Issue 39 October 2021 - Journal - Page 106
Spontaneous Breakage of
Thermally Toughened Glass
by Ben Wallace, Glass and Glazing Federation
• Sharp body impact, either accidental or malicious
• Poor glazing design (e.g. glass to metal contact)
• Poor workmanship (e.g. incorrect installation,
inappropriate assembly of fittings, unskilled labour)
• Inferior glazing materials (e.g. use of incorrect
gaskets, bushes, etc)
• Excessive loads either mechanical or thermal
• Incorrect processing of glass
• Inclusions in the glass
With construction products in the spotlight more than
ever it is important to have the right products installed
in the right places. A common glazing industry issue
that we regularly experience, both throughout the
UK and internationally, is the seemingly random
spontaneous fracture of thermally toughened
soda lime silicate safety glass. This is where the
installed glass shatters into thousands of pieces.
Thermally toughened soda lime silicate glass was first
manufactured in 1931.
Fractured thermally toughened glass may remain in
place, or it may fall out. The design of this product
means that the internal stress obtained during the
toughening process causes the glass to get the shattering effect. Whilst thermally toughened glass has
many benefits regarding additional strength when
thinking about wind loadings, impact, and safe breakage behaviours, it also carries risk. If fractured, glass
falling from height and/or acting as a barrier, can have
significant safety implications.
1 Ballantyne E.R.: Report 061-5: Fracture of toughened
glass wall cladding, I.C.I. House, Melbourne, CSIRO, Division of Building Research, Melbourne: 1961
Thermally treated glass therefore becomes associated
with unexplained, but noticeable, breakages and these
have been labelled “spontaneous fractures”, whereas
breakages from similar causes in other types of glass
are frequently referred to as “cracks”. In fact, thermally treated glass is less susceptible to breakages than
any other form of glass, but the fracture propagates
with a loud noise which may be accompanied by
falling particles and is therefore much more obvious.
In addition, large projects with multiple spontaneous
fractures could have significant financial implications.
This may involve multiple Expert Witnesses acting on
behalf of their clients trying to ascertain the cause of
In addition, with toughened glass, the origin of the
fracture, which is a source of information as to the
cause, is often lost. Of the various causes of “spontaneous fracture”, only that associated with the presence
of foreign particles in the glass is more likely to cause
fracture in thermally treated glass than in other forms
of glass, because they can disturb the very high builtin stresses in thermally treated glass. Spontaneous
breakage due to inclusions is possible in any of the
three different types of thermally treated glass
• Heat strengthened soda lime silicate glass – EN
It was in 1961 that E R Ballantyne issued a report on
the breakage of thermally toughened cladding
panels from a building in Melbourne, Australia. This
report showed that an inclusion of 'nickel sulphide'
was at the origin of the fracture. Further work
explained that the inclusion experienced a phase
change that caused the toughened glass to fracture.
Thermally treated glasses may fracture from a variety
of causes. These in order of occurrence are:
• Edge damage (e.g. caused during manufacture,
transportation, installation, service conditions)
Above, example of an inclusion in a toughened glass origin.
EXPERT WITNESS JOURNAL
Above, Magnified NiS Inclusion.
O C TO B E R 2 0 2 1