Serving all Illinois and Wisconsin

Tempered Glass Quality Standards

Updated: 06/20/2024

Tempered glass, often referred to as safety glass, is a cornerstone of modern architecture and design. Its superior strength and safety features, compared to standard annealed glass, make it the go-to choice for applications involving potential human contact.

But how can you be sure the tempered glass you’re getting meets the highest standards? In this Apex Tempered Glass article, we’ll break down the essential qualities of top-notch tempered glass, how to assess its quality, and the key industry standards (ASTM C1048-18 and ANSI Z97.1 for North America) that guide its production.

What Are the Qualities of Good Tempered Glass?

Tempered glass undergoes a specialized heat treatment process that significantly improves its strength and safety. Here’s what to look for:

  • Increased Strength: Tempered glass is typically four to five times stronger than regular annealed glass. This translates to a higher resistance to impact, making it less likely to break under pressure.
  • Safety in the Event of Breakage: In the rare event that tempered glass does break, it crumbles into small, dull fragments instead of dangerous shards. This drastically reduces the risk of severe cuts or puncture wounds.
  • Enhanced Thermal Resistance: Tempered glass can withstand higher temperatures than regular glass, making it a superior choice for applications near heat sources, like ovens or fireplaces.
  • Durability: Thanks to its increased strength, tempered glass is more resistant to everyday wear and tear, including scratches and chips.
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Tempered Glass Markings

The presence of specific markings and certifications ensures that the tempered glass meets established safety standards. So, what are the standards for tempered glass? Let’s explore them in detail.

Tempered Glass Markings

ASTM C1048-18

This American Society for Testing and Materials (ASTM) standard specifies the requirements for heat-strengthened and fully tempered flat glass used in general building and architectural purposes. It covers aspects like material properties, dimensional tolerances, testing procedures, and surface quality. ASTM C1048-18 differentiates between two types of glass:

  • Heat-Strengthened Glass (HS): This type of glass offers enhanced strength compared to regular glass, but it doesn’t shatter into small pieces upon breakage.
  • Fully Tempered Glass (FT): Commonly known as safety glass, this type meets rigorous surface and edge compression requirements, ensuring it fragments into small, relatively harmless pieces if broken.

ASTM C1048-18

Material Properties

ASTM C1048-18 isn’t just a jumble of letters and numbers — it’s the backbone of quality tempered glass. This standard outlines the ideal types of glass for tempering, like the soda-lime silicate glass you often find in buildings.

But it goes deeper, ensuring the glass’s chemical makeup and physical traits, such as clarity, light transmission, and heat resistance, are up to par for tempering.

Dimensional Tolerances

Tempered glass needs to fit seamlessly into frames and structures, and ASTM C1048-18 makes sure of that. It sets clear boundaries for acceptable variations in thickness, length, and width, accommodating the slight shifts that can happen during the tempering process. This way, you can be confident your tempered glass will slide right into place.

Surface Quality

Who wants scratches or blemishes on their tempered glass? Not us, and not ASTM C1048-18. This standard demands a high level of surface quality, with strict limits on scratches, digs, bubbles, and other imperfections. It even keeps an eye on haze and distortion, ensuring your tempered glass stays crystal clear and visually appealing.

Test Specifications

ASTM C1048-18 outlines mandatory testing procedures to verify the performance and safety characteristics of tempered glass. These tests include:

Test Specification Description
Thermal Stress Can the glass handle sudden temperature swings without cracking? This test finds out.
Modulus of Rupture How resistant is the glass to bending under pressure? This test measures its strength.
Center Punch Fragmentation If the glass breaks, will it shatter into small, safer pieces? This test makes sure it does.

ANSI Z97.1-2004


This American National Standard focuses on safety glazing materials used in human contact applications. It applies to tempered glass, laminated glass, and plastic glazing materials used in doors, windows, shower enclosures, and other architectural glazing products.

ANSI Z97.1 doesn’t specify manufacturing procedures but establishes performance specifications for safety glazing materials. These specifications ensure that even in the event of an accident, the glass won’t shatter dangerously.

For tempered glass, the key test is the Impact Test. But before any glass undergoes this test, it’s carefully inspected in its delivered state, with any temporary protective coverings removed but permanent coatings left in place.

To guarantee accurate results, the glass is then conditioned in a controlled environment between 65°F and 85°F for a minimum of 4 hours, allowing it to adjust to a stable temperature before being subjected to evaluation.

The Role of Safety Glazing Certification Council (SGCC)

The Safety Glazing Certification Council (SGCC) is a non-profit organization that administers a product certification program for architectural glazing materials, including tempered glass. Getting this certification isn’t required by law, but it’s a big deal because it means the product has been independently verified to meet important safety standards, such as ANSI Z97.1. When you see the SGCC label on tempered glass, you can be confident it meets established safety regulations.

How to Check Tempered Glass Quality? Visual Inspection

Even a quick look at your tempered glass can tell you a lot about its quality. Here’s what to keep an eye out for:

  1. Clarity and Smoothness
    • The glass surface should be free of scratches, bubbles, or anything else that looks out of place. These imperfections can weaken the glass and potentially lead to breakage.
    • Look for any waves or distortions in the glass, which can affect how clearly you can see through it and its overall appearance.
  2. Edge Finish
    • Look for smooth, polished edges without any chips, cracks, or unevenness. Rough edges could be a sign that the glass wasn’t handled or processed correctly, which might affect its strength.
    • Fully tempered glass should have edges with a distinct “frosted” or “crushed ice” appearance.
  3. Transparency
    • High-quality tempered glass should be clear, not cloudy or hazy. Even a slight haze could mean there’s internal stress or the tempering wasn’t done properly.
    • Check for any visible specks or bubbles inside the glass itself, as these could be air pockets trapped during manufacturing that might weaken the glass.
  4. Consistent Color
    • The color of the glass should be uniform throughout, with no splotches or unevenness. Uneven color could suggest problems with the tempering process or the materials used.
    • Keep in mind that some architectural tempered glass might have a slight greenish tint, which is normal for certain types of glass and doesn’t necessarily mean there’s a quality issue.
  5. Cleanliness
    The surface should be clean and free of dirt, dust, or other contaminants. These can make it harder to inspect the glass and potentially interfere with the adhesion of films or coatings applied to the glass.

Prioritize Safety: Choose High-Quality Tempered Glass Solutions

Safety should always be your top priority. Choosing high-quality tempered glass that meets industry standards gives you peace of mind, knowing your windows, doors, and other glass features offer superior strength and protection if they ever break.

At Apex Tempered Glass, we’re committed to providing top-quality tempered glass products for homes and offices throughout Illinois and Wisconsin. Contact us today, and we will help you choose the right tempered glass product that meets all the necessary safety standards (and looks great too).

Nina Tsoy

Finance And Operations

Nina leads Apex Tempered Glass, a company specializing in manufacturing all types of glass. Known for her strategic expertise and exceptional operational performance, Nina is committed to providing innovative solutions for any glass-related issue.


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*Serving the Illinois and Milwaukee areas.