VOCABULARY TERM: ORANGE PEEL

Orange Peel. A surface effect on coins that resembles a shingled roof, or pebbled or rippled look, or the skin of an orange (although at a smaller scale). This occurs in an area that should be smooth in appearance. In extreme cases, it can be seen on frosted or wire brush field areas but mostly appears on cameo type proof coin finishes. It has been observed on nineteenth century proof coins as well as ones of later issue and modern proofs.

Orange peel describes the macro-scale breakup in the field of a coin that is generally due to excessive grain size. It also refers to grain boundary slippage in the field that is visible with 3X power near the rim (border). Appearance is irregular where it should be smooth and mirror like.

Some opinions vary slightly but most mint professionals agree that it is due to the grain size and metallurgical characteristics of planchets. Metal grain size and orientation is affected by annealing or other heat treatments applied to planchets. Orange peel is also observed in non-coinage situations as described in a 1981 article Grain Size of Gold and Gold Alloys.

The classic manifestation of coarse grain size in gold alloys is the orange peel effect. This is characterized by the development during deformation of the material of a surface appearance similar to that of an orange skin. It results from a shifting of individual coarse grains relative to one another, in extreme cases to such an extent that the grain boundaries become apparent to the naked eye.

Sources:
CLASS 08.9

Ott, Dieter and Christoph J. Raub. Grain Size of Gold and Gold Alloys, Gold Bulletin, June 1981, pp.74. [Forschungsinstitut für Edelmetalle und Metallchemie, Schwäbisch Gmünd, Germany] 5195-(006)01

To read the complete entry on the Newman Numismatic Portal, see:
Orange Peel (https://nnp.wustl.edu/library/dictionarydetail/520149)

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