Paintings must have rigid stretchers so that the canvas will be taut, and the paint must not deteriorate, crack, or discolor.
画必须有坚硬的画架，这样画布才会绷紧，而且颜料不能变质、开裂或变色。
2
Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age related impairment of both cells and tissues.