Jewelers treat soldered objects with dilute acid solutions to remove oxides produced from heating and soldering as well as glassy flux residues. This process is called pickling. I believe the term comes from the chemical jewelers once used to do the job: Alum, such as is used traditionally for making pickles.
Many jewelers today are not aware that they can use alum for this purpose. Here is what I think happened: jewelers used alum and salts to pickle with, then in the 19th century mineral acids became cheaper and more available, which along with the rise in industrialization and the increasing need for speed caused sulfuric acid to become the standard pickling solution. Sulfuric acid however is dangerous to store, handle and mix up. It also wants to concentrate to a 49% solution, which means it evaporates until it reaches this dangerously corrosive proportion.
With the introduction of safety and (maybe litigation) pressures jewelers began to turn to dry salts which dissociate to form constant dilutions of acid. The most common one used by jewelers has various brand names (sometimes called a ‘safety pickle’). It is usually sodium bisulfate, which is used in industry for pickling metals of all kinds, in tanning leather, in toilet bowl cleaners and most importantly for jewelers: to reduce the pH of water in swimming pools or hot tubs.
The most easily available source is ‘Swimming pool acid” or “pH reducer” at your local hardware or grocery store. It is essentially the same material as dry ‘Safety Pickle’ and is mixed in the same manner, perhaps half a cup to 4 liters (1 gallon) of water. This is significantly cheaper than from the jeweler’s supply store. An even larger user is the cleaning industry and great drums of the material are apparently used by industrial cleaners for cleaning toilets. After all these changes people forgot about using alum as a pickle.
Alum has an advantage over ‘Safety Pickle’, it does not appear to outgas as much. Tools placed near a ‘Safety Pickle’ will rust, but tools next to an alum pickle do not. As well, you can place steel tweezers or wire into an alum pickle without causing the copper plating reaction on your work familiar to those jewelers who have done this with a standard pickle.
Some people have taken to using citric acid as a pickle. This is an industrial baking supply material. It takes more, several cups more to make a pickle. It works more slowly than the Sodium bisulfate and rumor has it that if you leave it alone, cold, for a week or two, it can serve as a growth medium for interesting molds. Phil Poirier of Taos feels that the citric acid pickle has the advantage of not adding sulfur compounds (and hence making sulfur dioxide gas when melted) to cast sprue buttons being cleaned for remelting. He says that he has reduced porosity in re-used metals by using citric acid.
In a pinch vinegar and salt make a very good safe home pickle, about one cup vinegar to one teaspoon salt or more. An emergency pickle is to sprinkle salt on a cut lemon and rub it over the metal to be pickled. I’ve even tried ketchup and Cola Drinks with salt for this with fair success.
So what do I use on an everyday basis?
Swimming pool acid does me fine.
A covered crock pot works very well for containing a pickle. One must take care not to splash the acid around and to keep the outsides clean as they tend to corrode if not cared for. A pickle solution may last for months if used with care, replenished with water when necessary, kept clean and so on. In a shared shop I suggest caulking all seams of the crock pot with silicone, caulking the heat control fixed on the low setting and regular wiping down. This makes the pickle solution last longer and prevents boiling the pickle dry accidentally on high.
For pickling long and tall objects use a long and tall container such as a bottle to pickle in. Bench jewelers who solder at their bench and want a small pickle close to hand have been known to use a coffee cup size heating pad to keep a small amount warm in a Pyrex® beaker. I personally prefer to separate chemical activity from my bench workspace and to set up a pickle close to the sink and rinse water.
An alum solution is also used for dissolving out broken drill bits from jewelry pieces. One simmers a concentrated solution of alum and a broken drill bit embedded in a piece turns to brown dust and dissolves out in about twenty minutes. Use a Pyrex® or Corning® type container – not a steel pot for obvious reasons.
If you ever get copper flashing on your metal from steel contamination in a pickle or when working with brass it may be quickly stripped from the metal by taking a scoop full of pickle solution from the pickle pot and mixing it with an equal amount of hydrogen peroxide from the drug store. This strips off copper flashing in less than two minutes. The mixture may be returned to the pickle pot after use as the peroxide breaks down to make water. Avoid using industrial strength peroxide solution as it is too dangerous.