Here's another entry from Dick Johnson's Encyclopedia of Coin and Medal Terminology. Interesting property of tin!
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.
To read the complete entry on the Newman Numismatic Portal, see:
Wayne Homren, Editor
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