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Toughened glass (also known as Fully Tempered Glass - FTG) begins with annealed glass. It is heated to 620°C - 675°C (90-140°C above the transition temperature) and rapidly cooled with jets of cold air. This causes the outer surface of the glass to solidify before the inner part. As the interior cools, it tries to shrink, but the solidified outer surface resists this force and goes into compression (usually between 90 and 150 N/mm2) and the interior goes into tension. The temperature distribution is usually parabolic, with a colder surface and a hotter interior. To get the best results with maximum temper stress, the surface should be solidified exactly at the point when the highest temperature difference occurs and the initial tensile stress is released. In this type of glass, surface flaws do not propagate under compressive stress, and so toughened glass can sustain higher stresses than annealed glass. [9] Glass with low thermal expansion, such as BSG, is more difficult to be toughened. [10]