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annealing brass
Could someone help me out, please. I'm trying to anneal brass but it's
turning copper color in the pickle. I put a piece of silver wire in to see if the pickle is bad and plating it with copper. No changes to the silver yet. Most of what I've read about annealing just says to heat until it is dull red and I've done this with silver. Is there a different procedure for brass and other metals? Thanks, CR |
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#2
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On Wed, 06 Jul 2005 21:59:05 -0700, in @õ "C Ryman" t wrote:
Could someone help me out, please. I'm trying to anneal brass but it's turning copper color in the pickle. I put a piece of silver wire in to see if the pickle is bad and plating it with copper. No changes to the silver yet. Most of what I've read about annealing just says to heat until it is dull red and I've done this with silver. Is there a different procedure for brass and other metals? Thanks, CR Connie What is happening is actually normal. Nothing wrong with your pickle other than it's working as it should, and you should not expect any odd behavior with silver or other metals. What is happening is different from when silver gets copper plated in the pickle (more on that in a moment) When you anneal the brass, you are oxidizing both the copper in the brass, and the much more volatile zinc. The zinc is more deeply affected, and after both oxides are removed in the pickle (plus whatever portion of the zinc was removed during annealing just by vaporization), what you get is that the zinc is oxidized more than the copper, so after oxides are removed, the remaining surface has more copper and less zinc than the original brass alloy, and thus appears copper. This would happen even with brand new freshly mixed pickle. The copper color you see is the actual color of the metal after it's been depleted of it's surface zinc. Ways to prevent it are few, since they amount to needing to prevent the surface zinc from being depleted. To start with, you don't need to heat the brass to an actual glow. It needs to get to somewhere around 850F (like silver and many gold alloys too, I might add, which also don't need to actually glow) You can help things by covering the metal with a good flux. By good, I don't mean the very active types that will help solders flow, but rather, long lasting types that will continue to cover and protect the metal as you heat it. Batterns or Prips flux are two that come to mind. You've heated the brass hot enough when the fluxes melt and glaze over. Depending on the brass, there may still be some depletion of surface zince, but it should be considerably less than if you've annealed by heating in just air till it glowed. Next, try a useful method for restoring the brass color to the metal if it DOES depletion gild. Use freshly mixed pickle, and add a couple good "glugs" of hydrogen peroxide. Best is the concentrated (and therefor somewhat dangerous if used carelessly, as it can burn skin) forms found in beauty and hair supply stores. This mix is not stable over time, and within a day or two, the hydrogen peroxide in the mix will have become inactivated, so then you've just got normal pickle again. But when first mixed, it's much more aggresive than normal pickle, and will nicely remove that surface copper coating, giving you a good brass color again. Don't leave the work in this longer than needed, as the brass will continue to be slightly etched. Silver is unaffected, but the same mix is a fine way to remove copper flashing from silver, if you've had a pickling problem with silver. Next, back to the silver thing, and your experiment with it, as well as problems when silver plates out with copper. Understand that this occurs not from an excess of copper dissolved in the pickle (though it cannot happen until there is at least some copper dissolved in it). And it's not "bad" pickle. Silver gets it's undesired copper coating when something is done that sets up what amounts to an electrochemical battery cell, with the silver as one electrode and the pickle as the electrolyte, just like in a car battery. This will occur when electrical contact is made between silver, immersed in the pickle, and any metal which is more electrochemically reactive than copper. The most common culprit is iron, occuring when you hold silver in the pickle with steel soldering tweezers, or have iron binding wire or some steel tumbling shot, or the like, stuck to the silver. When that happens, some of the iron starts to dissolve in the pickle, and in so doing, becomes ionized. Because the iron is in electrical contact with the silver, the silver conducts the resuling current and copper in the solution gets electroplated back out of the solution, as the iron dissolves. The iron replaces the copper in the electrical equation. If the iron and silver were not in contact, the whole thing would have no effect on the silver. It's the slight voltage set up by the contact between dissimilar metals and the dissolution of the iron, that is to blame. While iron is the most well known culprit, there can be others. In fact, brass and bronze will do it, since the zinc or tin in these alloys are also more reactive than copper. If the brass or bronze is fully copper colored, such as after your annealing, then nothing happens. But if the brass is still bright brass color, then zinc is available at the surface to the solution, and it can slowly and slightly dissolve. In so doing, if silver is in contact with it, there can be some plating out of the copper. Generally the effect is much less obvious and strong than with iron, in part because there is not much zinc available, so not much can dissolve, so not much copper gets forced out. Commonly, the copper deposits on the silver mostly close to the junction or contact between the silver and brass. For this type of situation where copper plates out of solution, whether due to iron or zinc or other contamination, it does NOT indicate bad pickle or contaminated pickle, and when this happens, you do not need to replace the pickle. Just remove the contaminant (the iron). Any iron already dissolved in the pickle no longer has any effect, and the plating will stop when the contaminant metal is removed, or even when it simply no longer can make an electrical circuit with the silver. Eventually, with enough contamination by both dissolved copper and dissolved iron or zinc or whatever else has ended up in there, the pickle may end up being a weird color, perhaps somewhat greenish, rather than the straight light blue from only copper, after pickling lots of sterling silver. When it gets like that, it may be best to replace the pickle, because eventually the chemistry gets complex, and subtle situations can end up occuring where you end up discoloring your silver or gold from not obvious interactiongs going on. When you pickle gets to this stage however, you'll definatly know it, since it also won't be working as well to remove flux and fire scale from your silver. Hope that helps. Peter Rowe |
#3
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C Ryman wrote:
Could someone help me out, please. I'm trying to anneal brass but it's turning copper color in the pickle. I put a piece of silver wire in to see if the pickle is bad and plating it with copper. No changes to the silver yet. Most of what I've read about annealing just says to heat until it is dull red and I've done this with silver. Is there a different procedure for brass and other metals? Thanks, CR You're doing nothing wrong. What's happening is perfectly normal. -- Abrasha http://www.abrasha.com |
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