Here's another entry from Dick Johnson's Encyclopedia of Coin and Medal Terminology.
Flow marks on a silver denarius of Antoninus Pius, from Forum Ancient Coins.
Flow Marks and Halos on Roman Coins.
Minute striations or corrugations on the surface of a freshly struck coin or medal. Flow marks are caused by the surface displacement of metal as the piece is struck, generally forming a microscopic pattern of ridges and channels – appearing to the naked eye as radial lines – which tend to form from the center outward toward the rim. They are best viewed in the field where no relief is present.
Flow marks on a struck piece are the cause of luster (also called mint luster) by reflecting light in a distinctive way; it is also the cause of cartwheel effect. The slightest wear or handling will diminish or erase flow marks (the minute channels fill with dirt, grease, human perspiration, oils, other contaminates or fluids the piece has come in contact with). This reduces the surface metal's unusual reflective quality, tarnish and wear begins.
Flow marks disappear rapidly as coins circulate or are handled; because of this they are an excellent criteria to distinguish true uncirculated pieces. (Collectors call the existence of flow marks a diagnostic of mint state where the piece has never been mishandled or circulated.)
In addition to wear or handling where contaminates fill the channels, any type of processing – polishing or whizzing – will remove flow marks. There is no known method of artificially returning flow marks once they have disappeared. They are very fragile, and once gone, cannot be restored.
Flow marks can be seen under low power magnification (3x to 10x) but are even more obvious under greater magnifications. Flow marks do not appear on proof coins because of the smooth polished surfaces, of both dies and planchets. Likewise in medallic work, any flow marks are removed by the finishing process. Thus flow marks are characteristic of uncirculated coins only.
White metal flow marks. An unusual situation exists for certain white metal alloys with high tin content. Flow marks on such pieces, where the metal stress is the greatest, will not tarnish but remain bright forever. The surface of certain white metal medals exhibit this phenomenon; the remainder of the piece will tone or tarnish (actually turn dark gray to black), but the flow mark high stress areas will remain bright. Thus the bright area is a reverse bright "shadow" in contrast to the naturally dark toned surface. See white metal.
To read the complete entry on the Newman Numismatic Portal, see:
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