Why we will not put a blade on your tempered glass

What tempering actually does

Tempered glass is annealed glass that has been reheated to about 1,150 degrees F and then quenched with high-pressure air. The surfaces cool and contract first, leaving them in compression while the core stays in tension. That locked-in stress is what makes the pane four times stronger and makes it break into blunt cubes rather than blades.

It is also what makes the surface different. The pane travels through the furnace on ceramic rollers, and two things happen. The glass takes a very slight wave from the rollers, visible as stripes under polarised sunglasses. And microscopic particles of glass and roller material fuse onto the surface. That second one is the problem.

Fabricating debris and the scratch nobody expects

Those fused particles are called fabricating debris. They are part of the glass surface, they are invisible until something drags across them, and there is no way to tell by looking whether a given tempered pane carries a lot or a little.

Run a steel blade across that surface and the blade catches a particle, tears it loose, and drags it. The result is a fine arc-shaped scratch, usually several inches long, usually in the middle of the pane where it is most visible. The blade did not scratch the glass; the glass scratched itself. That distinction matters not at all to the person looking at their scratched picture window.

Which panes are tempered

More than most people assume. Building codes require safety glazing in and next to doors, in panes with a low sill height, in bathrooms near tubs and showers, and in most large panes near walking surfaces. Every solar module is tempered. Every shower enclosure is tempered.

You can identify it by the etched bug in a corner, though on some units it is hidden behind the frame. When we cannot find a bug, we assume tempered and act accordingly.

• Entry doors, sidelights and any pane within 24 inches of a door edge
• Glass with a sill lower than 18 inches from the floor
• Panes over 9 square feet in most residential applications
• All shower and tub enclosure glass
• All solar module cover glass

What we do instead

Almost everything that people reach for a blade to remove comes off another way. Paint specks lift with a dwell-time solvent and a plastic scraper. Construction film responds to a longer soak. Mineral deposit needs a dissolver, and if it has progressed to etch, it needs a cerium polish rather than a scraper that will never touch it anyway.

Where a deposit genuinely requires mechanical work on tempered glass, we test a small area at the edge of the pane, show the client the result, and get a decision in writing before continuing. That conversation takes five minutes and it has saved several picture windows.