The term HST (Heat Soack Test) refers to the type of test to which all the glass façade panes must be submitted to avoid spontaneous breakage of the same caused by inclusion of nickel sulfide.
Although this event is quite rare, according to the European Standard EN 14179-1: 9-2005, all tempered safety glass that are used to be part of a façade of any building, must be subjected to this specific test.
But why is it necessary to submit the glass panes to this test?
Obviously, since these are building facades, the test is performed to guarantee the safety of people.
The aim of the HST is in fact to avoid, with a 99% reliability, the spontaneous breakage of the tempered glass caused by internal impurities. These impurities, in particular, are nickel sulfide particles (Nis), natural components of glass raw materials that are not visible by human eye (they measure from 0.05 to 0.5 mm), if present they can determine the breakage of a tempered safety glass.
When this happen, if the panel remains physically intact (ie the glass is not reduced to thousands of fragments) it is possible to immediately verify the position of the Nis that caused the break. The plot of the crack takes the form of a “butterfly”, and the “guilty” particle is always at the center of this “drawing” because it corresponds to the breaking point. (See photo)
Although this phenomenon is very rare, we must be aware that it is a spontaneous event and does not depend on the glass producer himself.
Precisely because it is not possible to foresee this break, it is therefore mandatory to proceed with the HST.
The test, specifically, requires that the glass panes are subjected to temperatures averaging around 290 ° for a few hours. The ones containing the sulphide are therefore identified because they are spontaneously destroyed.
Briefly, this test does not prevent the spontaneous breakage of the glass, but acts as a real accelerator: the combination of time and high temperature make sure that 99% of the glass that would be destined to break, gets cropped even before being installed as you do in various buildings, thus avoiding unpleasant and costly inconveniences.
Obviously the HST is also recommended forglass panes that are used for other types of projects such as parapets, windows, pompeiane, and so on, but it is mandatory by law only in the case of the façade panels of the buildings.