The golden funerary mask, seen Friday by AFP at the Egyptian Museum, showed the sticky aftermath of what appears to have been overzealous use of glue to fix the mask's beard in place.
A museum official, who spoke anonymously to avoid repercussions, told AFP the beard had fallen of accidentally when the mask was removed from its case last year to repair the lighting.
Museum head Mahmoud al-Helwagy denied that conservation workers had damaged the mask
"This is illogical and inconceivable," he told AFP. "These are conservation workers, not carpenters."
Antiquities Minister Mahmud al-Damaty also denied that the 3,000-year-old relic was treated carelessly.
"The job was done correctly," he told AFP, without explaining why curators needed to fix the mask.
Monica Hanna, an Egyptologist who inspected the mask, said what she saw had so shocked her that her group was taking the matter to the public prosecutor.
"We are presenting a complaint on mismanagement to the prosecutor tomorrow," said Hanna, from Egypt's Heritage Task Force, which has long battled mismanagement and looting of Egypt's legendary ancient artefacts.
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According to the museum official, "there seems to have been a lapse in concentration and the mask hit the case and almost fell" when it was removed from its case.
"So (the curator) grabbed it in his arms to break the fall, and the beard separated," he said.
The long braided beard fit into the mask with a peg, and had been separated before, the official said.
"This mistake can happen. But what caused it to get worse? The curator was scared and he fixed it hastily."
The epoxy glue dried very quickly, said the official.
"You should use material (that dries slowly) and then support it, maybe over several hours or 24 hours, so you can fix mistakes," he said.
"Renovation work needs an adhesive that is easy to remove in case there is any damage, without leaving any traces."
Museum director Helwagy told the official MENA news agency that epoxy glue is used internationally to fix artefacts.
The death mask of the enigmatic boy king is one of the crown jewels of the museum, which also houses the mummy of Pharaoh Ramses II.
The museum used to attract millions of tourists before a 2011 revolt -- centred in nearby Tahrir Square -- brought down president Hosni Mubarak and unleashed four years of tumult.