Unlike the other works that hang on the walls, the portrait is locked in a separate display window.

"It is the portrait's debut in China and probably the last time (it will be exhibited)," says Sanfo.

Painted in tempera grassa on a wooden panel, the 60cm-by-40cm portrait depicts a man in three-quarter view, with high nose, long beard and wearing a dark hat. On the hat there is a small curled white feather. A crack and several areas of abrasion to the surface of the painting can easily be seen.

"Analysis has shown the pigment used for the feather is different from the pigment used in other parts of the painting," says Sanfo.

Sanfo says the feather will be removed after the painting returns to Italy. Experts will meet in Paris in a few months to decide whether other damage like the abrasion shall be repaired. Three areas on the back of the panel marked with the numbers "1", "2" and "3" indicate wood materials have been taken from the three places for analysis.

People can also see the reversed inscription "PINXIT MEA" on the back, which according to Sanfo is proof the portrait was created by Leonardo da Vinci.

Zeng Yi, who planned the exhibition, says the portrait is of immeasurable value and the security at Shandong Provincial Museum, where the exhibition is held, is unprecedented.

On Oct 15, the first day of the exhibition, 15,000 people flocked to the exhibition, forcing the museum to control the inflow of people.

"It's a rare opportunity to appreciate classic European works ranging from the Renaissance to the present. From the works you can see the spirit reflected in the Renaissance is compatible with the spirit we are pursuing now," says Pan Lusheng, a member of China National Academy of Painting.

9 am-5 pm, until Oct 30, daily except Monday. Shandong Museum, 11899 Jingshi Donglu, Jinan, Shandong province. Free admission.