Thermal breakage or thermal stress in glass is caused by varying amounts of radiant heat absorbed across a single sheet of glass. Large wandering cracks that mysteriously appear in glass panels are the typical consequence of thermal stress. There are various conditions or even types of glass that contribute to a higher risk of thermal breakage. Although in most cases there is no way to avoid the source of thermal stress a variety of solutions are available to prevent the costly fractures that are produced by this phenomenon.
What is thermal stress?
When subjected to radiant heat glass expands, which increases the overall dimensions of the sheet. If a sheet of glass is heated in this manner at an even rate over the entire surface, there is less thermal stress and generally no fracture will take place. The problem stems from variations in heat absorption across the sheet, creating hot zones that want to expand and cold zones that remain in their original state. The pressures caused by thermal stress expose small imperfections that are usually located on the edges of clean cut glass and this is where the cracks begin.
Causes o f Thermal Stress
The main factor that influences thermal breakage is rapid heat absorption of one area of a glass sheet whilst the remainder is not affected. Circumstances which produce this situation include, shadows from awnings or blinds, artificial cooling or heating by air conditioning and even the small amount of glass concealed inside a glazing frame that is not exposed to sunlight. In most cases ordinary annealed glass is more likely to succumb to the pressure of thermal stress. Factors that increase this risk are coloured or tinted glass, reflective coated glass or when solar film applied to existing glass.
In the majority of cases thermal stress is unavoidable, however the following are solutions that will prevent thermal breakage from occurring:
Toughened or heat strengthened glass should be installed in locations subject to these conditions as it is capable of withstanding exposure to extreme thermal stress.
Annealed glass can be used in most situations, provided that all edges are ground to eliminate any imperfections on the perimeter of these sheets.
No solar film or signage application
The most common cause of thermal breakage to existing glazed panels is the application of coloured or reflective films to the surface of glass. Annealed glass can be in positions affected by sudden increases of radiant heat for many years without any problems. Once a film is applied, it changes the amount of heat retained in the glass rather than passing through it, therefore increasing the likelihood of thermal breakage.
In rare situations glass shelves or glass doors can be subject to thermal breakage. An example of this would be when a hot plate is placed on a cold refrigerator shelf. The difference in temperature creates enough thermal stress to crack the glass. The best way to avoid this is to use tempered or toughened glass for this application. The refrigerators website at www.refrigerators.org provides in depth tips and reviews on related refrigerator issues.