Wilkinson, David A. Glass and Effects by Fire. Paper presented at "Fire
Down Under" Conference (Auckland, New Zealand: 12 May 1993).
Abstract: The melting point of glass is between 800 degrees Celsius
and 950 degrees Celsius and its tensile strength resides almost entirely
on its outer skin. Glass does not have a fixed melting point; it grows soft
and increasingly liquid as temperatures rise. Sudden rises in temperature
can engender different effects than gradual increases.
Types of fracture:
thermal: unequal expansion of glass casued by rapid rise in temperature
leades to cracking. Thermal cracks do not have obvious "rib markings"
of impact cracks. Slow to medium temperature rise can cause a gentle, curved
appearance. In a rapid temperature rise, fracture surface will be less smooth
with some rib marks.
explosion fracture: demarked by "square" ends
impact fracture: creates a "cobweb" or "stone in the pond"
pattern with radial or concentric cracks and usually shards. Rib marks can
be seen near the point of impact. Secondary breaks can result if glass falls.
hihg-velocity impact: projectiles will often pass through glass so quickly
that radial cracks to do not result; instead a ìhaleî (hole
with chipping and sharp, small shards on the exit side) is formed with a
larger hole on the exit side of the glass.
toughened glass has been contracted under compression. When this glass
fracures, it commonly does so in "cubes." Hazy rib marks can be
formed by projectiles as the softer center core of the glass absorbs shock.
Laminated glazing in glass causes two differences: it takes longer to
create thermal fractures; total collapse seldom occurs.
Large, flowing curves can indicate a slower burning fire. The gradual
heating of glass as the fire smolders minimizes the shock and creates "wave"
stresses. Edges of glass will be smooth and rounded, with the most rounded
commonly found the towards the heat. Heavy smoke stains, usually matte,
are also common.
Close formed curves following the line of the frame can indicate a fast
burning fire. The sudden heating and rapid pressurization causes sever stress
on the glass. Fractures occur quickly and are small and sharp. Edges of
fractures will be sharp on the side closest to heat. Faint rib markings
may appear. Smoke staining may be less and color may be baked on and shiny.
A mixture of fracture types may be found due to variations in heating.
"Spalling" or "fire crazing" of glass (small, close
cracks) is created by rapid cooling, usually from the application of water.
Crazing is not to be confused with fractures from a rapid growth fire.
Glass condition and position can give indications to the behavior and
type of fire. Glass position should be noted during examination. If explosion
occured first, glass will be found a distance from the building and will
not be stained. If the explosion occurred after fire development, some staining
may be present. The inner side of glass should be more stained. Glass inside
does not necessarily mean forced entry. When a window breaks out, 30-40%
of it can be sucked back inside the structure due to depressurization.