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Abrasive Blasting

Abrasive Blasting. The pelting of a metal surface with fine grain particles, as sand, pumice or glass beads, to roughen it either for a matte state, or in preparation for some additional processing. The technique of abrasive blasting gives such a treated surface a "tooth" or roughness – creating microscopic cups that are capable of holding liquids; thus it is ideal as a first step in a finishing, coloring or patina process.

 

The use of abrasive blasting was adopted in the late 19th century by the French, who employed it as a first step in oxidizing and relieving a design, to highlight a medallic surface, a process that is even called French finish. They also created the artificial coloring, by applying a patina to a metal surface. This was the same technique applied to bronze statues, the adoption to medals and medallic items for a fine patina finish and color was a natural progression.

 

French practitioners first used sand and the process was known as sandblasting, In modern times, however, sand has been replaced for the most part by glass beads because the grit size could be smaller, creating a more uniform, and ultimately, a more pleasing appearance. When abrasive blasting is applied to a numismatic surface – as a die or a portion of a die – this creates a matte finish, which drastically reduces the reflectiveness of its surface, or the surface of the piece struck from a die that is so treated.

CLASS 09.1

 

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Source: D. Wayne Johnson Encyclopedia of Coin and Medal Technology
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