Types of Blade Forging and Construction

There are three basic types of Japanese sword blade that can be built by forging:

Simple: The blade is made from one piece of unfolded steel. Very simple and effective. Most high quality swords are made like this.

Damascus: A folding technique where the steel is folded over itself many (generally up to sixteen) times, leading to hundreds or even thousands of layers (one fold = 2 layers, two folds = 4 layers, three folds = 8 layers, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, etc.). More folds than sixteen are pointless, as the layers would become thinner than one molecule - which is impossible. This method was developed to removed impurities from the steel (it also removes carbon, which is bad, but can be compensated for to some degree), but modern steel is so pure that Damascus folding is no longer needed. It is now used for aesthetic reasons; Damascus steel looks really good; you can see the layers in the blade.

San Mai: Translates to "three layers". Layers of softer, lower carbon steel (or iron) is forge welded to layers of harder, higher carbon steel. The lower carbon steel forms the core (and sometimes the sides and/or back), and the higher carbon steel forms the edge. The hard edge will hold its sharpness, the softer core provides shock absorption; making the sword harder to break. Unlike Damascus, San Mai is still very practical. San Mai folds should be invisible; if you can see a San Mai fold the forging was flawed and the blade should be discarded - you'll have to trust your seller that the blade is San Mai - you can't tell by looking. San Mai can be revealed by etching the blade with acid, such as lemon juice, but I don't recommend trying this unless you know what you're doing; you can permanently stain the blade. San Mai swords are more durable (and more expensive) than simple blades. They are more durable (and usually less expensive) than Damascus blades. A blade can be either San Mai or Damascus, or it can be both; Damascus steel which is then San Mai folded. Very cool, looks good and lasts long.