermal and wind/snow load requirements 100% effective when properly heat strengthened100% effective when properly heat strengthened Made on the same equipment as temperedMade on the same equipment as tempered Remember, it is NOT a safety glazing materialRemember, it is NOT a safety glazing material

- not practical- not practical– – costly, not 100% effectivecostly, not 100% effective

HEAT SOAKING PROCEDURE HEAT SOAKING PROCEDURE

• An attempt to initiate or accelerate the phase An attempt to initiate or accelerate the phase change.change.

• Typically, exposes glass to temperatures Typically, exposes glass to temperatures between 280° and 300° C for two to four hours.between 280° and 300° C for two to four hours.

• Objective is to achieve a “break now, not later” Objective is to achieve a “break now, not later” result, based on the assumption that any lites result, based on the assumption that any lites with inclusions will break in the process.with inclusions will break in the process.

• Can be conducted on a statistical sampling Can be conducted on a statistical sampling basis, or on entire lots.basis, or on entire lots.

HEAT HEAT SOAKINGSOAKING PROCEDURE PROCEDURE• There may be Unintended Consequences: There may be Unintended Consequences:

Small inclusions which would have never caused Small inclusions which would have never caused breakage grow, don’t break during the process, but breakage grow, don’t break during the process, but continue the phase change and cause breaks latercontinue the phase change and cause breaks later

Tempered glass stresses are reduced, affecting Tempered glass stresses are reduced, affecting particle size and required safety glazing performanceparticle size and required safety glazing performance

Effects of handling, temperature, and breaks on low-e Effects of handling, temperature, and breaks on low-e coatingscoatings

• There is general agreement that heat soaking There is general agreement that heat soaking can eliminate some of the problem lites, can eliminate some of the problem lites, but NOT but NOT that it guarantees 100% elimination.that it guarantees 100% elimination.

““IS HEAT SOAKING WORTH IT ?”IS HEAT SOAKING WORTH IT ?”……..GLASS MAGAZINE, April 1991……..GLASS MAGAZINE, April 1991

• A question asked more than 15 years ago, and still being asked A question asked more than 15 years ago, and still being asked

today today

• Significant coverage in industry and materials science literature, Significant coverage in industry and materials science literature, including web-sitesincluding web-sites

• General agreement on most of the “science”, but not the General agreement on most of the “science”, but not the conclusionsconclusions

• Advocates of heat soaking are not absolute in their support of Advocates of heat soaking are not absolute in their support of how the process is represented. You may see:how the process is represented. You may see: ““avoids” or “reduces” or “minimizes” breakageavoids” or “reduces” or “minimizes” breakageYou will NOT see:You will NOT see: ““eliminates” or “prevents” or “guarantees against” breakageeliminates” or “prevents” or “guarantees against” breakage

Final CommentsFinal Comments

• Nickel sulfide inclusions in float glass are rareNickel sulfide inclusions in float glass are rare :: No batch compositions produced in North America No batch compositions produced in North America

intentionally include nickel intentionally include nickel Procedures to constantly monitor raw materialsProcedures to constantly monitor raw materials Nickel plates, rods, bearings, etc. have been Nickel plates, rods, bearings, etc. have been

eliminated from processing equipment eliminated from processing equipment Still, it is NOT possible to guarantee 100% Still, it is NOT possible to guarantee 100%

elimination of NiS inclusions elimination of NiS inclusions

Final Comments Final Comments (continued)(continued)

• Glass on site breaks for a reason, however:Glass on site breaks for a reason, however: Too often, when there is no apparent cause, Too often, when there is no apparent cause,

“spontaneous breakage” is blamed on an “spontaneous breakage” is blamed on an inclusion. inclusion.

In fact, it is far more likely that the breakage is In fact, it is far more likely that the breakage is due to glass edge and/or surface damage from due to glass edge and/or surface damage from handling and glazing, or vandalism, or handling and glazing, or vandalism, or structure movement. structure movement.

Final Comments (continued)Final Comments (continued)

• Heat soaking can eliminate, by destruction, Heat soaking can eliminate, by destruction, SOME of the nickel sulfide inclusions.SOME of the nickel sulfide inclusions.

• Heat soaking is NOT a guarantee for Heat soaking is NOT a guarantee for elimination of all inclusions.elimination of all inclusions.

• There is evolving knowledge of:There is evolving knowledge of: inclusion chemistry and behaviorinclusion chemistry and behavior heat soak cycle parametersheat soak cycle parameters statistical and breakage monitoring techniques statistical and breakage monitoring techniques

The Infamous “Cat-Eyes” or “Butterfly”The Infamous “Cat-Eyes” or “Butterfly”

• Does NOT prove that the Does NOT prove that the break was caused by an break was caused by an inclusion.inclusion.

• Is inherent in the break Is inherent in the break pattern of tempered glass.pattern of tempered glass.

• All inclusion breaks will All inclusion breaks will have “cat-eyes”, but not all have “cat-eyes”, but not all breaks with “cat-eyes” are breaks with “cat-eyes” are caused by inclusions.caused by inclusions.

Source – “The Achille Heel of a Wonderful Material – Toughened Glass by Dr. John Barry

?HEATSOAK GLASSHEATSOAK GLASS – Is it worth it! – Is it worth it!

Thank YouThank You

and have a nice day...and have a nice day...