Die Erosion. Wearing of a die's surface until flow marks are entirely eliminated from pieces struck from that die. The term is one of condition used by collectors studying pieces struck from dies rather than by metalworkers using the dies. Flow marks form on the surface of coins struck from a fresh die. Tiny furrows form on the surface, generally from the edge toward the center, caused by minute imperfections on the surface of the die. These furrows reflect light causing mint luster on freshly struck pieces (also cartwheel reflections when the coin is tilted). With continued striking these imperfections slowly disappear and the die's surface becomes smooth and bright. Continued use of the die causes very minute rounding of edges of detail and lettering. After long- term use a die may sink ever so slightly, beginning in the center and cause a very slight dome on struck pieces. The final stages of a die is when stress has caused a diebreak on the surface, usually near the rim. When the piece inevitably breaks away the die is no longer useable. See DIEBREAK.