Some examples of Flemish bond incorporate stretchers of one colour and headers of another. This effect is commonly a product of treating the header face of the heading bricks while the bricks are being baked as part of the manufacturing process. Some of the header faces are exposed to wood smoke, generating a grey-blue colour, while other simply vitrified until they reach a deeper blue colour. Some headers have a glazed face, caused by using salt in the firing. Sometimes Staffordshire Blue bricks are used for the heading bricks.