Mercury is the bad-boy of the periodic table, often called 'quicksilver', it is both mesmerising and toxic as Professor Andrea Sella of University College London vividly explains. In the fourth of our series examining the global economics of chemical elements Justin Rowlatt speaks to Tim Kasten of the United Nations' Environment Programme who is one of the architects of a new international treaty that aims to ban the metal from industrial uses by 2020. As we discover, that ban will affect everything from coal-fired power stations to small-scale gold miners in developing countries, to the illumination of the lowly office. We visit a fluorescent bulb recycling plant outisde Norwich and speak to small scale gold miners in Ghana about how the ban might affect them. But it is all in a good cause, as Justin discovers when he visits one of the finest fishmongers in London.