Heat Tempering

Thermal Tempering Advantages and Limitations

Below You find normal limitations in thermal tempering.

Lasimyynti Tammela can deliver smaller and thinner thermally tempered glasses.
Our minimum size is 100×100 mm, instead of standard 300×100 mm.
Minimum thickness 3 mm instead of 4 mm.
We can offer Heat Soak threathment which minimizes spontaneous breakage possibility.
(Read more in the end of this article)

Thermal tempering of glass offers significant advantages in strength and safety, but it comes with specific
manufacturing and design limitations.

Advantages of Thermal Tempering

• Enhanced Strength: Thermally tempered glass is approximately four to five times stronger than
  untreated (annealed) glass, offering greater resistance to impact, pressure, and bending loads.
• Improved Thermal Resistance: It can withstand rapid temperature changes and higher
  temperatures (up to 250°C difference) that would cause regular glass to break, making it ideal for
  oven doors, skylights, and areas with direct sunlight exposure.
• Safety upon Breakage: In the event of breakage, the glass shatters into small, relatively dull, cube-
  like fragments instead of large, sharp shards. This significantly minimizes the risk of serious injury, which is why it is considered a safety glass.
• Scratch Resistance: The process increases the surface hardness of the glass, making it more
  resistant to scratches and abrasion.

Limitations of Thermal Tempering

• No Post-Production Modification: Once the glass is thermally tempered, it cannot be cut, drilled,
  polished, or re-worked in any way. All shaping and processing must be completed before the
  tempering process.
• Risk of Spontaneous Breakage: Although rare, microscopic imperfections (such as nickel sulfide
  inclusions) within the glass can expand over time, leading to spontaneous, unpredictable breakage.
• Optical Distortion: The heating and rapid cooling process can cause slight surface irregularities,
  such as minor waves or wind spots, due to contact with rollers and airflow in the tempering oven.
• Vulnerability at Edges and Corners: The balanced stresses within tempered glass make its edges
  and corners the weakest points. A hard impact to these areas can cause the entire pane to shatter
  instantly.
• Complete Shattering: While safer in terms of injury risk, the complete shattering of the glass upon
  impact can pose a security risk, as it leaves an immediate, open gap in a window or door frame.

Size and Thickness Limitations

Thickness Limitations:

• Minimum Thickness: Glass that is too thin cannot be effectively tempered using the thermal
  process because it lacks sufficient mass to maintain the core temperature required to create the
  differential cooling and resulting compression. The minimum general thickness is typically 4mm.
• Maximum Thickness: While very thick glass can be tempered, the process becomes more difficult
  to control consistently, which may increase the risk of optical distortions or the effectiveness of the
  temper throughout the entire thickness. Common architectural thicknesses range up to 19mm.
  

Size Limitations:

• Minimum Size: There is a minimum dimension required for the glass to be handled effectively by the tempering equipment (rollers, tongs, etc.). A common minimum size is around 100mm x 300mm, although this can vary by manufacturer. Pieces that are too small may not be able to be processed thermally and would require chemical strengthening instead.

• Maximum Size: The maximum size is restricted primarily by the dimensions of the tempering
  furnace and handling equipment. While standard “jumbo” sizes are commonly around 2.4m x 3.6m
  (8ft x 12ft), some specialized furnaces can handle extremely large, “oversize” panes up to 3.2m x
  16.5m for unique architectural projects.

In summary, the specific size and thickness capabilities often depend on the manufacturer’s specific
equipment, so it is always best to consult with the glass fabricator if a project has unusual size
requirements.

Heat soaking

is a quality control process performed on fully tempered glass to minimize the risk of spontaneous
breakage after installation. This unexpected failure is typically caused by microscopic imperfections
within the glass, primarily nickel sulfide (NiS) inclusions, which are tiny, undetectable particles that can
expand over time.

The Process

After the standard thermal tempering process, the glass panels are placed in a specialized heat-soaking
oven and subjected to a controlled thermal cycle.

• Heating Phase: The temperature inside the oven is gradually raised to approximately 290°C
  (554°F).
• Holding Phase (Soaking): The glass is held at this peak temperature for a specified period,
  typically a minimum of two to four hours.
• Cooling Phase: The glass is then slowly cooled to ambient temperature.

How It Works

The high temperature in the oven accelerates the phase transformation and expansion of any NiS
inclusions present in the glass. This expansion creates sufficient internal stress to cause the defective
panels to shatter within the controlled environment of the oven.
Glass that survives the heat soak test is considered “safe” because the majority of critical NiS inclusions
have been forced to fail before the glass leaves the factory for installation.

Purpose and Benefits

The main purpose of heat soaking is safety and risk mitigation:

• Minimizes Spontaneous Breakage: It significantly reduces the probability of a panel breaking
  unexpectedly after it has been installed in a building facade, skylight, or balustrade.
• Prevents Accidents and Costs: By identifying flawed panels in the factory, it avoids the safety
  risks of falling glass, injury, and the high costs and inconvenience of complex, high-level
  replacements in the field.
• Compliance: Many building codes and architectural specifications for critical applications (e.g.,
  overhead glazing, structural glass) mandate the use of heat-soaked glass.

Limitations

• Not 100% Effective: While highly effective (reducing breakage probability significantly, such as
  from 1 in 10,000 m2 to 1 in 1,000,000 m2), the process does not completely eliminate the risk of
  spontaneous breakage.
• Added Cost and Time: The extra step of processing adds both cost and time to the glass
  manufacturing process.
• No Guarantee of Quality Identification: There is no reliable method to definitively tell if an
  installed piece of glass has been heat-soaked after the fact, other than through manufacturer
  documentation.