Saturday, June 9, 2012

In a pickle. All about pickling solutions and how to dispose of them.



It's Q & A time, and one group of frequently asked questions sent to my email inbox is all about pickle. 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.

What is 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 925 parts fine silver and 75 parts copper), concentrates 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. can be polished as a bright, fine silver "plating" over sterling.

Okay, you can't understand pickle without talking about fire scale, but back to pickle.

Here are some of the usual questions (besides what is pickle, which we covered above):

1) What's the best pickle to use?
Well, if you're working at home or in a home studio or if you're concerned about eco-friendly chemicals, then I would suggest vinegar based or citric based pickles. Sounds yummy.

Back in olden times, the first pickle cleaning solutions were made with alum, as in what you use to make, well, pickles. After that, pickle solutions became more aggressive. 10% sulphuric acid and acid substitutes like sodium bisulphate (aka Sparex #2 brand pickle solution for non-ferrous metals like copper, sterling, brass, etc), are your typical studio or shop choice. They're effective, but testy. Acid solutions and Sparex don't like steel tweezers or any steel being put into the pickle pot - it reverses the flow and puts copper back onto your metal. So if you wanted pink, coppery colored sterling, you got it. Taking the steel out of the pickle stops the reaction. Sparex splashed on clothing and surfaces can put pin holes in clothing and bubble floor finishes. It's durable and long lasting, but it also has to be neutralized with baking soda before disposing of it (see disposal of pickle later on in this post).

Instead, you can use home friendly pickle solutions like citric acid or vinegar and salt. Citric acid is a baking product and you can buy it in bulk or you can buy brand name citric acid pickle solutions like Black Magic Biodegradable Pickle. The brand name solutions seem to work better than just using straight citric acid and come with mixing instructions, usually something like a heaping tablespoon or more per cup of water. It definitely takes more citric acid than Sparex to make the same volume of pickle. But, on the plus side, this stuff is all natural, doesn't mind steel at all (no adverse or copper pink reactions to steel) and no neutralizing required. I've stored citric pickle in the pot for more than a month with no mold, etc and I've had students report that they've stored it for a couple of years in a jug and it was still effective. Interesting fact: if you mix some brands of citric acid based pickle you get a strong vinegar vapor that gets worse when heated. We're talking coughing and abused nostrils like a mild tear gas. Not fun. So, don't mix brands. Dump the old solution (see below), clean the pot with soap and water and add the new mix.

Looking for citric acid at the grocery store? Well, look for drink mixes high in ascorbic acid, aka citric acid, like Crystal Lite drink mix. It's refreshing, lemony, and it will strip scale from sterling. Delicious! However, drink mixes are also near opaque and very "warm lemony" smelling, and not very effective, so this is not my preferred natural pickle mix.

Another natural mix is vinegar and salt, one teaspoon of salt per cup of vinegar. Heat it as usual in your clean pickle pot and you have your own homemade pickle. And a lot more home friendly than Sparex or diluted acid.

No pickle and no ingredients? Well, in a pinch you can use almost boiling hot water to remove the flux and even a lemon, sprinkled with a pinch of salt, rubbed over your metal will clean it (especially if you boil off the flux first).

2) Do I have to heat pickle? Can't I use it cold? 
Yes you have to heat pickle. Yes you can use it cold. Wait a minute. What?

Well, yes, pickle is more effective if you heat it up. Don't boil it, just use a crock pot or pickle pot, set on low, to warm the solution up until it steams. When it's hot, your pickle will strip off the flux in a couple of minutes and get to work on firescale soon after. In five - ten minutes it's probably ready for more soldering. Leave it in longer to clean off the most firescale possible.

In fact, and this is where using cold pickle comes into play. You can leave your metal in the pickle for hours, even overnight. Cold pickle takes time to work, but in the shop, we'd leave metal in the pickle overnight to soak, with the pot turned off for safety, and come back to nice clean bits to work on.

Whatever you do, citric or sparex, natural or toxic, don't use a microwave to reheat your pickle once it's been used and has started to absorb copper. Especially if you share that microwave with food. I don't want a burrito that tastes like pennies, no matter how interesting that sounds.

Besides crock pots, other pickle pot solutions include used, no longer for baby, bottle warmers or mug warmers. And remember, once you use a crock pot for jewelry, it's not for chocolate fondue or slow cooking anymore - unless you're trying to make yourself a "last supper" you'll never forget.

3) When is my pickle done and used up?
When it doesn't work anymore. Well, duh. But you can tell it's done when it turns bright blue from all the copper it has absorbed. As it gets more and more blue, pay attention to how long it takes to pickle your metal. As you get more experience with pickling, your "pickle senses" will tingle with impatience when it's taking too long for something to clean. That's a sign that it's done. Just don't mistake that deadline you procrastinated on and the rapidly approaching arrival of your customer with bad or defunct pickle. Also, if the pickle gets contaminated and your metal is coming out gooey or coated with annoying substances, it's time for a new batch.

4) How do I dispose of pickle?
For Sparex or dilute acid pickles, you have to neutralize your pickle before you can dispose of it. Time for fun with science!

To neutralize an acidic solution (pickle) you add a base (like baking soda). Before you dump some baking soda in your pickle pot, check this out: adding a base to acid increases the volume of material rapidly, like a volcano experiment gone wrong from science camp. So, cool off the pickle and dump it in a larger container, like a bucket. Add baking soda until it stops fizzing and foaming. Now it's done.

For citric acid or natural pickles, no neutralizing is necessary. Just let it cool and you're ready for disposal.

But before you dump any pickle solution down the drain - stop! Natural or neutralized, green or blue used pickle is full of copper and local cities often have rules about dumping copper into the water supply. They don't like it. You have to dispose of it properly, like taking it in a labeled container to your hazardous waste dump and turning it in. They may laugh at you, but in your heart and behind those tears you'll know that you did the right thing. The geeky thing, but the right thing.

Off the record, and as non-confidential as any public blog post can be, there are other ways that jewelers have been known to use for disposing of pickle. As long as the pickle is neutralized or natural, some people dump it into kitty litter, which absorbs the liquid and turns the pickle into a solid (lumpy and nougat-like, but solid). I can't imagine that pickle laden litter is anymore toxic than the ammonia scented wonder-pee of rex feline domesticus, but then again, I'm a dog man. I actually stoop down, bag my dog's poo and dump it in the trash can everyday. Now, that's nasty. And they say I'm the master and he's the pet.

Another unusual, off the record solution for natural pickles that are full of copper is to use them to change the color of plants. Huh? Yup, some gardeners/jewelers will use a little copper-full pickle to amend the soil around, say, the hydrangeas, giving them a lovely new shade for the next bloom. I wouldn't try it with used Sparex unless you really, really hate that plant.

There you have it. If you want more, more, more information about the wonderful world of soldering, grab a copy of my book and/or dvd, Soldering Made Simple: Easy Techniques for Kitchen Table Jewelers, or sign up for a class with us at the Silvera Jewelry School in Berkeley, CA.

Have some tips about pickle - the jewelry kind, not the edible kind? Then please share it here. And thanks!


10 comments:

truthbetold said...

great article! thank you. So how about a tip on getting the copper off a goldfilled soldered ring. Super pickle (hydrogen peroxide) is a bad Idea because your solder will turn silver. What kind of polishing method is the best without grinding away too much of the metal?m I used a dremmel with a polishing tip and ZAM but the gold still looks a bit pink.

Joe Silvera said...

Hi - the best way to remove firescale from filled metals is to prevent it in the first place. Try using firescoff or Cupronil to eliminate or minimize any scale. By protecting the whole piece, the extra time spent fluxing will be recovered 10 fold in time saved during pickling and polishing.

Are you using low karat gold solder ( I use 10, 12 or 14 karat) or yellow tinted sterling solder for your gold filled? These can also help you avoid the dreaded solder line.

I don't know of any magic bullet for removing scale on filled metals other than polishing - always risky, but if you protect with scale retardant flux, avoid overheating, and use gold or tinted solder your gold-filled should be a lot more seamless looking and happy.

Happy soldering!

Ran said...

Thanks for great information you write it very clean. I am very lucky to get this tips from you.

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Woojoo said...

Ohmigaw! This is great! Exactly the info I needed. Thanks so much.

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Heather Anne Walter said...

Just set up a salt and vinegar pickle pot. It works very well, I am quite pleased. Thank you so much for the great info.

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Jesse Oberes said...

Thanks so much for your article. It's very informative and I hope you don't mind me sharing a link to it from my website.

kp mohan said...

I am manufacturer of small sculpture in India using the process vacuum investment casting Many times my metal % is 65 of silver and remaining copper After casting it is impossible to remove the fire scale in H2so4 pickling solution and many times heating them and using burnishing process it is also difficult to remove the fire scale and even it is economically costly process can anyone suggest me the right process for my job. I heard that theere is electro striping process if so what is the bath contained ? Prakash