Negative Shadow. Flow marks on tin and tin alloy compositions which form a permanent bright “shadow” in contrast to the remaining surface of a struck piece which has darkened from exposure to cold temperature. Tin has this unusual property; it does not work harden like other coinage compositions. Instead, its surface displacement during striking causes the flow marks to form a permanent bright area, somewhat like bright finish on other metals. Tin, tin alloys, including white metal, will darken in time, called darkening, from a mild condition of tin pest. This darkening is brought on from another property of tin – it is unstable below 18° centigrade (64° Fahrenheit). Prolonged exposure to cold temperatures first causes this dark toning on all surfaces that do not exhibit negative shadow. Continued exposure will cause unstable metallic tin (beta-tin) changing to the stable powdery form (alpha-tin) on the surface. The darkened area is in stark contrast to the bright areas of negative shadow. See tin, white metal.

class 06.5