Every pane of glass in a vacuum insulated glass unit must be fully tempered before assembly. This is not a specification preference — it is a structural requirement imposed by the physics of the vacuum itself. What is less well understood is that not all tempering is adequate for VIG production. Standard tempering lines, producing glass that easily meets architectural glazing flatness requirements, routinely fail the flatness tolerances that VIG demands. This is why manufacturing partner matters in VIG, and why Glaston technology is central to what VIG Horizon supplies.
Why fully tempered glass is required
When the vacuum is established inside a VIG panel, the atmospheric pressure acting on the outer surfaces of both glass panes is no longer counterbalanced by internal pressure. At standard atmospheric pressure (~101,325 Pa), this creates a compressive load on the panel of approximately 100 kN/m² — or roughly 10 tonnes per square metre.
Annealed glass (ordinary float glass) has a flexural strength of approximately 40–50 MPa. Fully tempered glass has a flexural strength of 120–200 MPa — four to five times higher. The ceramic spacer pillars, which are the only solid elements bridging the vacuum gap, create local stress concentrations at their contact points with the glass. Only fully tempered glass can withstand these concentrated loads over the unit's operational life without cracking.
Additionally, the tempering process must be completed before VIG assembly. Once the vacuum seal is formed, the glass cannot be cut, ground, drilled, or thermally processed. The finished dimensions of each pane must be correct at the time of tempering.
Critical sequence: In conventional IGU production, glass can be cut to size, then edge-worked, then incorporated into the unit. In VIG production, glass must be cut to final size, fully tempered, then assembled into the VIG unit. There is no post-assembly processing possible. This places a higher premium on dimensional accuracy at the tempering stage.
The flatness problem: what standard tempering does to glass
Tempering heats glass to approximately 620–680°C and then rapidly quenches it with air jets. The rapid surface cooling creates compressive stresses at the surfaces and tensile stresses in the core — the stress gradient that gives tempered glass its strength and characteristic fracture behaviour. This thermal process also introduces permanent deformations in the glass surface.
The most significant deformation is roller wave: a periodic undulation in the glass surface caused by differential heat transfer between the glass and the conveying rollers during the heating and quenching phases. Roller wave amplitude in standard architectural tempering typically ranges from 0.1mm to 0.3mm over a measurement span of 300mm.
For conventional architectural glazing, this level of surface variation is invisible and structurally irrelevant. For VIG, it is a fundamental production problem.
The 0.05mm tolerance: why it matters
The vacuum gap in a VIG panel is 0.2mm — maintained by ceramic spacers exactly 0.2mm in height. If the glass surface has roller wave of 0.15mm amplitude (well within standard tempering tolerance), the actual gap between panes varies from 0.05mm to 0.35mm across the panel surface. At the minima, the ceramic spacers contact both panes simultaneously — transmitting compressive load correctly. But in the peaks of the wave, the gap widens, and the spacer no longer contacts the outer pane — creating an unsupported span that concentrates stress.
More critically, the edge seal — the metal solder that hermetically seals the perimeter — must contact both glass surfaces uniformly to form a perfect vacuum bond. If the glass surfaces are not flat within tight tolerances at the seal location, the solder cannot form a continuous hermetic contact, and the vacuum will leak — silently and immediately degrading the unit's insulating performance.
VIG production requires glass surface flatness better than 0.05mm over any 300mm span. Standard architectural tempering delivers 0.1–0.3mm. The gap between what is needed and what standard production delivers is a factor of 2–6×.
What Glaston FC500 does differently
Glaston Corporation, the Finnish glass tempering equipment manufacturer, developed the FC500 tempering line specifically to address the flatness requirements of high-precision glazing applications — including VIG. The FC500 achieves superior flatness through several engineering approaches that differ fundamentally from conventional tempering systems.
The roller system in the FC500 uses optimised roller geometry, spacing, and surface treatments that minimise the differential contact forces that create roller wave. The heating system uses a combination of convection and radiation heating with precise zone control, reducing temperature gradients across the glass surface during heating. The quenching system delivers highly uniform air pressure and distribution, minimising the differential thermal stresses that cause distortion.
In February 2026, Glaston Corporation and Leadus Technology announced a formal cooperation partnership to advance VIG production. This partnership grounds Leadus's VIG manufacturing quality directly in Glaston's precision engineering — not as a supplier-customer relationship, but as a technology development collaboration focused specifically on the flatness and seal quality requirements of vacuum glass production.
"We are at the forefront of tempering technology, delivering world-class flatness in tempered glass. This is a key enabler for efficient and reliable VIG production."
Kimmo Kuusela, EVP Sales & Service, Glaston Corporation — February 2026Why this matters when specifying VIG
Not all VIG is produced on Glaston lines. A VIG unit produced on a standard tempering line — or on a line that does not achieve the required flatness tolerances — may look identical to a correctly produced unit. Its U-value may be measured as identical at delivery. But over time, the vacuum seal's integrity will reflect the quality of the hermetic contact established at manufacturing. Units produced outside the flatness tolerance will show faster vacuum degradation than units produced within it.
Asking a VIG supplier what tempering equipment they use is a reasonable and technically informed question. The answer tells you something real about the expected performance longevity of what you are specifying.