Generally we pickle our metals for two reasons, 1)to dissolve the
added flux/solder paste or fire coat and 2)to dissolve any surface
oxides, generally copper oxides, which form as a result of heating the
metal. Most substances burn (oxidize) in the presence of air
(oxygen), when they do, they change color. Precious metals due this
slowly or not at all (gold, platinum metal group). Alloys of gold
(less than 24K) or silver (sterling or .925 or lower) are frequently
alloyed with copper. Pure copper is a fairly stable metal, but we see
polished unprotected copper tarnish fairly quickly (copper pots,
copper wire and so on). Heat, which speeds up most chemical processes
does the same to copper oxidation - we solder it and get that red or
black oxide formation.
To ‘get rid’ of this scale (oxidation) we dissolve it with and acid.
The stronger the acid, the faster the action. Almost any acid will
work as long as the result (copper salt) is soluble. It turns out
that sulfuric acid (H2SO4) is an excellent choice. Copper oxide is
attacked by sulfuric acid and the resulting salt, copper sulfate is
very soluble in water (this is why the used pickle turns blue). Now
there are a few down sides to sulfuric acid. Concentrated sulfuric
loves water and will pull it out of most anything. Organic materials
are generally composed of molecules that have plenty of hydrogen and
oxygen (components of water) and carbon. If you have ever seen what
happens to sugar and concentrated H2SO4 (turning it carbon black), you
know what I mean. More dilute solutions are still very active, a
spill of a little battery acid (H2SO4) will eat holes in the cloth.
It is a liquid so it spills and is a problem to transport.
There are other strong inorganic acids like nitric acid (great to
dissolve copper, but it also dissolves silver), or weaker inorganic
acids (like citric acid - slow working even hot). There are also weak
inorganic or mineral acids like sulfurous acid (H2SO3).
What to do? It turns out there is a half acid, half salt of sulfuric
acid that is a powder and works almost as fast as sulfuric acid. This
is the acid salt or bisulfate. The most common one is sodium bisulfate
(NaHSO4). This powder, when dissolved, disassociates or ionizes in
water and acts very much like a toned down version of sulfuric acid.
It is the main ingredient in Sparex and is used for a variety of other
purposes, including solid swimming pool acid used to adjust the pH (or
acidity/alkalinity) of pools and hot tubs. I buy it 1n 20 pound bags
for about the same price as a small jar or Sparex.
Your question was about Sodium Metabisulphite (Na2S2O5). While this
is not an acid salt, when dissolved in water it acts a bit like a weak
version of sulfurous acid (NOT sulfuric). In addition to being a weak
acid it is a reducing agent. It is used in food preparation for
things like keeping cut apples from turning brown (oxidizing). It
might work -fairly slow, but would be safer than some other materials.
Sodium bisulfite (NaHSO3) is also a possibility. One problem you
could find with a piece is that since the sulfite is a reducing agent,
you may reduce some of the copper oxides to copper leaving a pink
blush on the metal.
There are some down sides to any pickle as well. They may attack the
grain structure on the alloy, they will quickly attack alloy
components like zinc. If you pickle brass, the surface zinc is
dissolved leaving a top coat of copper - great fun for patinas.