Climate change forces facelift for Michelangelo masterpiece

The restoration of the 16th century masterpiece in the Vatican is aimed at bringing back to light the vibrant colours of the work, which measures nearly 14 metres (46 feet) high.

Visitors will still be able to access the Sistine Chapel during the restoration but the fresco is obscured by a large scaffolding covered with a reproduction of the work.

Vatican Museums director Barbara Jatta described the veil being removed as "a bit like a cataract", during a press tour on Saturday of the project, due to be completed before Easter.

The Vatican said the substance was "invisible to the naked eye".

The fresco is being dabbed with distilled water through a layer of Japanese paper to remove the sub substance, identified as calcium lactate.

"Transpiration has increased in recent years because of climate change. Due to transpiration we produce lactic acid... which becomes calcium lactate," Fabio Morresi, head of scientific research at the Vatican Museums, told reporters.

Vatican Museums staff said measures had already been taken to reduce the number of visitors present at any one time in the Sistine Chapel, which is also the place where cardinals meet in closed-door conclaves to elect new popes.

Morresi described the difference in the fresco before and after the treatment as like "two different worlds" and said working on the masterpiece was an "emotional" process.

The Last Judgment, which was completed between 1536 and 1541, is the centrepiece of the Sistine Chapel, located just behind the altar.

Then pope Paul III was said to have been so impressed by the work that he fell on his knees and ask for forgiveness when he first saw it.

It covers an area of 180 square metres and has 391 figures.

The many nude and semi-nude figures in the painting caused scandal and many were covered up with cloths painted over the original following Michelangelo's death.

Some of the painted cloths were removed in 1994 during the last major restoration.

The current project is being sponsored by US donors.

The Sistine Chapel is being restored by section in a major overhaul that started in 2010.

Work is normally carried out when the Sistine Chapel is closed to visitors and without the need for scaffolding but Vatican Museums officials said this was not possible for The Last Judgment because of the scale of the work.

Morresi said the restoration felt personal for him because he was hired in 1988 when the last major facelift of the Sistine Chapel was just starting and he is now nearly at retirement age.

"It's marvellous... There's a piece of me in here," he said.