BEYOND THE FACADE | PART 1: Verifying glazing beyond specification
- Ayman Abziouad
- 15 hours ago
- 5 min read
Assessing glass type and performance through on-site measurement
Glass is one of the most safety-critical materials in a facade. Whether a pane is heat-strengthened or fully toughened determines how it will behave under load, how it will break if it fails, and whether it meets the specification for its position in the building. The difference between the two is not visible to the naked eye - but the consequences of getting it wrong can be significant.
At Henriksen Studio, we have the capability to verify glass heat treatment levels on-site using portable scattered light polariscope technology. It is a precision assessment tool that gives our clients and project teams objective, measurable evidence of what they actually have - not just what it says on the label.
The problem with glazing classification
Glass units arrive on site with stamps, stickers, and documentation that are intended to confirm their specification. In theory, this is straightforward. In practice, discrepancies can and do occur - between what is stated on a stamp and what is shown on a glazing sticker, between what was ordered and what has been manufactured, or between what a document says and what the glass itself reveals under examination.
These discrepancies matter because heat-strengthened and fully toughened glass are not interchangeable. Toughened glass is approximately four to five times stronger than annealed glass and, crucially, fractures into small, relatively safe fragments. Heat-strengthened glass is approximately twice as strong as annealed glass but breaks into larger shards, more like standard glass. Using heat-strengthened glass where toughened is specified - or vice versa - can result in a facade that does not meet its structural or safety brief.
A further and often overlooked consequence of incorrect heat treatment is vulnerability to thermal shock. Thermal shock occurs when different parts of a glass pane are exposed to significantly different temperatures simultaneously, creating unequal expansion and contraction across the surface. This generates internal stress that the glass must absorb - and if that stress exceeds the material’s capacity, the pane cracks or shatters without any external impact.
A common cause in occupied buildings is internal blinds: when blinds are partially or fully closed, they trap heat between the glazing and the room, causing the central area of the pane to heat up rapidly while the edges, which are held in a cooler frame, remain at a lower temperature. The resulting temperature differential can be enough to fracture a pane that has not been adequately heat treated. Fully toughened glass is significantly more resistant to thermal shock than heat-strengthened glass, which in turn is more resistant than annealed glass - meaning that where blinds are present or anticipated, the heat treatment specification carries real practical weight.
How the SCALP-05 Portable Scattered Light Polariscope works
The SCALP-05 is a portable, non-destructive instrument that measures the surface compressive stress within a glass pane. This stress profile is the defining characteristic that distinguishes one heat treatment level from another: fully toughened glass has a significantly higher surface compressive stress than heat-strengthened glass, and the SCALP-05 measures this directly.
The technique works by directing a laser beam into the edge of the glass and measuring how the scattered light behaves as it travels through the material. Because the optical properties of glass change under stress, the instrument can calculate the magnitude of the surface compression with a high degree of accuracy - and do so rapidly, without causing any damage to the unit.
The results allow our engineers to determine, with confidence, whether a glass unit has been:
Fully toughened, in accordance with EN 12150
Heat-strengthened, in accordance with EN 1863
Annealed (i.e. not thermally treated), where results fall below the expected thresholds for either of the above
When is this glazing assessment needed?
Surface stress measurement is a targeted tool, most valuable when there is a reason to question the heat treatment status of a glass unit. Common triggers include:
A discrepancy between the stamp classification and the glazing sticker or delivery documentation
Concerns raised during a quality inspection or factory witness visit
An unexplained breakage pattern on installed or delivered units, which may indicate incorrect heat treatment
A requirement to verify the heat treatment level of units already installed, as part of a condition survey or due-diligence exercise
A contractual or insurance requirement to demonstrate that installed glazing meets the specified standard
Because the SCALP-05 is portable and non-destructive, assessments can be carried out on-site, in a factory, or on units held in storage - without removing glass from the facade or causing any disruption to the building.
What the results tell you
A SCALP-05 assessment delivers a clear, documented set of surface stress readings for each unit tested. These readings are interpreted against the relevant European standards to give a straightforward classification: toughened, heat-strengthened, or neither. The output is a technical report that can be used to support a procurement decision, resolve a dispute, satisfy an insurer, or provide a permanent record of the glazing specification as installed.
Importantly, the assessment is not a pass/fail test in isolation - it is a piece of technical evidence. Our engineers will contextualise the results within the broader facade specification, explain what the findings mean for the performance and safety of the installation, and advise on any next steps where the results indicate a concern.
Verifying coatings and pane configuration: the Merlin tool set
At Henriksen Studio, we use the Merlin range of handheld glass analysis instruments to verify glazing units rapidly. The Merlin Lazer measures individual pane thickness and cavity width in single, double, and triple-glazed units, while the Merlin Low-E detector identifies the presence of low-emissivity coatings and - critically - confirms which surface of the unit they are applied to. Together, they give our engineers an immediate and reliable picture of what a unit actually contains, without removing it from the facade.
Why coating orientation matters
A low-emissivity coating works by reflecting heat back towards its source - keeping warmth in during winter and reducing solar gain in summer. For it to perform as intended, the coating must sit on the innermost face of the outer pane, within the sealed cavity. If a unit is installed the wrong way round, the coating ends up on the wrong surface, the thermal benefit is largely lost, and the facade will not perform as specified - despite looking exactly the same.
Why pane thickness matters
Individual pane thickness is a structural and acoustic matter. Panes are sized to carry specified wind and imposed loads, and their thickness directly determines deflection and breaking strength. A pane thinner than specified can take a facade outside its structural design envelope, and even a small shortfall can compromise acoustic performance across an entire facade element. The Merlin Lazer confirms on site that what has been installed matches what was engineered.
Confidence in what you have built
Glass is a material that is easily taken for granted - until something goes wrong. Whether it is the wrong heat treatment level, a coating facing the wrong way, or a pane that falls short of its specified thickness, the consequences are rarely visible until they manifest as a performance failure, a breakage, or a dispute. Verifying what you actually have, before those problems arise, is a straightforward act of due diligence.
At Henriksen Studio, our glazing assessment capability - from heat treatment verification to coating and thickness measurement - gives clients and project teams objective, documented evidence of the glazing as installed. If you have concerns about glazing specification on a current or completed project, or would like to arrange an assessment, get in touch with the team at Henriksen Studio.
