Italy's restorers have unveiled the secret hidden in the eyes of King Sevt III's unique bronze mask discovered in Bulgaria, archeologist Georgi Kitov has said.

The sculptors who have worked on the mask probably knew a lot about chemistry too, Kitov was quoted as saying by actualno.com.

Italian restorer Edilberto Formili has discovered that the eyes of the unique Thracian mask were made from a glass paste mixed with alabaster and iron, which produced the brownish tint in Sevt III's look. The bottoms of the eyeballs were painted in red, which was also restored so that they look more natural, Kitov added.

Before the restoration, archaeologists in Bulgaria thought that Sevt III's eyes were made of ivory, but that turned out wrong, Kitov said.

The unique bronze head will be displayed in Italy from February 14 until mid-May. Upon its return, Bulgarian restorers, who have taken up the bronze parts, will complete the renovation.

The work of the Italian masters that took about three months has cost Bulgaria EUR 66,000 according to media reports.

The head is believed to portray King Sevt III, whose vault was found by Bulgarian archaeologists in 2004.

Back then, the oldest and largest Thracian tomb disclosed so far on Bulgarian land emerged from beneath the Golyamata Kosmatka mound.

The archaeological team's head Kitov, called also Bulgaria's "Indiana Jones" explained that the excavations have revealed a 13-metre long passage and two halls walled up with stones behind the facade.

The tomb has amazed archaeologists with its first-of-the-kind doors made of marble and decorated with human figures, iron nail imitations and blue-and-red sculptural ornaments.