The central grove, or groves, in a sword is usually referred to as a “fuller”. The reason for this is that they are normally made with a blacksmith’s fuller, a type of stamping tool, rather than by grinding away material.
There are many reasons to make the fuller in this fashion, many of which you probably know.
- When using historic tools and techniques, forging is faster than removing metal.
- Less metal overall is used in the process.
- It helps to form taller I-beam like ridges on either side of the fuller, as the metal in the center is pushed up.
There is another that you may not be aware of. Before the invention of mass produced steel, swords were made primarily out of wrought iron. Steel, if available, would only be used on the edges where the extra hardness justified the expense.
Wrought iron has an interesting property not found in steel: it is fibrous. I can’t find a good photo, but if you see a piece of heavily corroded metal that looks like wood or bundles of straight wire laid on top of each other, you’re looking at wrought iron.
When working with wrought iron, the blacksmith needed to avoid cutting the fibers. Holes were punched, rather than drilled, so that the fibers would be bent around the hole rather than stopping abruptly, causing a weak spot.
Reading a blacksmith’s manual published in the early 1900’s, I learned that the same rule applies to fullers. Had they ground away the central grove, either end of it would have left cut fibers that would significantly weaken the sword at those points.
What about folding?
Yep, that’s a real thing. Even in the early 1900’s, they advise hot working wrought iron with hammers to mitigate the problem of impurities, also known as slag.
If I’m reading this correctly, the fibers are actually caused by slag. Folding metal not only helps remove some slag, it also aligns the rest into long fibers, which is much better than having a lot of short, broken fibers running in every direction. Fibers can also be straightened by “rolling”, which forces the iron through heavy rollers that use pressure to form the metal into bars or rods.