Pattern welding
Pattern welding is a process where metals of different composition (typically lower and higher carbon iron) are forged together. This process could be typically found in blades such as swords and knives.
Decorative and Functional
This process results in alternating bands of colour and are thought to be the waves mentioned when referrring to Dark Age swords.
It also results in a stronger, more homogenous blade than one forged of poorer quality metal without being forged entirely of the better quality iron/steel. Vikings were fond of pattern welded blades and would seek such blades in trade and would then have a local smith set them with fittings. Not thought to have been used on axes.
Swords with pattern welded blades would have edges of steel welded to them to provide a good quality edge.
History
Thought to have been first used by the Romans, and was relatively common by the year 700. The process appears to have disappeared by the end of the 12th century.
