Partially shaded glass

When heated by 50 °C, a glass with an edge length of 1 m expands by about 0.5 mm. This “thermal expansion” is not critical when the glass is evenly heated.

It is quite different if the glass is not heated evenly: Then some areas of the pane expand more, others less strongly. This results in stresses in the glass. These “thermal” stresses are greater the larger the difference in temperature in the glass becomes.

Float glass “tolerates” temperature differences of about 40°C. If uneven heating produces a higher temperature difference, breakage of glass is to be expected. Often one part of a pane of glass is exposed to direct sunlight, while another part is in the shade. Such "partially shaded" glass will always be heated unevenly.

The size of the tensions generated by the partial shading in the glass depends on a number of circumstances. Such factors include:

• Intensity of the sun irradiation,
• pane format and installation situation,
• geometric distribution of the glass surface portions in the sun and in the shade,
• Absorption of sun irradiation.

Increased absorption is mainly due to coated and coloured glass. For glass that is subject to heavy exposure due to partial shading, the use of thermally toughened safety glass may be a suitable preventive measure.