Between them, the members of the CRTC produce tiles in more than 50 colours which will last a lifetime. In and around Kent and Sussex, Wealden clay gives tiles a warm, deep, reddish hue that has become a feature of the local built landscape Variations in colour are obtained by controlling the kiln atmosphere whereby the iron content occurring naturally in clay undergoes a chemical change during firing, producing a wide spectrum of colours. On the East Side of Edinburgh and down through Eastern England as far as Suffolk, clay pantiles occur. These are generally made from the alluvial clays found around the Humber Estuary. Variations in colour abound in East Anglia, from the light buff, deep oranges and browns of the Cambridge Mix to pockets of the Norfolk countryside which are characterised by almost back tiles. This unusual colouring is the result of ‘smut’, a black glaze used on tiles in the area. In and around London, the city atmosphere with its pollution and smoke has resulted in roofs becoming covered in grime, which means that roofs which started out being the characteristic red and orange of clay roof tiles now look black and red. The main source of clay in the Midlands is Etruria Marl in the Staffordshire area where production methods have influenced the characteristic Blue and Brindled tiles. Blue tiles are produced by close control of the kiln atmosphere upon iron-rich Etruria Marl clay.