Pattern welding

[1] Often called Damascus steel, blades forged in this manner often display bands of slightly different patterning along their entire length.

Pattern welding developed out of the necessarily complex process of making blades that were both hard and tough from the erratic and unsuitable output from early iron smelting in bloomeries.

[6][7] By the 2nd and 3rd century AD, the Celts commonly used pattern welding for decoration in addition to structural reasons.

[3] By the 6th and 7th centuries, pattern welding had reached a level where thin layers of patterned steel were being overlaid onto a soft iron core, making the swords far better as the iron gave them a flexible and springy core that would take any shock from sword blows to stop the blade bending or snapping.

[7] During the Middle Ages, Wootz steel was produced in India and exported globally, including to Europe.

The similarities in the markings led many to believe it was the same process being used, and pattern welding was revived by European smiths who were attempting to duplicate the Damascus steel.

While the methods used by Damascus smiths to produce their blades was lost over the centuries, recent efforts by metallurgists and bladesmiths (such as Verhoeven and Pendray) to reproduce steel with identical characteristics have yielded a process that does not involve pattern welding.

[11] Modern steels and methods allow for patterns with much higher number of visible layers compared to historical artifacts.

A contemporary pattern-welded sword blade made by Danish swordsmith Ejvind Nørgård. The blade shows a chevron pattern with opposing twists and straight laminate alternating.
An image of a modern pattern welded knife blade, showing the dramatic patterning on the side below, and the layering of the steel in the spine above. Acid etching darkens the 1080 plain carbon steel more than it does the 15N20 low alloy nickel steel, producing alternating bands of light and dark on the surface.