Greetings Everyone,
I have been doing some research into different alloys, the most
current one is “Purple Gold”. I know that it is generaly an 80/20
mix of Gold and Alluminum. I also know that this alloy is generaly
brittle. So, here is my questions. Has anyone worked with purple
gold before? Does anyone know any other metals that can be alloyed
with the Gold and Alluminum to make it less brittle and more
Jewelry-Friendly?
Thank you for your time,
Ray Tomlin
Columbus Ohio
Purple gold is the intermetallic compound AuAl2. Most alloys are not
chemical compounds they are mixtures of two or more elements that
are more or less evenly distributed in a random fashion in the
crystal matrix. Intermetallics however have a very definite order to
the location of each atom of gold and aluminum in the matrix. Another
name for the intermetallic structure in alloys is ordering. This
ordering makes the matrix very rigid and it will not slip and slide
when stressed like a more random arrangement of atoms tends to do.
This what makes it brittle. AuAl2 is so brittle that it will shatter
like glass if dropped on a hard surface.
When you add or subtract enough atoms either of gold or aluminum or
add other elements the ordering which is the reason for the purple
color is lost and it quickly turns dull and whitish yellow. There
are at least two companies that are using patented alloys that are
close enough to the pure intermetallic that they retain some purple
color but not the rich saturated purple of AuAl2 but they also retain
too much of the stiff unworkable characteristics of the intermetallic
to be used as anything other than a casting alloy. A quick search of
the various patent databases will reveal these patents and other ones
for trying to somehow utilize the purple gold color they make
interesting reading but are not really useful to a goldsmith as most
are high tech processes that are definitely not studio applications.
James Binnion
@James_Binnion
James Binnion Metal Arts
360-756-6550
Purple gold is the intermetallic compound AuAl2. Most alloys are not chemical compounds they are mixtures of two or more elements...
Jim, your explanation was so eloquent and interesting, I read it to
my husband (a smart guy who is just interested in things, not a
metalsmith or anything of the sort). He asked me to forward it to
him, but he also asked what the difference is between the ordering
of the intermetallic and the crystal structure of a diamond. This is
all way over my poor head, so I pass the question on to you!
Noel
It is way over my head as well but I will take a stab at it. The
difference is in the realm of molecular bonding. It concerns how the
outer electron shells of the individual atoms interact with their
neighbors outer electron shell. There are two basic categories of
bond primary and secondary bonds. The primary bond is the stronger
of the two types
There are three types of primary bond Ionic, where one atom gives up
an electron from its outer shell to an other atoms outer shell to
form a more stable number of atoms in the outer shells the whys of
this are way beyond my knowledge but very often the donor atom is a
metal as they have an odd number of electrons in their outer shells.
The donor atom that gives up the electron becomes more positive (by
giving up an electron there are more protons in the atom so it takes
on a positive charge) and the one receiving the electron becomes
more negatively charged and this difference in charge is what holds
the atoms together. Salt (NaCl) is a good example of this type of
bond the sodium has 9 outer electrons and gives one up to the
chlorine atom that has only 7. The sodium becomes a positive ion (an
atom missing one of its electrons) and the chlorine becomes a
negative ion and the electrostatic force from the difference in
charge holds them together. This is a very strong bond.
The Metallic bond is also a primary bond but the absence of receptor
atoms means that these extra electrons form a cloud that are shared
by all the metal atoms in the matrix. The constantly roving
electrons are what make the metallic atoms excellent electrical
conductors and good thermal conductors as well. Because of this loose
connection the atoms in the matrix are more able to move relative to
each other and this is what gives metal its ability to be ductile.
The third primary bond is a Covalent bond where the atoms each share
an electron from its outer shell with the adjoining atom. This type
of bonding tends to lock the position of each atom relative to its
neighbor in a very specific position. This is the bond of the carbon
atoms in diamond. it is the strongest type of molecular bond.
The gold and aluminum atoms in the Al2Au intermetallic are still
bonded via the metallic bond so they are not truly a chemical
compound with an ionic bond like table salt (NaCl) or aluminum oxide
(AlO2) or a covalent bond like the carbon in diamond. When you have
exactly 2 aluminum atoms to every gold atom because of their
relative sizes and the number of electrons present they want to stack
in a specifically ordered arrangement. It is a somewhat poor analogy
but think of a box filled with soccer and basketballs they will not
tend to stack in a particular order in the box but if you were to mix
basket balls and tennis balls the tennis balls would tend to
position themselves in the spaces between the basket balls. This is
sort of what happens in intermetallics. This ordered stacking makes
it much harder for the atoms to move around in the matrix and makes
it a more rigid structure and more brittle
There are actually 5 different intermetallics that form in the gold
aluminum system AlAu, Al2Au, AlAu2, Al3Au8 and AlAu4. Only Al2Au is
purple the rest are whitish tan. The color has to do with the
spacing of the atoms in the crystal matrix and what wavelengths of
light are reflected by the matrix and what ones are absorbed by it.
I hope this helps to understand the difference and doesn’t just make
it less clear,
Jim
James Binnion
@James_Binnion
James Binnion Metal Arts
360-756-6550
James,
Thanks for the description and help with my question. It truly
helped me understand the complexities of this particular Alloy. Now,
all I need to do is find a source for Alluminum Casting Grain so i
can have fun experimenting.
Ray Tomlin
Columbus Ohio
Ray,
You can get pure metals from places like Goodfellow
http://goodfellow.com but you want to be sitting down when you look
at the prices. You want at bare minimum 99.95 purity to use for
alloying. Pure metals with a guaranteed analysis are surprisingly
expensive. You can use materials that are not provided with an
analysis but you may not get the results you want. In many metals it
is costly and difficult to get only the pure metal without
significant traces of other metals. It takes surprisingly small
amounts of some metals (lead, arsenic and antimony come to mind when
talking about gold alloys) to contaminate your alloy to the point
where it is too brittle to be used.
Jim
James Binnion
@James_Binnion
James Binnion Metal Arts
360-756-6550
Jim,
Many years ago I had a friend who worked for a company called
Materials Research. The manufactured equipment for chip makers. They
had the ability to produce what they called “5-9” gold, 99.999%
pure. It was extremely expensive to get to that level of purity, as I
recall several times the cost of the market price of gold at the
time.
Joel
Joel Schwalb
www.schwalbstudio.com
Jim,
thanks again! Although, i am still tring to get my head around the
intermetallic alloy idea. It has been a while since i have studied
chemestry, and i am only now learning metalurgy. BTW, i love your
work!
In Troth,
Ray Tomlin
Columbus Ohio