Alexandre Farto aka Vhils is a Portuguese artist, known for his striking huge murals that have appeared on city walls from Brazil and the US, to Senegal and Vietnam. He uses a bas-relief carving technique, which involves using chisels and even hammer drills to scrape away at the fabric of the wall, revealing the history in the layers below the surface. Abi McNeil talks to Vhils as he works on his latest project – a 31 metre long mural for the Paris headquarters of Unesco.