'Using a piece of slate from the Bwlch quarry, Manod because during World War II, the paintings of the National Gallery collection were hidden in a vast cavern under Manod Mountain. The slate comes from that cavern (technically it is called a "gallery" which tickles me). I know the slate is grey but the cavern is unbelievably black when you're standing in it and your torch goes out. I've always been intrigued by the idea of a totally dark cavern with brilliant, colourful, sensual masterpieces hidden in it. It gives me hope and makes me feel that if I root around in the darkest bits of my brain my fingers might just brush against the edges of a secret masterpiece.'