How do you restore a vandalised masterpiece?
13 May 2014 Last updated at 17:48 BST
Mark Rothko's painting Black on Maroon has gone back on public display at London's Tate Modern gallery, 18 months after it was vandalised with graffiti.
The 1958 painting was defaced by Wlodzimierz Umaniec in October 2012. He was sent to prison as a result but has now apologised for his actions.
Rachel Barker was one of the conservators who have spent 18 months restoring the painting - she demonstrated how the painstaking process involved creating replicas of the painting to test different solutions for restoring the damaged area.