A gallery staff member paints a wall near an installation entitled "Four Ways" by Richard Long at the Lisson Gallery in London May 22, 2014. Long's solo exhibition opens tomorrow and runs through July 12, 2014. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

Gallery technicians assemble a sculpture made from stainless steel bicycles entitled "Forever" by Ai Weiwei at the Lisson Gallery in London May 22, 2014. Ai's solo exhibition runs from May 23-July 19. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

Pedestrians walk past an installation entitled "Four Ways" by Richard Long at the Lisson Gallery in London May 22, 2014. Long's solo exhibition opens tomorrow and runs through July 12, 2014. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

A gallery technician assembles a sculpture made from stainless steel bicycles entitled "Forever" by Ai Weiwei at the Lisson Gallery in London May 22, 2014. Ai's solo exhibition runs from May 23-July 19. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

A visitor photographs "Forever" by Ai Weiwei at the Lisson Gallery in London May 22, 2014. Ai's solo exhibition runs from May 23-July 19. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

"Marble Couch" by Ai Weiwei is displayed in a courtyard at the Lisson Gallery in London May 22, 2014. Ai's solo exhibition runs from May 23-July 19. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

A pedestrian walks past a bicycle sent from Chinese artist Ai Weiwei locked outside the Lisson Gallery in London May 22, 2014. The bicycle identical to one outside Ai Weiwei's home and studio has been shipped from Beijing and installed in front of the gallery. The artist has been placing flowers in a bicycle's basket outside his home in Beijing daily since November last year and will continue to do so until the authorities return his passport. In solidarity with Ai, who remains unable to leave China, the London gallery will place fresh flowers in its basket each morning during Ai's solo exhibition which runs from May 23-July 19. REUTERS/Suzanne Plunkett (BRITAIN - Tags: ENTERTAINMENT) 

Japanese artist Tatsuo Miyajima poses for a picture inside his installation titled "Life Palace (Tea Room)" at the Lisson Gallery in London September 26, 2013. Miyajima's show "I-Model", consisting of numerical LED displays will be on show until November 2. REUTERS/Andrew Winning (BRITAIN) 

Chinese artist Liu Xiaodong poses for a portrait in front of his painting titled "Green Pub" at the Lisson Gallery in London September 26, 2013. "Green Pub" is part of Xiaodong's new body of work stemming from his invitation to work in Britain from the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: ENTERTAINMENT) 

Chinese artist Liu Xiaodong poses for a portrait in front of his painting titled "Green Pub" at the Lisson Gallery in London September 26, 2013. "Green Pub" is part of Xiaodong's new body of work stemming from his invitation to work in Britain from the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: ENTERTAINMENT) 

Justin Aubrey, Margaret Finnegan and their dog Mati pose in front of Chinese artist Liu Xiaodong's painting of them, titled "Green Pub" at the Lisson Gallery in London September 26, 2013. "Green Pub" is part of Xiaodong's new body of work stemming from his invitation to work in Britain from the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: ENTERTAINMENT) 

Justin Aubrey, Margaret Finnegan and their dog Mati pose in front of Chinese artist Liu Xiaodong's painting of them, titled "Green Pub" at the Lisson Gallery in London September 26, 2013. "Green Pub" is part of Xiaodong's new body of work stemming from his invitation to work in Britain from the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: ENTERTAINMENT TPX IMAGES OF THE DAY) 

A man takes a photograph next to "Disarm (Sentinel) 2013" by Mexican artist Pedro Reyes at the Lisson Gallery in London March 28, 2013. REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

Mexican artist Pedro Reyes poses with one of his artworks, a bass guitar made out of recycled guns, at the Lisson Gallery in London March 28, 2013 REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

A detail of "Disarm (Magnetic Cello) 2013" by Mexican artist Pedro Reyes is seen at the Lisson Gallery in London March 28, 2013. REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

A visitor looks at artworks made from recycled guns by Mexican artist Pedro Reyes at the Lisson Gallery in London March 28, 2013 REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

Mexican artist Pedro Reyes poses with a bass guitar made out of recycled guns, next to an artwork called "Disarm (Harpanet) 2013" at the Lisson Gallery in London March 28, 2013 REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

Mexican artist Pedro Reyes poses with a bass guitar made out of recycled guns, next to an artwork called "Disarm (Harpanet) 2013" at the Lisson Gallery in London March 28, 2013 REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

Mexican artist Pedro Reyes poses with one of his artworks, a bass guitar made out of recycled guns, at the Lisson Gallery in London March 28, 2013 REUTERS/Luke MacGregor (BRITAIN - Tags: ENTERTAINMENT SOCIETY PORTRAIT CONFLICT) 

Adam Rattigan, a student from Walsall, England, walks past "Daniel" from 2012 by Julian Opie at the Lisson Gallery installation at the Frieze Art Fair in London October 11, 2012. REUTERS/Suzanne Plunkett (BRITAIN - Tags: SOCIETY) 

Artist Anish Kapoor poses next to his piece "Intersection 2012" which forms part of his new solo show at the Lisson Gallery in London, October 9, 2012. The show of new work by Kapoor marks the 30th year of his working relationship with the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: SOCIETY) 

Artist Anish Kapoor poses next to his piece "Intersection 2012" which forms part of his new solo show at the Lisson Gallery in London, October 9, 2012. The show of new work by Kapoor marks the 30th year of his working relationship with the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: SOCIETY) 

Artist Anish Kapoor poses next to his piece "Two Blues 2012" which forms part of his new solo show at the Lisson Gallery in London, October 9, 2012. The show of new work by Kapoor marks the 30th year of his working relationship with the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: SOCIETY) 

Artist Anish Kapoor laughs as he poses next to his pieces "Hollow 2012" (R) and "Two Blues 2012" which form part of his new solo show at the Lisson Gallery in London, October 9, 2012. The show of new work by Kapoor marks the 30th year of his working relationship with the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: SOCIETY) 

Artist Anish Kapoor poses next to his piece "Hollow 2012" which forms part of his new solo show at the Lisson Gallery in London, October 9, 2012. The show of new work by Kapoor marks the 30th year of his working relationship with the Lisson Gallery. REUTERS/Andrew Winning (BRITAIN - Tags: SOCIETY) 

A worker attaches a banner showing Chinese artist Ai Weiwei, to the Lisson Gallery in London May 11, 2011. The gallery is presenting a major exhibition of Ai Weiwei's work from May 13 to July 16. REUTERS/Andrew Winning (BRITAIN - Tags: ENTERTAINMENT POLITICS SOCIETY) 

A worker attaches a banner showing Chinese artist Ai Weiwei, to the Lisson Gallery in London May 11, 2011. The gallery is presenting a major exhibition of Ai Weiwei's work from May 13 to July 16. REUTERS/Andrew Winning (BRITAIN - Tags: ENTERTAINMENT POLITICS SOCIETY)