Det går tilsyneladende på, at glasset er testet for nikkelsulfid (NiS), ved at hærdningen foregår på en måde, der får glasset til at sprænges i ovnen, hvis det indeholder NiS.

Du kan måske bruge nogle af de følgende websteder:

http://www.faelleskonserveringen.dk/Print.asp?NyhedsID=4

Varmetestet hærdet glas.
Hærdet glas kan i særlige tilfælde spontangranulere. Partikler af nikkelsulfid inde i glasmassen kan give spændingskoncentrationer, som ved varmebelastninger kan få glasset til at bryde. En varmetest (heat-soak) afslører så godt som alle ruder med nikkelsulfid ved at de revner.


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http://www.glasindustrien.dk/Web/Site/Glasterminologi

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http://www.beaumontrose.co.uk/tech_heat.htm
The heat soak test is carried out on tempered glass and is used to minimise the risk of spontaneous breakages in situ, caused by Nickel Sulphide inclusions
How? - The glass is placed in a chamber and is subjected to high temperatures and then cooled in a predetermined cycle, this accelerates nickel sulphide expansion, and causes glass containing nickel sulphide inclusions to break in the heat soak chamber. Reducing the risk of potential breakages when the glass is in situ.
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http://www.glassonweb.com/articles/utils/print.php?id=97

Heat soak test
It has been known for many years that tempered glass can be subject to spontaneous breakages since NiS inclusion changes the dimensions under the heat-treating process that can cause glass breakage.


The heat soak test or heat soak process is used to minimise the risk of spontaneous breakages of heat-treated glass caused by NiS inclusions. In this process, glass panels are placed inside a chamber and subjected to an oven temperature of 290ºC to accelerate nickel sulphide expansion. This causes glasses containing nickel sulphide inclusions to break in the heat soak chamber, thus reducing the risk of potential field breakages. The broken glass can then be recycled and re-manufactured. Two tests options are possible: statistical and full. The statistical heat soaking process involves a specific number of glasses from a statistically selected order. NiS breakage in this process does not exceed 0,1%, with a probability of 95%. On the other hand the full heat soak process involves testing all glass in the order, virtually eliminating any chance of NiS breakage.