As stated in the preceding section, it is the uneven heating of glass (high temperature gradient) that leads to the introduction of thermal stress of sufficient magnitude to induce thermal fracture in glass. One way that a high temperature gradient can be introduced in glass is through uneven heating of the glass caused by exterior shading. Guidelines have been widely presented for the identification of critical exterior shading patterns. These guidelines can be summarized as follows.
It is generally accepted that shading patterns that result in straight line shadow boundaries on the glass where the shadow pattern encompasses most of the glass are acceptable for clear annealed glass. Even straight-line shadow patterns can cause problems for tinted, or coated annealed glass depending upon the dimension of the shadow and the time that it remains on the glass.
Shading patterns that result in angular or L-shaped shading patterns of moderate width around the perimeter have been judged to be acceptable for clear annealed glass and only marginally acceptable for tinted or coated glass.
Shading patterns where the shadow area is very narrow around the perimeter of the glass, have been judged to be marginally acceptable for clear annealed glass and not acceptable for tinted or coated glass.
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