Bill,
The japanese technique prevents the metal from just sitting in air
as it solidifies, absorbing more oxygen. With many alloys, oxygen
absorbtion is not significant during melting as the flame protects
it. But the only oxygen or gas thats given off while the solidifying
metal is under water, is gas that’s merely dissolved, like CO2 in soda
pop. It doesn’t affect any oxygen that has chemically combined with,
for example, copper, to form copper oxides. Copper containing alloys
cast this way will have less oxides in them than those just poured
into an open air mold, but they’ll still have the same oxides in
them from any oxygen absorbed during melting or pouring, until the
metal reached the water. it’s better, but not perfect. The water
bath (actually for the metal, a steam bath) is also a controlled
cooling rate which can be beneficial for a uniform crystal structure.
46irst, the 18K purple gold alloy is 18K gold, which is 75% gold,
and 25% aluminum. It’s NOT a mix of 18K gold with aluminum.
Technically, the perfect mix is more like 22 percent aluminum, but
that’s not a big difference with a strict 18K proportion, and doesn’t
affect the color.
As to the japanese method of pouring ingots, I expect it might help
some, but I doubt it would help enough. i’ve not tried it. When I
did try to make this alloy, using a torch, I couldn’t even get the
alloy to melt clearly enough to pour. Just the attempt to melt the
two metals together formed, as they combined, a progressively cruddy
gunky mass, not a pourable liquid. Remember that aluminum has far
more of an afinity for oxygen than does copper or precious metals,
and the oxygen it finds combines with it chemically to form an
oxide, it’s not just absorbed, free to be expelled upon cooling. I
expect (but don’t know for certain) that even only partial oxidation
of the aluminum in the mix would prevent the proper formation of the
ordered array structure that gives the exotic purple color. so far
as I know, all the folks I’ve heard of who’ve made this stuff with
good results have melted the mix under truely oxygen free conditions,
such as induction or electric melt furnaces with inert gas shield, or
under vaccuum. Note I say melted, not just the ingot mold, which is
what the japanese method really is. I don’t think a torch is capable
of doing it. Hydrogen fuel, maybe, but again, I’ve not tried it in
many years, so I’m not really sure of that. On the other hand, I do
recall having little difficulty melting just aluminum itself, which
is of course a much lower temperature, in graphite crucibles in a gas
melting furnace, for varous casting projects. Excess oxidation didn’t
get in the way with that. It’s always possible that something
similar might work. But for my part, I have my doubts. Try it,
please. I’d love to find I’m wrong. 20
I’ll mention at this point one other reference to purple gold some
may find of interest. In the 70s, I bought a small paperback book on
various patinas and the like which mentioned this alloy, by a
metalsmith/teacher who’s name I’ve long forgotton, as well as the
book itself, which has long disappeared. What he’d done was to raise
holloware vessels in sheet aluminum, and gold electroplate them.
Then he’d fire them in a kiln, watching them carefully until the gold
just started to dissipate into the aluminum surface. Times right,
he could pull the piece from the heat at just the point where the
gold incorporated a suffient amount of aluminum to form that purple
gold color. Too much, and the gold was gone, too little, and of
course there was just the gold color still. an interesting concept,
but again, not one I’ve tried. I don’t recall the temps he was
using, but expect it to be perhaps in the 600 - 800 range, perhaps?
Just a wild guess…
Peter