Fire scale is caused by oxygen combining with copper present in the silver-copper alloy. A sterling silver alloy contains 925 parts out of a thousand parts silver and 75 parts out of a thousand parts copper.
fire scale - the layer of oxidized metal on the surface of gold or silver. It may extend below the surface. Caused by heating an item during soldering.
Fire scale is caused when you heat silver or gold and the copper alloy mixes with oxygen in the surrounding air. One way to prevent fire scale is to coat the entire piece you are working on with flux.
Fire Scale: Discoloration of copper alloys when heated in air. In the case of sterling silver this discoloration is extremely difficult to remove.
Flex Shaft: ...
Soldering Sterling Silver
To help prevent fire scale and melting,turn off your bench light when you solder. This way you can see more accurately what shade of red your pieces are at and know if you are overheating them.
 See also: Copper, Jewel, Sterling, Polishing, Sterling silver
  
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