Built in 1755 beside Foxiang Ge ("tower of Buddhist incense") — the landmark and the highest building in the Summer Palace — Baoyun Ge on the Hill of Longevity is the only surviving Qing imperial architecture entirely built in bronze. About 207 metric tons of bronze was used for its construction, according to Wei Lijia, a cultural heritage conservator at the Summer Palace administration.

Qing royals used it for important Buddhist rituals and Emperor Qianlong later ordered the building of its replica in the Mountain Resort in Chengde, Hebei province. Unfortunately, that replica was dismantled and melted down by the invading Japanese army in 1944 to make bullets.

The original pavilion in the Summer Palace also endured an attack by Anglo-French Alliance Forces in 1860 who burned and destroyed many key pieces of architecture at the Summer Palace. Baoyun Ge became one of the few survivors due to its bronze structure, but its interior settings were ransacked. In 1900, dark days came again as it was hit by the Eight-Nation Alliance Forces during the Boxer Rebellion. Out of its 70 latticed panels on doors and windows, 31 panels were later found to have disappeared.

Chinese relic conservators have long searched for these lost panels. In 1993, Maurice Greenberg, an entrepreneur from the United States, donated 10 windows, including 20 panels, back to China after he bought them in Paris. Another panel was returned by a French antique authentication expert in 1995.

"The whereabouts of the last nine missing panels remains unknown," Wei said. "We hope the donors' good deed might inspire others to find and bring other lost pieces back home for the restitution of the pavilion."

This year marks the 110th anniversary of the Summer Palace opening to the public, and a series of celebratory activities has been launched alongside the return of the panel.