It's semi-final week in Stoke-on-Trent and just five potters remain, determined to secure their place in the final. For their main make, the potters face their most technically demanding challenge yet - creating a decorative chandelier in bone china. This delicate and translucent clay was first produced in Britain at the end of the 18th century as a cheap alternative to Chinese porcelain. The potters must pour liquid clay into plaster moulds to make the pieces for their chandeliers in a process called slip casting. For their spot test, steady hands are required when judges Kate Malone and Keith Brymer Jones test the potters' banding skills. They must paint consistent and fine lines onto plates which are revolving on the wheel. And for the throw down, they have just 15 minutes on the wheel to make the largest closed sphere they can, a hollow ball of clay which could collapse at any moment.