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Saturday, June 9, In a pickle. All about pickling solutions and how to dispose of them. So, since for every 1 student who asks, at least 10 more were wondering about the same thing, I thought I would share some helpful and hopefully not too boring information about that funny cleaning solution we jewelers call pickle.
Pickle is the solution used to clean metal, especially after soldering - to remove baked on flux and oxides that have formed on the metal. The oxides are typically concentrations of copper in the metal or alloy that come to the surface during annealing and soldering.
The pickle strips these oxides, called fire scale from the surface. For example, the copper in sterling an alloy of parts fine silver and 75 parts copperconcentrates on the surface of the metal in a not too handsome black or copper colored stain when it is heated with a torch in an oxygen rich environment, or what we lay people call "air".
Even though soldering or annealing in a vacuum or room filled with inert gas may help to stop fire scale, I'm addicted to breathing oxygen and don't have any immediate plans to stop. First point about pickle that is often confusing to students is that pickle removes fire scale, not fire stain.
Fire scale is the oxide that sits on top of the surface, drawn up and through the metal by the heat and oxygen. Fire stain, is the dark grey or copper colored oxide that is still inside the metal, visible just under the surface.
Fire stain can't be removed by normal pickling and must be sanded off with abrasives. So, when students first use pickle, they're stunned to still see copper stains on their sterling, even after soaking it for a long, long time.
That's the fire stain. All the surface scale has been cleaned away, but the stain inside the surface is still visible and not going anywhere without persuasion, no matter how much the students beg or plead with the teacher. Fire Stain on Sterling Silver Even more confusing, sometimes sterling will come out of the pickle looking matte white like fine silver, but with a slightly clouded blotchy mottling.
This is from a process called depletion gilding. Repeated heating and pickling of your sterling strips away copper oxides fire scale and depletes the sterling at the surface of some of the copper content in the alloy.
Eventually, the sterling will be go from black or copper stained with firescale to matte silver white. This thin fine silver layer, or loam, is a frail mask over the fire stain, which is still there, underneath.
In fact, it is that scale that is making the matte silver color look blotchy and cloudy. As soon as any abrasive polishing is done, the fire stain is revealed and back with a vengeance. Incidentally, depletion gilding metal is used to coat everything from castings to tableware, covering incidental fire stain, and if burnished with tumbling, etc.Top Notch 2, Second Edition Unit 7 Workbook Answer Key UNIT 7 Exercise 1 1.
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