Subject: purple gold compendium

From: Mark Reynolds <[email-masked]>
Date: 19 May 1995 19:32:15 GMT

My first attempt at purple gold transformed 7g of 24k & 
1.855g pure aluminum into a horrible crumbly mess that 
was irreversibly fused to the crucible!
The following is a compendium of information I received
as a result of my plea for help... Thank you to everyone who
took the time to respond!

[email-masked] (PeterWRowe) writes:

<Re: purple and blue golds...

<We had a good thread going on this subject in rec.crafts.metalworking a
<couple weeks ago.  Here's the gist of it:

<Purple gold is an alloy of 75% gold and 25% aluminum.  However, this forms
<what is called an intermetallic compound, which is, by nature, about as
<metalic and workable (in the sense that our normal gold alloys are) of
<BRITTLE GLASS CRAP.  It's very pretty, but it is not, by itself, other
<than a: a curiosity and b: a problem in electronics, where it forms where
<not desired as contact corrosion between al and au contact points, such as
<gold plated al contacts etc.  
<Anyway, the only place I've ever heard if being used is in the following
<manner:
<A thin gold coating on an aluminum object (the item described in the
<article was a vessel) is quickly fired to just under the melting point of
<the aluminum, about 1200 F.  It is carefully watched, and quickly removed
<just when the metals diffuse forming the purple color.  Additional heating
<destroys it.  The point is, at this point the purple intermetallic is a
<durable surface coating, nothing more.  And it's on an aluminum, not gold
<object.  The gold coating was, in the article I've got, applied using
<organo-metallic liquids, ie the liquid gold low-fire glaze materials used
<for putting gold ornametation on glassware, or the gold coating of low
<fired ceramics.  I see no reason why a good gold electro plate would not
<also work for this.  But I have not, personally tried it.

<Blue gold.  As you must be aware, certain manufacturers have perfected
<this, as seen in various photos of finished italian and other goods.  But
<they ain't talking.  In the U.S., a jeweler named Stephen Kretchmer has
<worked out more gold colors than most jewelers can imagine.  He had a good
<short article in the current JC-K magazine which shows several blue gold
<items.  He also shows a blob of purple gold, which is interestingly and
<significantly the only colored item pictured in his article which was not
<shown as a finished item.  Derive whatever meaning from this that you
<wish.   But back to the blue.  The metal itself is not blue, it is
<basically a yellow or perhaps white alloy (again, people aren't talking) .
 <The blue color is a surface patina made possible by additives in the
<alloy, and can be developed on just the blue portions of the piece without
<affecting other colors, as shown in several of Kretchmers pieces.  The
<recipes I've seen for blue gold (turn of the century) show iron as the
<unique additive, but don't mention treatments.  The source I had for this
<was British texts, and it is possible the dark greyish cruddy metal their
<recipie produces was blue enough for them, in contrast to other gold
<colors.  At the time I played with this, I discarded both these iron
<blues, and the aluminum purples as a mistake.  It never occurred to me to
<try surface patinas on the metal till I started working with Niobium and
<Titanium, and the exquisite colors I can anodize on those metals has
<removed my need to get gold colors.  With Niobium you can get a myriad of
<colors all on the same piece, and gold can't do that...   In the thread
<discussion on rec.crafts.metalwork someone, not a metalsmith but an
<engineer suggested that several additives might allow a glue patina, his
<thoughts being iron and cobalt.  Cobalt in a ceramic glaze gives intese
<blues, and heat discoloration of cobalt platinum alloys (The only platinum
<alloy I work with which will heat discolor, but it casts so well we use it
<anyway sometimes) is bluish.  So If I ever get around to exploring blue
<gold again, those are the starting points.  Other than heat and a variety
<of oxidizers (ever tried birshwood casey gun bluing?  gives interesting
<results on other metals sometimes too...) I have no real idea what they
<are doing to get the blue patina, but it sure does look good.  As a last
<resort, you could always get Stephen Kretchmer to install and patina your
<blue section...

<Peter Rowe

[email-masked] (Richard Warren) writes:

<The composition Au-20%Al is a so-called intermetallic compound with
<chemical formula AlAu2. This has a melting point of  1060oC. Presumably
<your melted Al was at a temperature below this so that when you added the
<gold it immediately started forming the compound instead of going into
<solution in the melt. It may therefore work to melt the gold first, raising
<the temperature so that is well above 1060oC say 1100 and then add the Al.
<A problem may arise because the Al develops a tough oxide film which
<hinders direct contact between Al and Au making it difficult to stir it
<down. A way round this might be to make an Al-Au master alloy first with a
<small amount of the gold e.g. Al+20%Au which it should be possible to make
<by melting the Al first and then adding the 20% Au. This preliminary melt
<you would need to make at about 800oC to avoid formation of the AlAu2
<compound. This master alloy should not be so susceptible to oxidation and
<therefore should be easier to stir down into the melt that you make from
<the remainder of the Au. I hope this is clear.

******Charlie Fredregill, [email-masked]  writes:

<Mark - According to Sharr Choate in "Creative Casting" the alloy 
<is 75% gold, 23% aluminum, 1.5% thorium, and .5% tin. She indicates 
<that the gold should be brought up to a white-hot molten state before 
<the addition of the other metals. However, the wording is a bit 
<obscure, because she is talking about lowering melting points by the 
<addition of cadmium and zinc.

 [email-masked] (James T. Stanley) writes:

<Melting point of the Al2Au phase (purple phase) is 1060 C compared to 660 C for 
<pure Al . Raising the temperature of Al to 1060 C will bring about rapid oxidation 
<of Al unless you use a protective atmosphere or suitable flux. I would suggest 
<melting the gold first. Also wrap the aluminum in gold foil and plunge the package 
<into the molten gold holding it under the surface of the gold using a ceramic 
<rod until melting occurs. The aluminum should be a single piece rather than a 
<powder since the powder adsorbs larqe quantity of of oxygen on the surface.

<*************************************
<*                                               *
<* James T. Stanley, Professor of Metallurgy *
<*                                                *
<**************************************

 [email-masked] Writes:

..snip...<aluminum oxidizes very quickly because
<of it's low melting point.  If the aluminum is heated in the
<presence of oxygen, it wil oxidize and go crumbly as well as
<become brittle.  If you're using an acetylene torch, what you 
<should do is heat the gold first, draw the heat off and add the 
<aluminum and mix it in.  Then reheat it until it goes molten and 
<pour.  Or if you have a furnace, mix a small ratio of aluminum
<to gold power, and then enrich it with aluminum filiings in a 
<process of heating and mixing.  A reduction furnace works best
<for this application.   
-snip-
<Much of the problem of combining these two metals is the significant
<temperature difference (400 degrees C).  You see, once you melt the 
<aluminum at a high enough temperature to melt gold, the aluminum has
<already developed advanced oxidation.  But if you melt gold so that 
<the gold is a free flowing liquid and add the aluminum as filings to
<the liquid gold, then the heat from the gold is more than sufficient
<to melt the aluminum.  And the mixing becomes very simple.

<: I used the crucibles at school. they are heavily coated with borax,
<: does aluminum stick to borax? Might that be one of the problems?
<: Someone else said that I should find a low temperature flux for the
<: aluminum, because the borax only kicks in effectively at higher
<: temperatures appropriate for gold & silver. Do you Know of any flux
<: appropriate for Al? and I hear you when you say to heat the gold first,
<: I suspected as much.  If I added the aluminum to the melted gold
<: would I still need  the low temperture flux? Could the higher & lower
<: temperature fluxes be used together effectively?

<Aluminum requires special flux (#33 flux).  However, Borax should be
<fine for a mixing.  If you mix the aluminum to the gold fast, then
<there shouldn't be a problem.  If you add the aluminum to the gold,
<you should not need to use the low temperature flux and nor for that
<matter will it be of any help.  BTW, you can use different fluxes
<with each other, but whether you actually gain anything out of doing
<that is another question.  Also, try to avoid acid based fluxes 
<whenever possible.  I don't know if #33 is acid based, but since it
<it a low temp flux I suspect that it probably is. 

******Jim Foley [email-masked]  writes:

<Purple gold is very very brittle.  You will NOT be able to make a solid
<ring of purple gold.  There is a way to make a purple gold coating...  
-snip-
<The only way that I have heard to make a ring with purple gold is to first
<make an aluminum ring and then sputter or electrodeposit gold on it and then
<anneal the real at the right temperature so the gold diffuses into the aluminum
<making purple gold while still having a solid ring.  Very tricky.

-- 
    ******   Mark Reynolds [email-masked]    *******
            <Boy--I'll remember THAT name and make sure
            <to stay away if I ever see it in a shop! Egads!
    ****** <--Lani Teshima-Miller-r.a.b. FAQ Maintainer *******