Itoin is a copper seal made in China in Ming Dynasty and brought into Japan over the Sengoku period (period of warring states) and the Momoyama period.
The height is 3-4 centimeters on average, and the weight is around 50g. Most are made of high quality bronze, and a deep color of the ground appears. The color of brass or brown is rare. It is hollow due to a process of making with split molds, and thus has a lighter weight than it looks. The face of a seal has a form of thin plate. There are various kinds of chu (finger grip of a seal), such as animal-shape chu or man-shape chu, mainly, and more than 200 kinds has been recognized. A bore to pass a ribbon through is provided without exception. Most of the seal have a square or circular stamping surface, but may have one in other various shapes.
Most of inmon (words or symbols engraved on the seal) are yokoku (raised characters.)
They seem to be kanji (Chinese characters), but 90 percent of them cannot be interpreted.
Barely readable characters includes those relate to letters such as '封,' '封信,' and '平安家書,' and Yuin (seal carved favorite words or phrases.)
Some of the words engraved on the seal were not even horizontally reversed, indicating that how it turns out when it is sealed had not been considered. Small number of shosei-in (seal on which twelve Chinese zodiac signs are carved) (design) are found.
These inmon and chu are limited in variation, but are combined in various ways. There are few itoin that do not overlap with others at all.
A reliable theory is that itoin was originally used as seal of approval when trading raw silk in Japan-Ming trade. In this theory, raw silk was sealed with one itoin each time of bundling up, and the itoin was regarded as a receipt stamp in the Japanese side. In one theory, it is assumed that the time is the tally trade (between Japan and the Ming dynasty) in the Muromachi period, and in another theory, the later Keicho era.
Other theories include that of a paperweight, that of netsuke (miniature carving attached to the end of a cord hanging from a pouch), that of imitation of a Goreo seal (Korean seal), and that of a purchase stamp (products in bulk.)
The theories also included that of production in Japan, but casting in China has been determined since it was affirmed that it was stored in the Shanghai Museum.
In the Edo period, it was called 'kodoin,' or 'kochu-in,' and also called as common names 'eto-in,' 'ju-in,' or 'Hakata-in.'
It was Tokifuyu YOKOI who first called it 'itoin' when he released 'A study of Itoin' in 'Kokogaku Zasshi' (Archeology Magazine) published in 1897.
Busho (Japanese military commander) in the Sengoku period (period of warring states) preferably used itoin, and especially well known is Inbanjo (license with a seal) with mimizu-in (seal of a worm) of Hideyoshi TOYOTOMI. In the early Edo period, men of refined taste such as Nobutada KONOE and Enshu KOBORI, painters such as Tanyu Kano, Mitsuoki TOSA, Oukyo MARUYAMA, Buncho TANI, Aigai TAKAKU, and Chinzan TSUBAKI preferably used itoin as their own Rakkan-in (seal to be put on a piece of calligraphy by a calligrapher when the piece is completed). Fuyo KO, known as insei (master of sealing), loved itoin, and included impressions of seal on the inpu (compilation of seal marks) he compiled. Collections in "Kodo Ini" that Fuyo specified the original condition by comparing with other writings are entirely impressions of seal of itoin.
Itoin is reliable as the origin of netsuke, and it is assumed that Japanese original netsuke has been developed with the shipped itoin hung on an obi. Some netsuke in the early days is quite similar to itoin.
Nowadays, itoin has been a target as collection and fancy among dilettantes.