Heat Soaked & Heat Strengthened – Differences Clarified

These two glass industry terms are often and easily transposed. Understandably less so with people working in the glass sector or with experienced specifiers, but they can create considerable confusion for buyers, contractors and end users.

It is fair to say, and a popular opinion in glass circles, that the industry would have been well advised to have chosen more distinct labels for one or both of these categories. More on that below.

The confusion is, as much as anything, simply linguistic, even the most experienced glass specialists can occasionally slip up by transposing one ‘S’ word for the other. No surprise then that errors and confusion can be found on drawings, tender submissions and material schedules, from operatives that encounter glass and the ever increasing combinations in glass laminate compositions, far less frequently.

As architectural glass use has progressed and developed, by design and necessity, the frequency that one or both of these terms is used has become far higher. Both have been around for decades, but the combinations of laminate glass using toughened glass, annealed glass, structural interlayers, acoustic interlayers and concerns over both breakage and break patterns, have extended the specified content and included processes and materials in more numerous combinations.

The simplest way to distinguish, is that one is a process and one is a product. Hopefully by providing a broader explanation of each, the correct use will be more easily understood.

Heat Soak (Testing) – is a quality fail safe process. Only used on toughened (sometimes called tempered) glass. Annealed glass float, the raw product, can contain chemical inclusions of nickel sulphide (NiS). During the toughening process, the NiS, if present, becomes unstable, and can result in spontaneous breakage of the toughened glass at a later date. By using the Heat Soak test, the toughened panels are again exposed to heat and held at a given temperature for a given time duration. The vast majority of glass with NiS inclusions will fail during this process and will be remade and re-tested.

Although somewhat anecdotal, most glass specialists experienced a rise in NiS failures during a period a few years ago. Various theories exist as to why that may be, but certainly logic suggests that the huge increase in market demand for toughened laminated glass, allowed the post failure inspection of a shattered toughened sheet, to identify the indicative ‘butterfly’ break pattern seen in NiS breakages (Figure 1 below). Where previously, many failed monolithic toughened panels were assumed to have been damaged by forceful contact, and were simply disposed of without painstaking sifting of the glass fragments to find the butterfly segments.


NiS butterfly glass break pattern

(Figure 1 – NiS butterfly break pattern)


That increase in NiS failures, led the construction industry to become more cautious in the use of laminated glass where both sheets are toughened and in an overhead position. In theory, a laminated glass of toughened/PVB interlayer/toughened that suffers associated NiS failures and is bottom edge retained only could succumb to gravity after breakage, and tear along the channel retention line, risking the broken section falling from height. Some contractors and engineers, as a matter of policy, do not now permit tough/PVB/tough combination in any situation that could potentially result in the glass falling from height. Even with the addition of Heat Soak testing and that associated breakage of both toughened panels is incredibly rare.

Heat Strengthened Glass – Is in effect (and possibly would be better titled), ‘semi-toughened’. It shares the same principle processes as toughened glass. Heated to the same temperature, usually in the same furnace equipment, the difference is the slower speed of the cooling period. It isn’t necessary here to go too much further on details of the temperatures and periods, as long as we distinguish the process by the speed of quenching. The net results are markedly different – Toughened glass is around 5 times stronger than the annealed float glass that entered the furnace. In contrast, Heat Strengthened is perhaps twice as strong.

The attraction of Heat Strengthening, is the break pattern in the event of a panel failure. Whereas toughened glass shatters into (very measured and controlled) small fragments, Heat Strengthened glass breaks similarly to annealed float glass, in large segments. Specifically in a laminated composition, it is the break pattern of Heat Strengthened glass that invites specification in overhead situations. The retained surface area is more stable and the glass likely to remain in a safe state until replacement.

Of course, there are some drawbacks. Most notably the lower structural performance of Heat Strengthened glass. If we were to take a bottom channel retained structural glass balustrade as an example, a compliant contemporary build up would be toughened/interlayer/heat strengthened sheet. When that specification is calculated against the applicable line load and stresses, the introduction of the Heat Strengthened element will mean that the overall glass thicknesses we have all become familiar with (17.52mm, 21.52mm etc) are unlikely to perform as needed. The next step is to introduce one of the range of structural (sometimes ionoplast) interlayers, to assist the performance of the laminate composition. In most cases, that will not be sufficient and the final result will be use of thicker, heavier glass to achieve the same load performance as a toughened/toughened combination. Needless to say, that necessitates an uplift in material cost.

The next limitation is a step progression from that need to increase glass thickness for load performance. There are difficulties with achieving Heat Strengthened properties when processing thicker float glass. Broadly speaking, 12mm float is the maximum that can practically be converted to Heat Strengthened. Beyond that the glass will inherit the properties of toughened glass. It can be realised then, that if the engineer’s calculations require a total glass thickness of say 30mm, and the use of a Heat Strengthened sheet in the laminate (to provide the preferred break pattern) is demanded and factored into the calculation, it will necessitate a ‘triple laminated’ of toughened/laminate/toughened/laminate/heat strengthened. Experience also indicates that both laminates would need to be the higher performing (higher cost) structural type.

If we are looking at this from a UK supply perspective, it should also be borne in mind that not all glass processors have yet set, or renewed, their furnace plant to the newer demands for heavier Heat Strengthened product. Many can only produce using a maximum of 10mm float. There are those processors who have adapted or invested to produce Heat Strengthened from 12mm float, but the industry must be mindful of this practical supply limitation when specifying large volumes of higher load glazing that will require an element of Heat Strengthened material. It would be expected that market trends will be accommodated when plant investment is made and in time, 12mm Heat Strengthened could be more widely available, but this is a reality based on our research at the time of this blog.

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