A Japanese sword whose whereabouts were unknown for 150 years until it re-emerged last year will make an appearance next month at the Kyoto National Museum.

The longsword was made by Masamune, a renowned Japanese swordsmith active toward the end of the Kamakura period (1185-1333). His blades earned widespread acclaim among collectors beginning in the late 16th century. Nine Masamune blades have received the National Treasure distinction from the Japanese government.

According to a 1919 guide on famous swords, the sword, nicknamed the Shimazu Masamune, was first recorded under the care of a lord in Kinokuni, now Wakayama prefecture. It moved through the centuries to come under the ownership of the ruling Tokugawa shogunate by the early 1860s. Of Masamune's surviving katana, or swords exceeding 60.6 cm in length, the Shimazu is unique.

"Among Masamune's remaining katana, it boasts outstanding soundness. Its fineness of iron, the power and energy of the ripple pattern, its general shape--all of those parts are above the standard," wrote Toshihiko Suekane, a researcher at the Kyoto National Museum, in an email to Japan Real Time.

On the origin of the blade's moniker, Mr. Suekane said, "Because of the name 'Shimazu,' it is thought it is related to the Shimazu clan who ruled Satsuma," a domain that spanned parts of modern Kagoshima and Miyazaki prefectures in Japan's southern island of Kyushu.

On the occasion of the shogun's marriage to a sister of then-emperor Komei in 1862, the shogun offered the blade to the imperial house in celebration of the festivities, according to the 1919 guide. The marriage was meant to strengthen the position of the shogunate in light of rising antigovernment activity, but little over six years later the Tokugawa system collapsed, replaced by an imperial state that lasted until the end of World War II.

In the confusion of the 1860s changeover of power, the whereabouts of the sword were lost to collectors and historians. According to the museum's website, over the last century and a half the sword passed down through the Konoe family, a line of aristocrats closely tied to the imperial house whose number includes prewar Prime Minister Fumimaro Konoe. The sword was obtained from the family by an unrelated donor, who gave it to the museum in 2013. The donation was the first anyone had heard of the sword since the 1860s.

The Shimazu Masamune will appear as part of an exhibition on famous Japanese swords at the museum from Oct. 15 to Nov. 16. The exhibition is called "Kyoto: Splendors of the Ancient Capital."

Image: Shimazu Masamune sword, donated by Befumon Okuda. Courtesy of Kyoto National Museum.