Definition

• Surface Displacement.  The movement of surface metal mass toward and into die cavities during striking. Generally surface metal flows from the outermost rim inward toward the center of the struck surface to fill the greatest cavity. Sometimes, but not always, striations will form on the surface; these are called flow marks and are always inward from the rim toward the center point like the spokes of a wheel. (Flow marks are the cause of cartwheel effect – an optical illusion of light reflecting off the flow marks.) Metal must fill every die cavity during striking or the struck piece will not be fully struck up (it will look like an uncirculated worn coin).
  Increasing the pressure of the coining press will generally increase the amount of metal displaced until every portion of the die cavity is filled. Metals ranking high in coinability provide this surface displacement readily. Obviously, harder alloys rank lower in the ease of surface displacement than softer alloys. Those low in coinability may not even fill die cavities. Also certain alloys – white metal in particular – have an unusual property of work hardening the area of surface displacement – which remains bright while the remainder of the surface will tone forming a reverse "shadow" of brightness (see white metal).

  excerpted with permission from

  An Encyclopedia of Coin and Medal Technology

  For Artists, Makers, Collectors and Curators

  COMPILED AND WRITTEN BY D. WAYNE JOHNSON

  Roger W. Burdette, Editor

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