The firescale layer deepens with prolonged heating, over heating or repeated heating.
““Fire scale or fire stain is a reddish purple toned ‘bloom’ or ‘stain’ that appears on silver/copper alloys such as sterling silver when they are heated in the presence of oxygen. Oxidation (the copper and oxygen type – there are other definitions of oxidation) is the interaction of the copper with atmospheric oxygen and heat.
Oxidation (of copper) w/o flux, Oxidation with flux Last updated: 06/21/17, 05/01/17 Some of my links are to Amazon and their affiliate program.
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In the case of fresh fruit, the skin usually provides a barrier against oxidation.
This is why most fruits and vegetables arrive in good condition at the grocery store.
The result is a form of rust we see as brownish spots or blemishes.
When it involves oxygen, the process of oxidation depends on the amount of oxygen present in the air and the nature of the material it touches.
True oxidation happens on a molecular level — we only see the large-scale effects as the oxygen causes free radicals on the surface to break away.
When sterling silver is heated in the air, the oxygen in the flame and in the atmosphere attacks some of the copper alloy near the surface.
This forms a copper oxide on the surface of the metal.
Technically, however, with the discovery of electrons, oxidation came to be more precisely defined as the loss of at least one electron when two or more substances interact. (Incidentally, the opposite of oxidation is reduction — the addition of at least one electron when substances come into contact with each other.) Sometimes oxidation is not such a bad thing, as in the formation of super-durable anodized aluminum.