Some background: If you take pure sodium (Na) and pure Chlorine
(Cl), and put them together (and duck), you will find a new thing - a
compound - that has it’s own, new properties that are different than
it’s parts. They have “bonded” together. If you take that salt
(which is called “salt”), and some charcoal powder, and blend them
together, you have a “mixture”. The two are blended, but the salt is
still salt, and the charcoal the same, and the two can be
mechanically separated. Subtly different is when you put that salt
into water - the salt is still salt, and the water is still water,
but the salt “ionizes” - NaCl becomes Na(+)Cl(-). That’s a
"solution". An example is if you dissolve silver in Nitric acid, you
get a solution of Silver Nitrate. If you Pour saltwater into it, you
get Silver Chloride which, being insoluble in water, precipitates out
of solution. The NaCl can go INTO the mixture and react. Finally
(I’m gettin’ there) If Na and Cl bond, you have a compound. If
Carbon bonds with Carbon, you have a Crystalline Lattice. So, is an
alloy a compound, a mixture, a solution, or a crystalline structure?
1st question is, What Alloy? Amalgams are mostly mixtures - heat
them up, and they will separate. Steels are compounds (carbon
ferrite), crystaline iron, and a mixture of free carbon and iron.
Most alloys (like brass or sterling) are a new thing called,
amazingly, “alloys”. They are not EXACTLY any of the other
interactions. Thus, if you mix gold with copper and silver, both of
which dissolve in Nitric Acid, you get a thing that is impervious to
nitric, but it is not “NaCuAg” - it is largely the crystal, but it’s
also more than that. That is why, when you mix iron with this black
powder called carbon, you get that mystery called “steel”, and why you
can manipulate it in many ways simply by using heat (controlling the
crystallization). What brings all of this to my mind is couple of
statements that the alloying ingredients can be “leached out” over
time- it simply isn’t so. Nickel dissolves in HCl - put even
powdered white gold into it, even for a year, and it will remain
unchanged. The Nickel is no longer nickel, and the gold is no longer
gold, but it is also not AuNi. It is a new thing, called white gold.
John, I must disagree with you on this. Merriam-Webster defines an
alloy as “a substance composed of two or more metals or of a metal
and a nonmetal intimately united usually by being fused together and
dissolving in each other when molten; also : the state of union of
the components”
Most of what we refer to as alloys are solid solutions.In a solid
solution there are places in the crystal matrix of the main
constituent where an atom of the other metal has substituted for one
of the main constituent atoms. Because atoms vary in size the
substitution causes a distortion in the crystal matrix and
eventually the matrix cannot sustain any more distortion and this is
where it can no longer dissolve any more of the second metal. What
are commonly referred to as alloys can contain solid solutions of
fairly evenly distributed component atoms or may be mixtures of
solid solutions and compounds or mixtures of several different solid
solutions. These distinctions can change due to temperature and
pressure and cooling rate and the presence of other elements.
For example a sterling silver alloy that is composed of silver and
copper can be examined under a microscope and two phases can be
found. One the alpha phase is a solid solution of silver and about
2.8% copper. The second phase is typically the eutectic alloy of
silver and copper that is about 28% silver to 72% copper that
surrounds the alpha phase because it has a lower melting temperature
than the alpha phase and solidifies after the alpha phase. These are
two separate solid solutions of silver and copper that exist as
separate crystals in a mixture at room temperature. It is possible
to heat the mixture of solutions to the point where only a single
solid solution exists because of the change in solubility of copper
in silver with an increase in temperature. This is like mixing sugar
in water. At room temperature up to a certain ratio sugar will
dissolve in the water but once you exceed the solubility limit the
sugar will no longer dissolve and sits at the bottom of the glass.
If however you heat the water more sugar will go into solution. As
the water cools the solubility limit of sugar in water will decrease
and sugar will fall out of solution. The same phenomenon happens
with solid solutions like copper and silver but there are some added
twists because they are solid solutions For example if you heat the
sterling alloy to 1382 F (about 750 C ) and hold it there for a bout
30 min. all the copper will go into solution and if you quench it
rapidly from this temperature and then look at it under a microscope
you will find it is composed of a single solid solution of alpha
phase and is no longer a mixture. By rapidly cooling the alloy the
copper cannot precipitate out of solution.
Here is another example. Gold and copper form a continuous series
of solid solutions. Because the gold and copper atom are almost the
same size they can substitute into the crystal matrix without
causing much distortion. This means that any ratio of gold and
copper will form an alloy where they are totally dissolved in one
another. However if you slowly cool the 18k alloy of copper and gold
it will form a compound AuCu and at somewhere around 12k the same
slow cooling will form the compound AuCu3. However if you cool them
rapidly (quench) they will remain solid solutions and not form the
compounds.The compounds are not ductile or malleable and can even
shatter like glass when dropped.
And finally if a solution of gold and copper is made that is 6k or
less then you can dissolve the copper out from the gold in nitric
acid all the copper will be taken up into solution in the nitric
acid as copper nitrate and you will be left with pure gold.
The bottom line is that alloys can segregate or become brittle due
to improper heat treatments or improper technique in manufacturing.
They also can have a portion of the mixture dissolved out of them
under the right chemical and physical conditions
Some good references for on precious metal alloys is
"Introduction to Precious Metals" by Mark Grimwade it is out of
print but can be found on line through used booksellers. Also there
are good basic papers on stress crack corrosion and chemical effects
on gold in the back of the most recent Hoover and Strong catalog and
many good papers on metallurgy and such in the sixteen or so volumes
of the “Proceedings of the Santa Fe Symposium” that is put out by
Eddie Bell of Rio Grande
Jim
Anyone who knows Jim Binnion personally would agree that he is one of
the most technically educated people you will ever meet. However I
believe that his disagreement is actually agreement with an overtone
of embellishment. I was striving 1) to keep it short, and 2) to not
get quite so technical. The gist of what I meant, though, was what
you wrote. The alloying of metals is more intimate that blending salt
with carbon, or even mixing salt into water, and also that there is
little generalization to be made about alloys - that “What alloy?” is
the essential question. However, it remains, that in practical terms,
the everyday alloys used in the jewelry industry are firmly bonded.
One thing which seems to have been overlooked in this discussion is
the crystallography of the metals. In any metal which has been cast
at some time there is a crystalline structure which, depending upon
the amount it has been worked since casting and the various component
parts of it, will be more or less permeable to gases and liquids.
Unless the alloy is at the ‘eutectic’, the point at which the amounts
of the constituent parts exactly dissolve in one another, there will
be a surplus of one or more metals which will crystallise out
separately and simply form a ‘mixture’ in the ‘alloy’. These crystals
will be available to be attacked by anything which can get to them.
In the horological field we have a problem which is known as ‘brass
embrittlement’ which is very similar to the problem you have
described. This is caused by cleaning the brass parts of clocks and
watches in Ammonia-based cleaning solutions and, as in this case,
makes the grains of the metal simply fall apart so that the metal
turns to dust. In this case, it is thought that the mecanism involved
is that ammonia vapour which is released as the cleaner does its work
seeps into the gaps between the grains of the crystal structure and
becomes trapped there. When the part is then exposed to damp air over
a period of years, the ammonia combines with hydroxyl radicals from
the moisture to form Ammonium Hydroxide which crystallises and
increases in bulk and, in so doing, forces the crystals apart
physically until the whole matrix crumbles.
The amount that the brass has been ‘worked’ seems to have some
effect on this process and, surprisingly, old brass which has been
hammered after casting to compact and harden the metal, is often
affected more than unworked castings. Presumably the hammering
distorts the crystals so that the bonds between them at an atomic
level are weakened so allowing microscopic cavities to develop.
So, thinking along these lines, could it be possible that exactly
the same process is causing the embrittlement but with Chlorine ions
diffusing into the crystal lattice and reacting with the Zinc to form
Zinc Chloride (perhaps even Copper Chloride also)?
Best wishes,
Ian
Ian W. Wright
Sheffield, UK
I have found this to be an interesting thread , there is an
informative article in the Stuller Settings metal book about
alloy’s that provides a good introduction in these matters .
Mark Clodius
And anyone who knows John Donivan knows what a great wealth of
experience in bench work he has and shares with others 
Anyone
who knows me knows I can get a little picky about the details
John, it is with your last statement there where I have a little
quibble
However, it remains, that in practical terms, the everyday alloys
used in the jewelry industry are firmly bonded.
This should be true if the alloys are properly processed, however
more often than we would like alloys are not processed quite right.
If alloying is done in the crucible just before casting the
resulting alloy can be not mixed very well or worse yet foreign
items can be introduced into the melt like old investment odd bits
of metal like iron from bench sweeps etc. or crucibles are not kept
separate for different metals so bits of say nickel white get into
yellow because only one crucible is used for gold. Anyhow there are
many ways that contaminated metal can end up in a ring and this can
lead to the kind of failures that we have been talking about.
Impurities tend to segregate in the interstices between the crystals
and cause areas that are susceptible to inter-granular corrosion or
brittleness.
Regards,
Jim