Next time you melt some metal look at the way the flux acts,
it does not form a skin over the metal it tends to collect at the
edges of the molten metal which means that the center portion is
not covered by the flux.
I’m not sure of this. It’s always seemed to me that there is a
tenancy for a thin film of the flux to sometimes (emphasis on
sometimes, perhaps relating to temperature) remain on the metal
surface, at least more than totally slagging off to the side. Not
sure of this, but sometimes it seems that way…
I am well aware of this. But it is physically possible to weld
aluminum with oxyacetylene and flux.
Actually, with the right flux, welding, tig welding works just fine.
Takes some practice and finesse, but works. The problem with purple
gold is that one is attempting to bring together two materials with
wildly differing melting points, so that by the time the gold melts,
the aluminum is WAY past it’s melting point, and far more reactive
than it normally is when only melted as in welding just the aluminum.
The problem is increased because aluminum and gold are not miscible,
so there is not likely any significant eutectic effect of the
aluminum causing the gold it’s in contact with, to melt lower that it
otherwise would.
However, keep in mind that purple gold is best known in the
electronics/electricians circles as a scourge which forms
spontaneously, at room temperature, sort of a corrosion product it
would seem, where gold and aluminum are in contact, which then messes
up the electrical conductivity of the connection. This ability of the
purple material to form spontaneously over time may present yet
another avenue for exploration. You know, something along the lines
of mixing aluminum powder with gold powder, pressing it together, and
going away for a year? Who knows. I don’t. Might work.
Just remember, in the midst of all the negative input, that purple
gold HAS been successfully used before. Steven Kretchmer published
work with it, apparently soldered to other gold alloys (though I make
that assumption only on the magazine photos I saw), in the 80s. This
stuff isn’t some new thing. And that small metals patina paperback
book I’ve got somewhere around here, in which a university professor
who’s name I now forget, described his method of making a vessel in
aluminum, plating it with gold, and firing it till it formed the
purple skin, is something I bought in the 70s, I think.
I agree a gas shield seems to be a necessity, a "glove box" or
otherwise. Although you can sand cast aluminum without a gas
shield. I have used silicate sand and crushed limestone to make
usable aluminum castings. Would there be anything in the sand
mixture that when heated by the molten metal would produce inert
gasses?
Aluminum by itself does not require all that fancy atmosphere
control. You can melt and cast it with a torch, even, though a
furnace of some sort works better. At it’s normal melting temps, the
aluminum forms a stable skin of oxide that helps to prevent excessive
further oxidation, and various fluxes can be used that also help
create a gas shield over the surface. This differs a lot, though
from what will be encountered with melting it together with gold, due
to the much higher temperatures at which gold melts.
Peter Rowe seems to have the best solution in the idea of
plating aluminum with a layer of gold then heating until it colors.
Appears straight forward, nice layer of pure to prevent oxidation.
I’m gonna have to find that pamphlet. My memory of the process was
that with a thin enough gold layer, the combination took place at a
fairly low temperature, with nothing actually melting. I don’t
recall the actual temperature. But the aluminum is never melted.
Off on a tangent here but the use of ammonium chloride to
remove impurities is a new one to me. The chloride I assume bonds
with the impurities? How much iron can be removed this way, traces
or larger quantities? I have used potassium nitrate which is
supposed to be capable of removing impurities in a similar manor
but it hasn't proven to be useful. The "nice blue noxious smoke" is
it always present, or is it the purple of gold chloride burning
off?
Ammonium chloride is a so-called refining flux. The ammonium half
degrades leaving chlorine gas, which is highly reactive. it combines
with any baser metals present to form chlorides. The key is that the
chlorides generally are not soluble in the molten metal, and they
then slag off. I wouldn’t use it to remove large amounts of iron, but
it will reduce, at least, small amounts. The classic situation is
that after refining gold to pure, melting down the ingot, or when re
alloying pure to karat gold, there can sometimes be traces of iron or
other impurities present that result in brittle cracky ingots. The
Ammonium Chloride “fixes” that problem. It’s used as a component in
some melting fluxes. Matt’s super flux for casting is a normal borax
or boric acid based powdered melting flux, but it then adds charcoal
or graphite powder as a deoxidizer, and a goodly bit of ammonium
chloride. Used to melt, say, old scrap gold items, it tends to reduce
the problems caused by bits of solder or old used traces of what
originally were casting deoxidizers in the original alloys. The blue
smoke is literally chlorine, or some compound of it. Very noxious. It
is not a gold vapor of some sort, since it will form just as blue
when melting silver. It’s always there if you use enough of the
ammonium chloride to form visible amounts of smoke. Often (such as
with the matts flux) the actual amount of ammonium chloride is quite
small. By the way, even table salt has some of this same effect, and
it too is sometimes used as a refining flux, though I’m not quite
sure just what happens to the sodium metal when the salt breaks down
to release chloride ion…
Potassium Nitrate is another type of refining flux, but it’s an
oxidizer. Removes things that will be removed from a melt if their
oxides are not soluble in the melt. Chemically, it’s akin to
pumping oxygen over the molten metal. This can have uses when melting
pure gold, but not so much when melting alloys containing baser
metals like copper.
Peter Rowe
