G'day Pam Chott et al.
-Is there any difference in action or safety among the various
"alum" compounds available?
The chemical action of various 'alums' depend upon the dissociation
of the sulphate ion from the rest of the molecule; it is the
sulphate ion which dissolves the copper sulphide or oxide or the
silver sulphide in sterling or low carat golds.
-Would prolonged immersion in alum solution affect the metals
or the solder joint?
Yes; many solders contain zinc to lower the melting or flow
temperature of the solder, and the sulphate ion in the alum would
help dissolve out the zinc in time, causing porosity and lowering
the strength of the solder. !0 minutes immersion in warm alum
solution should be sufficient
-Are there conditions that would produce plating of dissolved
Yes. If the solution of alum has been used for pickling it will
contain a small amount of copper. So if any iron metal enters the
solution some of that copper will be replaced by iron ions and the
copper metal will plate out on the workpiece, causing a pinkish
-Are there metals or stones which should not be placed in alum
Any stone which is affected by acids - pearls, mother-of-pearl,
haematite, copper hydroxy phosphate, etc. And of course, iron.,
but there are others.
-Any situations where acid might be preferred over alum
Depends upon the acid. warm 50% Nitric, for instance will remove
deep firestain plus some of the metal
All the substances used as pickle in jewellery work depend for their
action upon their ability to dissolve copper oxides and sulphides
and silver sulphide. They are to be regarded as dilute acids and
have the same action. I personally use 10% sulphuric acid, simply
because I have plenty, find it efficient, and am well used to
Don't be scared of the word "acid". Just handle them with care,
remembering that the hydrochloric acid content of your own stomach
is strong enough to make holes in cotton clothes. Which is why
certain things cause an excess which may eat holes in the stomach
lining - called ulcers!!
Cheers for now,
John Burgess; @John_Burgess2 of Mapua, Nelson NZ