A post-Roman period technique of ironworking used particularly in the manufacture of weapons, mainly swords, developed to overcome the problems of brittleness caused by trying to diffuse carbon into iron. It produced blades that were both strong and decorative. In the manufacture of a sword, for example, the central part would typically be a core of carbon steel, with soft iron welded to it. Wire and strip metal, sometimes in varying combinations of type and color, were welded together and hammered out to produce a blade with patterned effect. The pattern derives from the difference in the carbon content between the uncarburized cores and the carburized surfaces of the welded strips, which is exposed during the forging and grinding of the weapon. A sword of this quality could have taken some 75 hours to make. The finest examples have been attributed to Frankish workshops, although notable examples are also known from Anglo-Saxon and Viking contexts.