Many paintings in the London museum's collection have also been cleaned and restored.
The painting dates from 1846 and was restored in 2010.
The painting was restored in 1951 and again in 1989-92.
In 2005, the painting was restored and rehung at the high school.
After a period of neglect, the painting and building were restored in the 1980s.
The painting previously had been restored in 1915 and 1938.
The painting was restored for the 400th anniversary of the artists death.
The reason, experts said, was that the painting had been badly restored.
The painting was restored, but traces of the cuts still remain.
The dome's paintings were restored at the beginning of the 20th century.