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Thermal cracks

Mechanical glass tensions caused by heat (when single-glass or insulating glass units are used for glazing) emerge in case of the difference in temperatures between two points of the glass surface.

Causes of the difference in temperatures:

  • sunrays (the impact of the sun depends on the orientation of the glazing plane with regard to the cardinal points (North, East, etc.);
  • conditioning and heating of the air in the premises (glass cracks may also appear due to the heating or cooling devices in the premises).

Theoretically, sun rays would not make any impact on thermal tensions of glass if the entire plane of glass were equally illuminated by sun and the temperature on the entire glass surface were distributed evenly. However, this happens very rarely in practice, as insulating glass units are fixed in the frame glazing groove and/or are partially in a shadow. That is why only thermally hardened or tempered glass should be used for glazing of non-transparent zones.

Heating and ventilation devices are most often installed near windows; therefore, they become a cause for thermal tensions because they form hot or cold zones on the glass surface.

The Table presented below shows typical cases of thermal tensions caused by the differences in temperature. Temperature differences caused by the sun are in many cases the result of the absorption characteristics of glass. Therefore, the emerging thermal tensions can be assessed taking into account the orientation of the building, climate zone and other parameters described in the Table (frames, shadows, curtains, etc.).

When the glass absorption index (EA) exceeds 50 %, the risk of thermal tension must be assessed and most often glass has to be processed as required in such cases.

Ordinary thermal crack

Glass view with the lines of breakage process / Beginning of breakage
Evently distributed thermal load – small / medium intensity
Type of glass Polished glass, figured (ornamental) glass, rolled sheet glass, laminated glass. In case of armoured glass, deviations are possible due to the grid.
Examples Partial inside covering of glass under the shining sun;

Too deep chase;

Glass units with sound and heat insulation are mounted in the glass pack with the sun control function, without covering under direct sunshine.

Beginning Right-angled edge crack;

Right-angled transition angle;

No curvilinear cracks in the beginning.

Lines of process Change of direction near the cold/heat zone (crack/incurve), then the line is curvy.
End Straight line;

Also often with an angle.

Other features Curvilinear cracks from rarely and they happen to appear in the zone of the first change of direction;

Large thermal crack

Glass view with the lines of breakage process / Beginning of breakage
Evently distributed thermal load – high intensity
Type of glass Polished glass, figured (ornamental) glass, rolled sheet glass, laminated glass. In case of armoured glass, deviations are possible due to the grid.
Examples Welding torch is in close proximity to the glass;

Hot air ventilator is in close proximity to the glass;

Laying of asphalt, when glass is covered with the protective cover inevently;

Partial internal covering of glass in strong sunshine.

Beginning Right-angled edge crack;

Right-angled transition angle;

No curvilinear cracks in the begining.

Lines of process Straight line of edge crack;

Change of direction near the cold / heat zone, most often turning into several cracks, later the line is curvy.

End Straight line;

Very rarely with an angle.

Other features Curvilinear cracks are possible, primarily in the zone of the first change of direction.

Curvy lines may form, primarily in the zone of the first change of direction.

Very large thermal crack

Glass view with lines of breakage process / Beginning of breakage
Evently distributed thermal load – very high intensity
Type of glass Polished glass, figured (ornamental) glass, rolled sheet glass, laminated glass. In case of armoured glass, deviations are possible due to the grid.
Examples Welding torch is in close proximity to the glass;

Laying of asphalt, when glass is covered with the protective cover unevently;

Hot air ventilator is in close proximity to the glass.

Beginning Right-angled edge crack;

Right-angled transition angle;

No curvilinear cracks in the beginning.

Lines of process Straight line of edge crack;

Change of direction and intensive distribution near the cold / heat zone;

Later the line is curvy.

End Straight line;

Very rarely with an angle.

Other features Curvilinear cracks are possible, primarily in the zone of the first change of direction;

Curvy lines may form, primarily in the zone of the first change of direction.

Even thermal crack I

Glass view with the lines of breakage process / Beginning of breakage
Evenly distributed thermal load – from low to high intensity
Type of glass Polished glass, figured (ornamental) glass, rolled sheet glass, laminated glass. In case of armoured glass, deviations are possible due to the grid.
Examples Partial darkening with inside blind in close proximity to the glass;

Partial covering with interior decoration in close proximity to the glass;

Shadow from the roof;

Dark spots on the glass (stickers, advertising, etc.).

Beginning Right-angled edge crack;

Right-angled transition angle;

No curvilinear cracks in the beginning.

Lines of process Along the cold / heat zone;

Later the line is slightly curvy.

End Straight line;

Without an angle;

Most often progressive (depending on partial covering).

Other features Possible deviation of cracking edges;

Rare curvilinear cracks;

Possible curvy lines.

Even thermal crack II

Glass view with the lines of breakage process / Beginning of breakage
Evenly distributed thermal load – from low to high intensity
Type of glass Polished glass, figured (ornamental) glass, rolled sheet glass, laminated glass. In case of armoured glass, deviations are possible due to the grid.
Examples Partial covering with inside blinds in close proximity to the glass;

Dark sports on the glass (stickers, advertising, etc.);

Big leaf of a plant or similar abject from the inside, directly on the glass.

Beginning Right-angled edge crack;

Right-angled transition angle;

No curvilinear cracks in the beginning.

Lines of process Straight line of edge crack, change of direction near the zone of cold / heat,

Possible breakage near the zone of cold / heat.

End Straight line;

Without an angle, most often progressive cracking.

Other features Frequently curvy cracks;

Possible deviation of cracking edges;

Curvy lines can form, first of all in the zone of the first change of direction.

Thermal crack of the worm

Glass view with the lines of breakage process / Beginning of breakage
Thermal spot load – very high intensity (Only in case of very large and especially thick glass units)
Type of glass Polished glass, rolled sheet glass, laminated glass, thick glass laminated with pitch.
Examples Welding torch is in close proximity to the glass;

Hot air ventilator is in close proximity to the glass;

High spot heating in the area of an extremely thick showcase glass or similar glass area.

Beginning In the centre of the glass area;

Beginning not near the edge;

Impossible to distinguish between the beginning and end.

Lines of process In the centre of the glass, in the form of a snake or worm, without significant change of direction.
End In the centre of the glass area;

Beginning not near the edge;

Impossible to distinguish between beginning and end.

Other features Possible small deviation of edges;

Possible curvy lines;

Often possible to observe not from any angle.

Thermal crack in the form of a palm tree / fan

Glass view with the lines of breakage process / Beginning of breakage
Evenly distributed or spot thermal load – big intensity
Type of glass Polished glass, figured (ornamental) glass, rolled sheet glass, laminated glass. In case of armoured glass, deviations are possible due to the grid.
Examples Partial inside covering of glass under intense shining sun;

High heating of the edge (blowlamp, hot air ventilator); Heating pipe in close proximity to the glass.

Beginning Right-angled edge crack;

Right-angled transition angle;

No curvilinear cracks in the beginning.

Lines of process Straight line of edge crack;

Change of direction near the cold/heat zone;

Further sequence of crack in the form of a palm tree.

End Straight line;

Very rarely with an angle.

Other features Curvilinear cracks form very rarely;

Curvy lines are frequent and they happen first in the zone of the first change of direction.

Hybrid crack I

Hybrid crack I: Glass view with the lines of breakage process / Beginning of breakage
Thermal / mechanical load
Type of glass Polished glass, rolled sheet glass, laminated glass, figured (ornamental) glass.
Examples Diverse effect through surface load (vacuum, wind) for already thermally loaded glass.
Beginning Gentle angle of edge cracking;
Gentle angle of transition;
Curvy cracks in close proximity to the glass edge; Unrecognisable cracking centre.
Lines of process Thermal beginning of cracking with the change of direction near the cold/heat zone, later curvy forms and after as a mechanical crack from straight to angular lines.
End Straight lines;
No angles.
Other features Possible curvy cracks on the surface.

Hybrid crack II

Glass view with the lines of breakage process / Beginning of breakage
Thermal / mechanical load
Type of glass Polished glass, rolled sheet glass, laminated glass, figured (ornamental) glass.
Examples Diverse effect through surface load for already thermally loaded glass of too small size.
Beginning Gentle angle of edge cracking, gentle angle of transition;

No curvy cracks of the glass edge;

Unrecognisable cracking centre.

Lines of process Thermal beginning of cracking with the change of direction near the cold / heat zone, later curvy forms and after as a mechanical crack from straight to angular lines.
End Straight lines;

No angles.

Other features Possible curvy cracks on the surface.

Hybrid crack III

Glass view with the lines of breakage process / Beginning of breakage
Thermal / mechanical load
Type of glass Polished glass, rolled sheet glass, laminated glass, figured (ornamental) glass.
Examples Diverse effect through thermal (partial shadow) and mechanical load (wind pressure) to already mechanically loaded glass (crack).
Beginning Gentle angle of edge cracking, gentle angle of transition;

Curvy cracks of the glass edge;

Recognisable cracking centre on the free edge.

Lines of process Mechanical beginning of cracking with the change of direction through thermal load near the cold / heat zone, later curvy forms and after as a mechanical crack from straight to angular lines, also possible distribution.
End Straight lines;

No angles;

Can reach the edge.

Other features Possible curvy cracks on the surface.