Edge finishing & seaming
A freshly cut glass edge is a nasty little thing: razor-sharp, chipped, and secretly weak. Edge finishing is where that raw edge gets tamed, safer to handle, stronger under load, and either invisible in a frame or turned into a clean design feature. How far you take it depends entirely on where the edge ends up.
Why edges need finishing
Cutting glass is basically controlled cracking: you score the surface, bend it, and let a fracture run through the thickness. It works beautifully, but it leaves behind a raw edge (a cut edge or raw edge) with two things wrong with it:
- Micro-cracks and chips: The cutting process creates tiny surface flaws along the edge face. These flaws act as stress concentrators, meaning a raw edge is substantially weaker than a finished edge when loaded in bending. Under thermal stress or structural load, raw-edge glass is more prone to breakage originating at the edge.
- Sharp corners: The intersection of the edge face and the glass surface (the "arris") is an acute, razor-sharp corner that poses a laceration hazard during handling, glazing, and cleaning.
Edge finishing removes or smooths these flaws. The level of finishing required depends on how the edge will be used, a glass edge buried in a frame needs only minimal finishing, while an exposed polished edge in a structural glass application requires the most thorough treatment.
Edge finish types
Choosing the right finish
| Application | Minimum edge finish | Notes |
|---|---|---|
| Glass concealed in frame (curtainwall, window, door) | Arrised or seamed | Edge not visible; handling safety only |
| Glass shelves, table tops, furniture | Flat ground or polished | Edge visible and touched by users |
| Frameless glass doors and partitions | Polished | Edge fully exposed; optical quality expected |
| Structural point-fixed glazing, bolted | Polished (+ countersunk holes) | Edge quality affects strength at bolt holes |
| Tempered glass (any exposed application) | Must be finished before tempering | Cannot be cut or re-finished after tempering |
| Mirrors | Beveled (decorative) or polished | Bevel creates decorative light reflection |
| Laminated glass edge in structural sealant joint | Clean flat ground or polished | Interlayer edge must be sealed to prevent delamination |
Edge finish and structural strength
Finishing is not just about looks and safe handling, it changes how much load the edge can actually take. Anywhere the edge sees real stress (thermal or structural), the finish quality shows up directly in the numbers.
How big is the difference? A raw cut edge can be as weak as 20 to 30 MPa in bending. Grind and polish that same edge and it climbs to 60 to 80 MPa or better, because you are physically removing the little flaws that cracks like to start from. For annealed glass in structural applications where edge stress is expected, the design standard must account for edge finish quality when calculating allowable stress.
For tempered and heat-strengthened glass, the compressive prestress at the surface (including the edge) provides a large safety margin regardless of edge finish, but a properly finished edge before tempering still yields better final edge quality because the tempering process amplifies any pre-existing edge defects.
Edge deletion for coated glass
Low-E coatings and other soft-coat (MSVD) coatings applied to the glass surface extend to within a few millimeters of the edge. When a coated lite is used in an IGU, the coating must be removed from a perimeter band, typically 6–15mm, before the primary seal is applied. This process is called edge deletion.
Edge deletion ensures the PIB primary seal bonds directly to bare glass rather than to the coating, which would produce a much weaker adhesive bond. Without proper edge deletion, IGU seal failure is significantly more likely.
Edge deletion is performed using:
- Mechanical grinding wheels: A rotating abrasive wheel removes the coating from the perimeter band in a single pass. Fast and reliable for production environments.
- Laser ablation: A laser removes the coating with precision. More common for complex shapes or special coatings.
Edge quality inspection
Edge quality is inspected visually and by touch. Common defects that indicate inadequate finishing include:
- Shells (conchoidal fractures): Curved chips breaking out of the edge face. Visible as irregular curved voids in the edge.
- Hackle marks: Parallel ridges on the edge face left by the cutting process. On a raw edge these are expected; on a polished edge they indicate insufficient grinding depth.
- Micro-chips at arrises: Tiny chips at the edge-to-surface intersection, most visible on tempered glass with a polished edge. These are evaluated against acceptance criteria in ASTM C1036 and related standards.
- Roller wave on edge: Slight undulation on the edge face caused by the grinding conveyor rollers. Visible under raking light.
For high-specification projects, especially structural glazing, bolt-fixed glass, and architectural glass fins, fabricators may be required to submit edge samples for approval before production, and edge inspection may be included in the quality control hold points.