Types of Glazing

Common types of glazing and their features include:

• Annealed glass
  
  
  • Most common and inexpensive
    
    
  • Easiest to break
    
    
  • Tends to break into large pieces (shards) with very sharp edges and points
    
    
• Tempered glass
  
  
  • More difficult to break (three to five times the strength of annealed glass)
    
    
  • Breaks into small, cube-like pieces that are much less dangerous than shards of annealed glass
    
    
  • Susceptible to spontaneous breakage when subjected to thermal shock (cool water from fire hose during hot fire) conditions and high winds
    
    
• Heat-strengthened glass
  
  
  • Not categorized as safety glass
    
    
  • Twice the strength of annealed glass
    
    
  • Increased impact resistance when subjected to thermal shock and high winds
    
    
  • Not likely to break spontaneously
    
    
  • Breaks into larger pieces that tend to remain in the supporting frame
    
    
• Laminated glass
  
  
  
  • Has a thin plastic interlayer (usually polyvinyl butyral resin) between two or more layers of tempered glass
    
    
  • Cannot be cut from one side only because of the plastic interlayer
    
    
  • Sustains repeated blows before the plastic interlayer fails, creating an opening
    
    
  • The plastic interlayer holds the broken pieces of glass in place
    
    
• Wired glass
  
  
  
  • Annealed glass with wire mesh embedded inside
    
    
  • Typically used to satisfy building codes for fire protection
    
    
  • Impact resistance and breakage path similar to annealed glass. Helps hold the broken pieces of glass together and retains them in the frame
    
    
  • Remains in place during a fire even if exposed to the fire’s heat and subjected to a stream of water from a fire hose (thermal shock)
    
    
• Ceramic glass
  
  
  • An alternative to wired glass where fire protection is required
    
    
  • Has minimal thermal expansion and the ability to withstand extreme heat
    
    
  • Forms large shards when broken
    
    
  • Less impact strength than annealed glass
    
    
  • Laminated versions are available for areas requiring safety rating
    
    
• Transparent mirrored glass
  
  
  • One-way glass
    
    
  • Prepared by applying a special chrome alloy to the surface of the glass
    
    
• Plastic glazing
  
  
  • Acrylic or polycarbonate plastic
    
    
  • Acrylic plastic is clearer than polycarbonate
    
    
  • Acrylic plastic has about 17 times the impact strength of glass
    
    
  • Polycarbonate plastic has about 300 times the impact strength of glass
    
    
  • Acrylic and polycarbonate plastics are about 50 percent lighter than glass
    
    
  • Scratches easily
    
    
  • Deforms under heat
    
    
  • Acrylic plastic burns vigorously
    
    
  • Polycarbonate plastic burns, but not as vigorously
    
    
  • The amount of plastic glazing may be restricted by local fire codes (plastics give off toxic fumes when burned)
    
    
  • Can be cut and drilled easier than glass
    
    
• Glass-clad plastic
  
  
  • Multiple pieces of glass, plastic (usually polycarbonate), and interlayers of urethane bonded together under heat and pressure
    
    
  • Glass surface protects against scratching, marking, and flammability; plastic provides impact strength.
    
    
  • Impacts break the glass surface into a spider web pattern, but the plastic core remains intact.
    
    
  • Materials can delaminate, degrading visibility
    
    
• Window films designed for safety and security (rather than films designed to reduce heat gain or loss)
  
  
  • Make glass more shatter resistant by holding the pieces of glass together when the glass breaks.
    
    
  • Thicker than films designed to reduce heat gain or loss and have more aggressive adhesive
    
    
  • Can be used as a remedial solution on existing windows
    
    
  • Mounted on the interior side of the glass
    
    
  • Cleaning requires more care than glass without film
    
    
  • Large windows may require more than one sheet of film. The seam will reduce the security performance.

Security glazing is tested and rated for resistance to specific threats, including:

• Burglary and forced entry. The material is not expected to prevent access indefinitely, but will resist penetration for a specified period.
  
  
• Bomb blasts. The expectation is that the material will offer some increased protection against the shock or blast wave and fragments propelled by the blast. This type of glazing should not become fragmented and should stay in the specially designed frame.
  
  
• Ballistic attack. Specially designed glazing is intended to protect people from direct bullet injury and flying glass fragments. Such glazing is tested and rated in accordance with UL 752.
  
  
• Electronic surveillance. Specialty glazing designed to weaken electromagnetic signals that might be picked up from the outside and to prevent electromagnetic signals beamed from the outside from penetrating the glazing.