As described here, from 1790 until 1876 (when formal censorship ceased), all woodblock prints had to be examined by official censors, and marked with their seals.
From about 1805 until 1876 (which roughly parallels the last half of the best of ukiyo-e, and the period from which prints are most commonly found today), many (and in some periods almost all) woodblock prints contained a date seal, using the oriental cyclical calendar system.
To understand the ability to do such accurate dating, one needs to understand that the Japanese used (along with several other calendrical systems!
) the Chinese zodiacal calendar, which is a twelve-year cycle, with each year having an associated animal, e.g. (Anyone who has ever eaten in a Chinese restarant will no doubt recall this from the placemats.) During large parts of the censor seal period, the seals included a zodiacal year indication, along with the numeral for the month.
With time, it becomes easy to tell if a single round seal is a nanushi seal, or some other kind.
They are sometimes found with a zodiacal date seal and/or an aratame seal.
(Also not covered here are certain obscure censor seals used for special forms, e.g.
fan prints.) Hover the mouse pointer over an image to see what it reads; for all images, click on the image to see a larger version.
In the second, from 1805 until 1841, a large number of different forms (not fully explored here - see either reference listed at the foot of this page for more) can be found: the kiwame alone, or with a zodiacal date seal; on occasion, the zodiacal date seal alone; various seals from the Gyoji, wholesale publishers who acted as censors.
From 1805 until the end of the period, a separate seal giving the date in zodiacal form was often (but not always) added.
(One uncommon type from this period, one which is easy to confuse with later seals, was used during the period 1815 to 1832, and includes a zodiacal date in the top part of an oval seal, along with an aratame ("examined") character below.) In 1842, the whole system was reformed, and replaced by individual censors called Nanushi.
They marked prints with their individual round seals, bearing characters from their names.
the artist's working period, or the form of his signature, or the form of the censor seals, which changed often) usually allow one to ascertain the exact date. number, contents, shape, etc) of the censor seals used in the period from 1790 to 1876 (during which woodblock prints were censored) falls into six main periods.