Resources
Why Mirror Glass Cannot Be Tempered After Silvering
The order of operations is fixed by physics, and it constrains sizes, lead time and what you can change late.
Short answer: tempering heats glass to roughly 600°C and then quenches it. The silver and copper-free protective layers on a mirror do not survive that, so a tempered mirror must be tempered first and silvered second. The consequence for buyers is that the size, shape and hole positions are locked before the reflective coating goes on — and after that, nothing can be cut.
The order of operations
Ordinary float glass can be cut, drilled and polished at almost any point before installation. Tempered glass cannot: once it is toughened, cutting or drilling it makes it shatter. Every hole, notch and edge profile has to exist before the furnace.
Silvering is a wet chemical process applied to a finished glass surface. Put a silvered sheet through a tempering furnace and the coating degrades. So the sequence for a tempered mirror is fixed:
1. Cut to final size, with every hole and notch 2. Edge work and polishing 3. Temper 4. Silver, then apply the backing paint layers
There is no route that reverses steps 3 and 4, and there is no route that adds a hole after step 3.
What that means for a purchase order
| Decision | When it must be final |
|---|---|
| Panel size and shape | Before tempering — cannot change after |
| Hole and notch positions | Before tempering — cannot change after |
| Edge profile | Before tempering |
| Backing class | With the order, affects the backing stage |
| Frame finish, packaging, branding | Late — these are downstream |
The practical effect is on change requests. A size revision that arrives while panels are in edge work is an inconvenience; the same revision after tempering means starting the glass again. This is the main reason tempered mirror walls carry a longer lead time than an equivalent non-tempered panel, and why we hold minimum order quantity on those specific items when everything else in the catalogue runs from a single piece.
Tempering and backing are not substitutes
Two different problems, frequently conflated in specifications:
- Tempering changes the fracture pattern — blunt granules rather than long shards.
- Safety backing holds the fragments in place after the glass breaks.
A specification that says "tempered" and expects fragment retention has not asked for what it wants. A specification that says CAT II backing and expects the glass to be harder to break has the same problem in reverse. Where both matter, both go in the specification.
Where low-iron glass fits
Low-iron glass is a separate variable again. Standard float carries a slight green cast from its iron content, visible on the edge and across large panels. Low-iron substrate removes most of that, which matters when panels butt together in a wall and the colour shift between them would show.
It is a substrate choice, not a safety property. Low-iron glass is not stronger, and specifying it does not change the tempering sequence above.
Where this shows up in the catalogue
Our tempered gym and studio mirror walls are the products where this sequence bites: the panel layout has to be final before the glass is toughened, and they are the only items in the catalogue that carry a 50-piece minimum rather than running from a single piece.
For wet areas the more common answer is not tempering but coating: copper-free mirror glass removes the copper layer that drives edge corrosion, which is the failure that actually shows up eighteen months after handover.
What to send us
For anything tempered, send the panel layout with hole positions before we quote — it is the one specification that cannot be revised later without remaking the glass. The ADA, convex and safety-backed range covers the commercial builds, and we will come back with the drawing and a price within 24 hours.