Hi I have heard allot about piled stucture in the roman era and I was wondering how it was different from pattern welding?
Thnx
Z

It is only a personal theory, but, I suspect piling was most likely done to reduce brittleness in period pieces of bloomery iron. Many "piled" sections artifacts have very low levels of martensite/bainite (phases of metal grain structure that hardens and tempers well.) This suggests to me that they actually reduced carbon content during the piling process.

I have yet to form a sincere opinion on the historical purpose of pattern welding. It seems probable that it was a way of combining iron from different sources, and saving the known best "tool edge" materials for that edge region in the finished piece. The cosmetic beauty of it was commented on by early 5th century, and I suspect cosmetics were a real motivation for pattern welding then as it is now in our modern extremes of it.

Right, I think I'm pretty sure what pattern welding is. But what is "piled structure" is it just merl folded over a bunch of times or something else?

Great post Kirk! I have been looking for Roman era examples of it.
I had read that Romans saw the Germanic tribes make blades with techniques that I would interpret as early pattern welding / forge combining different materials. This does not seem to be written down until around 200 A.D. But, I know that evidence of this in blade cross section from finds in North Italy Celtic villages goes back to around 5th or 6th century B.C.