In theory, glass is very strong - much stronger than many metals. In practice, it is severely weakened by fine cracks caused by abrasions through handling and corrosion by water vapour. So glass behaves as a brittle substance with a tensile strength only 0.4 per cent of its theoretical value. And the safety factors set for designers reduce this by a further factor of ten.

Shaping is needed for most of the float glass used in transport and for some uses in building. Because it is brittle at room temperature, it is inherently different from metal in its bending characteristics. And because it is transparent the engineer must worry about the inside of the bend, where the glass may wrinkle. Optical distortion sets the limit for most shaping. A further restriction is that patterns - trademarks, antennas, de-icers - which are silk-screen printed in ceramic inks on the glass, must survive shaping unscathed.