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Porosity.  A surface full of tiny pores. Sand casting is notorious for creating porous surfaces from the contours of multiple grains of sand forming the wall of the mold. Other porous surfaces are formed by minute voids, tiny blowholes (from escaping gas) and sometimes from shrinkage during cooling. When intended as part of the design such a surface is called stippled and is a form of texture. When unintended the porosity can be so uneven or unsightly the cast piece would be rejected. Actually surface porosity is multiple concavity of thousands of micro cups.

In addition to sand casting and the other causes listed above, porous surface is also created by abrasive blasting (sandblasting), matte dip (tiny acid etched cavities), matte tools, and by edm, electrical discharge machine, which bangs out diminutive explosion craters to form the surface. Porosity is a characteristic of most casting and is a diagnostic for determining some cast versus struck surfaces.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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