Wu Zhu () is a type of Chinese cash coin produced from the Han dynasty in 118 BC when they replaced the earlier San Zhu (三銖; "Three Zhu") cash coins, which had replaced the Ban Liang (半兩) cash coins a year prior, until they themselves were replaced by the Kaiyuan Tongbao (開元通寳) cash coins of the Tang dynasty in 621 AD. The name Wu Zhu literally means "five zhu" which is a measuring unit officially weighing about 4 grams however in reality the weights and sizes of Wu Zhu cash coins varied over the years. During the Han dynasty a very large quantity of Wu Zhu coins were cast but their production continued under subsequent dynasties until the Sui.
The production of Wu Zhu cash coins was briefly suspended by Wang Mang during the Xin dynasty but after the reestablishment of the Han dynasty, the production of Wu Zhu cash coins resumed, and continued to be manufactured long after the fall of the Eastern Han dynasty for another 500 years. Minting was definitively ended in 618 with the establishment of the Tang dynasty. Wu Zhu cash coins were cast from 118 BC to 618 AD having a span of 736 years, which is the longest for any coin in human history.
History
Western Han dynasty
"Wu" means "five" (5) and zhu was an ancient Chinese unit of weight equal to 100 grains of millet. A "five zhu" cash coin would weigh about 4 grams. Originally Ban Liang cash weighed 12 Zhu as a Liang (tael) was 24 Zhu, however over time the weight of Ban Liang cash coins gradually decreased so the Wu Zhu cash coins were introduced as a new standard unit (after the earlier San Zhu, or "3 Zhu" cash coins) under the reign of Emperor Emperor Wu. The introduction of the Wu Zhu also fixed the standard exchange between bronze coins and gold as 10,000 bronze Wu Zhu cash coins would be worth 1 Jin of gold.
The first Wu Zhu cash coins had unfiled edges but the second series issued under the reign of Emperor Wu had them filed. In 118 BC the central government of the Han dynasty ordered both the Commanderies (郡, jùn) and the Principalities (國, guó) to cast Wu Zhu coins, these Wu Zhu coins are referred to as "Jun Guo Wu Zhu" (郡國五銖, jùn guó wǔ zhū) coins which at most have a diameter 33.3 millimetres and a weight of 5.8 grams. A notable feature of Jun Guo Wu Zhu coins is that they have a rim around the square center hole of the reverse side of them, these rims were added to prevent people from scraping metal off the coins which would reduce their value. Another notable feature of these early Wu Zhu's is that they tend to have edges which are unfiled, making these cash coins to generally have rough circumferences, they are notably also heavier than later cast Wu Zhu's. In 115 BC Emperor Wu decreed that all Wu Zhu cash coins should be cast with a value of 5 cash coins, these coins are known as "Chi Ze Wu Zhu" (赤仄五銖, chì zè wǔ zhū) or "Zhong Guan Chi Ze" (鍾官赤仄, zhōng guān chì zè) because of their "red" or "purple" edges as they were filed which resulted in the colour of the copper being visible, another feature of these cash coins is that the "Wu" (五) character tends to be composed of some rather straight lines.
Starting from the year 113 BC, the central government regained the exclusive authority to manufacture coinage, from this point Wu Zhu cash coins started being produced by the Three Offices of Shang Lin (上林三官, shàng lín sān guān). These Wu Zhu coins had a nominal value of one coin as opposed to the Chi Ze Wu Zhu's which had an unrealistic nominal value of five. The majority of the Shang Lin San Guan Wu Zhu's contain a raised line above the square center hole on the obverse side of the coin.
Under the reign of Emperor Xuan which lasted from 73 BC until 49 BC the Wu characters were less in size and notably was written with slightly crooked strokes that don't extend to the horizontal lines of the top and bottom ends. A number of these Western Han dynasty Wu Zhu cash coins also displayed dots which represent "stars" and crescents which represent the moon on the interior rim of the coin as well as other symbols which were considered to be auspicious being some of the earliest examples of cash coins used as Chinese amulets and charms.
In the 123 years after 118 BCE, when Wu Zhu cash coins were initially introduced, over 28 billion coins were cast for circulation.