See now, for a good blade, one that will not betray the man in battle, rods of hard and soft iron must be heated and braided together. Then is the blade folded over and hammered flat again, and maybe yet again, many times for the finest blades … So the hard and soft iron are mingled without blending, before the blade is hammered up to its finished form and tempered, and ground to an edge that shall draw blood from the wind. So comes the pattern, like oil and water that mingle but do not mix. Yet it is the strength of the blade, for without the hard iron the blade would bend in battle, and without the soft iron it would break.
Source: Goodreads quotations from Rosemary Sutcliff