Ordinary or annealed glass is very weak under tension, but very strong under compression.  When annealed glass breaks, it breaks into jagged and sharp-edged pieces which are very dangerous.  To make glass strong, it is tempered.  Tempered glass is a grade A safety glazing material as defined by International Standards.  When it breaks, it becomes small fragmented pieces called cullets which are less dangerous.

Tempered glass is made by uniformly heating the glass to just below its softening point and then rapidly cooling it by a uniform blast of air to both its surfaces.  This causes the outside of the glass into compression and the center into tension.  It is this outside shell of compressive strength protecting the center tension that gives tempered glass its strength.  However, it is also due to this that tempered glass cannot be cut or drilled.

• Tempered glass is up to 500% stronger than annealed glass.
• Tempered glass can withstand temperatures up to 300ºC compared to 50 - 60ºC for annealed glass.
• When broken, tempered glass shatters into small harmless fragments.

Measurements

• Minimum Size:  The smallest panel of glass that can be toughened must equal 300 mm in the diagonal measurement.
• Maximum Size:  2140 mm x 7000 mm.
• Thickness:  3 - 19 mm toughened.

Limitation on Hole Position

• Diameter of holes should not be less than the thickness of the glass.
• The distance from the edge of the hole to the nearest edge of the glass must be at least equal to 1.5 times the thickness of the glass (for 3 to 6 mm) or 2.0 times the thickness of the glass (for 8 to 12 mm) (figure 1).
• Distance between holes must be at least 50% of the hole diameter (figure 2).
• Hole diameters must not be greater than 33% of the panel measurement at the narrowest point (figure 3).
• Hole diameters must not be greater than 33% of the panel measurement at the narrowest point (figure 4).
• In a panel with holes, the minimum width of panel must be 8 times the thickness of the glass (figure 5)

Edgework on Holes and Cut-outs

Holes will generally have a ground finish with arris, Countersunk holes are available in glass of 5 - 19 mm thickness and are countersunk to 45º.

Cut-outs will have a flat ground finish with arris.
Cut-outs can be flat polished on a per request basis.

Rectangular Cut-outs

• Distance between hole and glass edge must be at least 50% of the cut-out dimension in the direction in question (figure 1).
• Height of the hole must not be more than 33% of the glass height (figure 2).
• Width of the hole must not be more than 33% of the glass width (figure 2).
• All rectangular cut-outs must have radius corners of semicircular ends with a minimum radius of not less than the glass thickness (figure 3).

Side and Corner Cut-outs

• All cut-outs must have holes of radii corners not less than the thickness of the glass (figure 3).
• Interior dimensions of the cut-out must never be less than the thickness of the glass (figure 4).
• The width of the cut-out must not be less than the thickness of the glass (figure 2).
• The depth of the cut-out must not exceed more than 33% of the panel (figure 2).
• External corners on cut-outs must be rounded (figure 6).

Special Hole Requirements

If a hole is placed in a corner closer to the recommended dimensions, a saw cut can be made to minimize the stresses and chances of breakage (figure 5).