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Alum question [was: broken drill bits]


#1
I have used alum for years.  It works well.  It is called
aluminum alum.  It is a powder you can buy at your local drug
store.  <<etc>>

Well, this brings up a question. All references here toalum
over the past weeks seemingly referred to “aluminum alum”. All
I can find in local pharmacies, grocery stores, and cooking
specialty stores is “ammonium alum”. The ammonium alum does
indeed pickle silver and copper-based materials, but does it do
as well? One dictionary referred to alum as “a double sulphate
of aluminum and potassium” or “a class of double sulphates
analagous to the potassium alum”. Good ol’ Sparex pickling
compound is “sodium bisulfate” – note the different spellings
of “sulphate/sulfate”.

Does this imply that all these are related? Are they similar in
the way they behave in solution, particularly with a piece of
metal thrown in?

Can someone with a background in chemistry explain all this so
that I, who last graced a chemestry lab nearly forty years ago,
can possibly grasp the meaning? :wink:

While you are explaining, please explain where the sulfuric acid
comes from with sodium bisulfate, and is there sulfuric acid
produced in an “alum” solution???

Thanks …


#2
  One dictionary referred to alum as "a double sulphate of
aluminum and potassium" or "a class of double sulphates
analagous to the potassium alum".  Good ol' Sparex pickling
compound is "sodium bisulfate" -- note the different spellings
of "sulphate/sulfate". 

G’day; The difference is purely linguistic: ‘sulphate’ is the
spelling taught in Britain and ex-British ‘colonies’ (like New
Zealand and Aussie) Sulfate seems to have come from America so
far as I can see, but may be derived from another language. That
is the only difference. Their chemical formulae both end in the
ion, SO4 (subscript 4) as in H2SO4: sulph(f)uric acid, Na2SO4
disodium sulph(f)ate; NaHSO4, sodium hydrogen sulph(f)ate and
so on. Potash alum is written: K2SO4.Al2(SO4)3.24H20 All very
confusing. But regard it as two chemicals joined together;
potassium sulphate and aluminium sulphate plus 24 molecules of
water.

The combination of an acid and a base make a salt. When a salt
is dissolved in water it breaks up into the negatively charged
acid ion and the positively charged base (or metal) ion and the
two bits go flying off in different directions. But in that
solution, remember there are billions of these ions, and
whenever the positive and negative ones meet up they rejoin, only
to break apart again; a continuous process. Of course, the
business isn’t quite so simple as that (things never are, are
they?) But I’m trying to simplify things not introduce more
confusion.

So in Sparex and in alum solutions, it is the acid sulphate ion
that joins up with other bases that may be around, which cleans
up the discoloured surfaces, like copper oxide in sterling and
low carat golds and it is also the sulphate ion that combines
with and dissolves the iron in broken drills. It doesn’t go for
silver and gold. Alum or Sparex are used by jewellers as a
’pickle’ simply because they are far less aggressive than
sulphuric acid (However, I do use a 10% solution ) and the salts
tend not to provide the aforesaid jewellers with lacy shirts if
they get careless.

Finally, in case you wondered, that blue colour in well used
pickle is of course, copper sulphate. Cor luv a duck, I do go on,
don’t I? But cheers,

        /\      John Burgess
       / /
      / /      Johnb@ts.co.nz
     / /__|\
    (_______)  In sunny temperate Mapua NZ -

Autumn’s here and winter’s close


#3

John, One of the master jewelers I worked with, Ray Grobe,
apprenticed in the late 30’s. He said the duties of all the
apprentices for the first year were to sweep the floors, empty
the trash, pull wire and mix the pickle. The pickle was Sulfuric
acid and water. The way they mixed was REAL interesting. He said
they would pour the acid into the water and stir it with their
finger until it got warm. Ouch!! Regards,
TR the Teacher