Understanding a Japanese sword is not an easy task. There are specific aspect that one has to learn to recognise and appreciate. Let's see the main aspects we have to consider when handling an original samurai sword:
A Japanese sword has been transmitted from owner to owner with great consideration. It is often said that owning a Japanese sword merely means we are its temporary keeper; it will survive to us and somebody else will take over the task. In order to preserve its integrity, a Japanese sword must be handled in a specific way; these rules offer protection both to the sword itself and to us, since we are handling a dangerous sharp blade:
Look at the sword's shape
Dimensions (length, thickness, blade depth) and geometries (curvature, structure, kissaki length) of the blade can give us a lot of information, especially on its age. Also, we have to learn to appreciate the beauty of the sword's shape. Is it powerful? Is it graceful? Everything is well balanced?
Look at the steel structure
The jigane is the result of how the iron has been folded and hammered. It can create beautiful patterns that resemble the wood's grain. Understanding the quality of jigane will result in understanding the quality of the blade. the best way to see it is with a soft light reflected on the sword's side. A window will work.
Look at the temper's line
The hamon has been designed by the sword-maker and it is his main artistic interpretation of the sword. Understanding hamon will bring us to understand the school or even the maker. The hamon can be seen pointing the sword to a light source and moving the blade in order to reflect the light in your eye. Appreciation of a good hamon is very similar to appreciation of conceptual art: something beautiful which is not really representing anything. Hamon and jigane will play together, forming beautiful activities (hataraki).
Look at the tang
The nakago is the only part of the Japanese sword which is left unpolished, hence its oxidisation will give us information about the sword's age. Also, if we see a well made nakago we can be sure the sword is quite important, as not many sword-smith would take care of a hidden part of their swords. Also, this is the only part where a Japanese sword can be signed and dated.