HST provides invisible evidence visible proof

7/30/2019 HST provides invisible evidence visible proof

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Apart rom the appreciated lightness andtransparency, glass inside buildings and onacades convey the impression o beauty,and strength. But even that strong materialcould ail. Reality shows that in certain cases,where parts o acade glazing are breaking andcome tumbling down resulting in additionalcosts that need to be covered, also to a lack o confdence and the question o responsibility.

Invisible evidence,HST providesvisible proof

92 intelligent glass solutions

Although manu acturers and constructors havedone their very best to ensure the high qualitystandard o their prestigious systems ailureo the dynamic glass material could become aharsh and bitter reality.

In the ollowing article we analyze one o the main reasons or this problem: the nickelsulphide-induced ailure o glass. We ocus on

the importance o relevant documentationand actual evidence, we also show methodsto ensure legal certainty to manu acturers,suppliers and all other intimately concernedparties o Heat Soaked Tempered Glass (HSTG).

Spontaneous breakage / NiS inclusionsSpontaneous glass breakage is a phenomenonby which toughened glass may spontaneously

C A S E S T U D I E SC A S E S T U D I E S

Thomas Rainerbora ident Gmbh


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93intelligent glass solutions


break without any apparent reason. In manycases this is caused by the inclusion o nickelsulphide (NiS) small particles that in ectthe glass melt inside the urnace during themanu acturing process.

These NiS inclusions (stones) are generally o 50 to 400 m in size, ar too small as to be visibleto the human eye. However, once embedded inthe tensile zone o the tempered glass substratethey can develop their own troublesome anddestructive orces.

Depending on time and temperature, NiStrans orms rom -confguration (high-temperature phase) to -confguration (low-temperature phase). During the temperingprocess, the glass (with the embeddedNiS particle) is heated above the glasstrans ormation temperature which convertsthe particles to -NIS. But when the glass getsquenched (rapidly cooled down) to create thedesired internal stresses, it takes only a smallamount o NIS to re-trans orm rom -NIS to

the -type so that the rigid tempered glasscontains initially a bigger part o -NIS. But therecrystallization to -NiS is slowly continuing,even at room and/or outdoor temperature.And due to the act that the particles changetheir structure during this process, urtherinternal stresses could appear even a ter manyyears. Depending on their size and positioninside the tensile zone, the glass could ail i their stresses exceed the stength o the glass beakage results.

HSTGRegardless o whether the contamination isa liated to the raw materials or the actualproduction equipment, its prevention oravoidance would require a disproportionatelyhigh and thus expensive - e ort. Hence, NiScontamination which leads to the ailure o glass presents a ubiquitous and unpredictablerisk or all manu acturers o tempered glass andall subsequent partners along the value chain.It naturally ollows that special provisions arefxed by law or overhead installation o glassor the usage o such in public areas (Europeanstandard DIN EN 14179-1). It includes amongstothers, that only a specially treated glass typemust be used: HSTG.

In order to make spontaneous glass breakagesdue to NiS inclusion less likely and therebyreduce the requency o this unwantedphenomenon, toughened glass shouldundergo a special treatment: the appropriatelynamed Heat Soak Test (HST).

This is a process which, when properlyper ormed, can destructively uncover mostNiS inclusions, i there should be any present. This Heat Soak Test involves the glass panesbeing placed in a special heat soak chamberand subjected to a temperature o 280 - 300C(540 570 F) over a time period o 2 to 4 hours(depending on local regulations).

The heat treatment accelerates the NiStrans ormation process and causes theexpansion o potential inclusions that lead to adesired breakage o the in ected pane.

Thus, the HST process ensures only a minor risk o ailure or the residual panes. This remaining

risk, commonly known as probability o ailure is only slight, but it still di ers rom zeroprobability. It is estimated with about 1% perannum. According to Bauregelliste 2002/1o the German Construction Authority, thisprobability o ailure is attested as productcharacteristics.

Fig 2: The diagram shows that the probability of failure for Heat Soaked temperedglass reduces to one third of the one of tempered glass.Source: Prof. Dr. J. Schneider, Technical University Darmstadt

Fig 1: Development of NiS-inclusion in the manufacturing process of tempered glassSource: Prof. Dr. J. Schneider, Technical University Darmstadt


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The legal situation in Europe and legaldecisions made in GermanyAccording to EN 14179, manu acturers mustprove that every as o ESG-H (heat-soaked

tempered glass) declared pane has passedthe Heat Soak Test. According to the productstandards, which may di er rom country tocountry, the manu acturer is held responsibleor the correct execution o the HST processand all relevant documentation. In the case o breakages a ter installation and resulting indamages, the manu acturers evidence will bethe basis or any legal decision making. Onlycomplete and unambiguous documentationkeeps the manu acturer indemnifed against allliability rom accidents and injury.

Reality shows that a lack o evidence createsan ambiguous situation in many legal cases.In the event o damages due to alling glasselements, re erees might veri y at frst, i therequired HSTG has been used. In the next step,the manu acturer or constructor has to provideevidence that the used charge had completelypassed the Heat Soak treatment usually bysubmitting the entire documentation, theso called oven protocols. This completedocumentation will support decisions onliability in cases where damage has occured; it

consequently ollows that incomplete evidenceinevitably leads to considerable claims orcompensation.

So decided the Regional Court in Dresden(Germany) that those glass elements areinsu cient, whose Heat Soak treatmentcannot be proven. It is not possible to assignthe manu acturers documentation tothe respective shipment (OLG DresdenAz 14 U 912_08).

Another case led to the manu acturers atewhen it was decided the oven protocol mustbe replicable to every single glass pane (LGDsseldor Az 1 O 472-08). The frst instance orinvestigation is the construction company whomust submit the certifcates o compliance.I they can provide evidence by means o complete documentation, sometimes, thesurveyor draws samples by inspecting theglass manu acturing plant. In many cases thathave resulted in claims or compensation, thecomplete documentation could not be oundor reconstructed and so was simply orged.In any case: whoever will be ound culpableor compensation purely on the basis o incomplete or inexplicable documentation,

Evidence of broken glass under UV light conditions


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will certainly be acing considerable fnancialstrain. As usually, the company is not onlyheld responsible or replacing the de ectiveglass, but also or secondary losses: property

damages, reproduction costs, downtimecompensation et al.

One possible solution:Evidence on the glass itselfBut how is it possible to urnish clear andunquestionable evidence that the glass and better: every single glass pane - hassuccess ully passed the Heat Soak treatment? Itwas only a ew years ago that thermochromicinks were established in order to prove thecorrect treatment on the glass itsel . This ink gets applied on the glass itsel be ore and thenchanges colour during the process. There is a new, sophisticated technologyavailable to the ree market in the orm o the HST checkmarker ink. Apart rom thethermochromic attribute it o ers a secondeature: under HST conditions, the UV-active ink creates a resistant uorescence marking insidethe glass material. Thus, it guarantees severaladditional benefts: A ter applying the ink witha pen or a stamp, every pane is individuallymarked. Under the Heat Soak process, thismark changes colour in the production

environment this is a visible re erence thatthe pane has passed the necessary process. This sur ace coating can be removed wheninstalling the glass pane in order not to a ectthe product aesthetics. But the proo remainsthroughout the product li e-cycle in theorm o a long-term resistant, unambiguousuorescence marking which can be visualizedonly under UV-C-radiation. Even a terspontaneous breakage o the pane, evidencethat the Heat Soak Test has been undertakenand passed, can be very easily provided.

Manu acturers o heat soaked tempered glassdo not only beneft rom peace o mind withthis legal certainty, the invisible evidence on thepane itsel is also an appreciated selling point.

boraident GmbHbora ident GmbH is an innovative technologyand engineering company located in Halle(Saale), Germany. It develops products andsolutions or customer-specifc applications onglass and other transparent media. In the areaso laser marking, laser structuring and opticaland chemical glass sensoric, bora ident o erssophisticated high-per ormance products -with patented technologies or a reliableglass handling.

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HST provides invisible evidence visible proof