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<p>[QUOTE=&quot;Tom Maringer, post: 184106, member: 7033&quot;]Oxidation on iron is usually called rust. It is NOT considered desirable because the iron oxide is generally not strongly adherent... that is... it flakes off easily and therefore craters out the surface... bad. But the good thing is that pure iron rusts much less than steel does! The idea is to &quot;blue&quot; the iron. I either heat-blue the coins in a pan on the stove (which gives a deep purple-black) or else take them to a gunsmith to be hot-blued chemically (which gives a flat-black). Either way, I then do a coat of paste-wax on the coin to help prevent rust. Even so... sometimes it happens.</p><p><br /></p><p>Anodizing is another form of oxidation! In the cast of the reactive transition metals (Ti, Ta, Nb, Hf) the oxide skin is both adherent and transparent. The most common method is to form an oxide layer by electrolytic means, similar to the process used in electroplating. The work to be colored is attached to the positive connection of a power supply (the anode), and usually another piece of titanium is connected to the negative side (the cathode) of the supply. Both are submerged in a mildly conductive solution, such as phosphoric acid (cola soft drinks), TSP - TriSodium Phosphate (dishwasher detergent). When power is applied to the contacts, a uniform layer of titanium oxide forms on the anode.&nbsp; As the voltage increases, the thickness of the layer also increases. Certain colors will appear at specific voltage levels. The &quot;change&quot; from one color to another is not sharply defined, but rather shades gradually through a limited spectrum.</p><p><br /></p><p>The colors are interference colors... like the colors you see in an oil-slick. The apparent color imparted to the metal is caused by interference between certain wavelengths of light reflecting off the metal and oxide coated surface. Light passing through the oxide layer, then reflecting off of the metal, must travel farther than light reflecting directly off the surface of the oxide. If one wave pattern is out of synch with the other, they will cancel each other out, making that particular color &quot;darker&quot; or not visible at all. If the thickness is such that a specific wavelength of light following one path closely synchronizes with that of the other path, then the wave strength (amplitude) will be increased, and that particular color would appear brighter. When the wave patterns cancel each other, it is called destructive interference, and when they match, it is constructive interference. It is possible that the thickness will create a combination of effects at the same time. The thickness of the coating is controlled by the voltage settings in the anodizing process. </p><p><br /></p><p>Each metal or alloy has its own specific color range at different voltages... so you need to calibrate for the target color for each different metal. But the results are generally repeatable if other factors are kept constant. The colors on the Austrian niobium coins should be quite durable... but wear can reduce the thickness of the oxide layer, and thus alter the color. You can actually use very fine polishing compound and blend colors by careful rubbing![/QUOTE]</p><p><br /></p>

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