purple-gold-experiments

The first time I read about purple gold was in a Swiss magazine called Arum in 1984.

If I remember correctly, they had made some with the correct proportions on a space craft in micro gravity, and it formed a amazing crystal like formation, which was the subject of the article. ( I stand to be corrected)

Fast forward to 1994, and I then approached a company called Mintek in South Africa.
I worked with Professor Elma van der Lingen and she had access to the argon induction melting machine needed to melt the two very different metals together.

She said she would give it a try, and so I gave her 30 grams of fine gold and she added the aluminium for me and produced two buttons of very beautiful purple gold.
Because of her 'can do' attitude, the following Hidi ( how I do it) became possible.
Also, everything I have written here is subject to correction. I have no experience in metallurgy and the theory there of.

Basically, purple gold is and alloy of fine gold at 80% and aluminium at 20% per weight.
When the two are melted together under an inert atmosphere, normally Argon, they form
an inter - metallic compound that is very brittle and not malleable at all.
There are a few websites and You tube videos that claim to have made malleable purple gold, but the bottom line is that an inter metallic compound is hard and brittle. Period.

One can cut it with a normal jewellery saw blade and also file it with a fine file or sand it with buff sticks and sanding disks.
I have extensive gem cutting experience, and if I have to compare a gem material to it in terms of 'feel' of cutting, I think Opal would come closest.
It has the same brittle feel to it and polishes as easily and at about the same rate as does opal.

The purple color is variable in relation to the percentage of aluminum.
At about 10% more gold, the colour disappears and changes to a silvery gold colour.
Lower percentages of gold also changes the colour.
Because the ration of 80% fine gold is slightly above that of 18 kt gold, purple gold can be hallmarked and sold as 18 kt gold.

Although both gold and aluminium don't suffer from much tarnishing, purple gold does.to some extent.
Not much, only after several years is there a slight darkening of colour.
A soft rouge brings the 'new look' luster back.

Purple gold has a higher melting point the its parent metals, which means it has a high thermodynamic stability.
The formation of purple gold was a problem in the electronics industry, because when the fine gold wires that are typically used, were joined to an aluminum substrate, at elevated temperatures diffusion occurred and the joints would become brittle and break.
This mechanical failure was known as 'purple plague'.

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I purchased an old but still very serviceable dental vacuum oven and I got to thinking whether it would be possible to make some purple gold for myself.
Keep in mind that the following methodology that I used is neither scientific nor anything remotely formal.
I just was doing some modern alchemy, and seeing what comes out.
My ancient vacuum oven.
The nice part of this machine is that it heats from 0° to 1200° C in about 5 minutes, so the experiment is quickly ready to be melted.

I took two soldering blocks and created some shallow depressions in each one, so that when the were fitted together, they formed a semi spherical hollow in them.

Then I weighed out a quantity of fine gold and aluminum to the 80% Au and 20% Al ratio.

The only flux I had was high temperature brazing flux so I spread that liberally on both soldering blocks.
Not the best flux I'm sure, but hey, you use what you got.

I closed them up and I set my oven for 1200° C.
The melting point for fine gold is about 1000°C and that of aluminum is about 660°C so I figured 1200C should be the middle point that both metals would be liquid.

It just seems to sag when it melts.
I am sure that if I had an induction oven with an inert atmosphere like Argon, then one could use a mixing stick to mix it more thoroughly ( if such a thing exists).

The resultant blob was certainly not going to win any beauty competitions.

When I pried it out of the solder pad, there was definitely some purple color at the bottom.

When I filed the top crust off, it revealed quite a nice purple color.
Of course, I was not expecting a non porous material at all, because I was certain that even though the metals had been melted in a vacuum, the aluminum at least would have reacted in some manner or other at 1200° C and cause bubbles.

I tried different ratios to see what color changes, if any, would occur.

As can be seen from the picture, that was certainly the case.
The sweet spot appears to be at the 80% gold ratio.

One of the pieces had a giant cavity in it.
So I put it back into my soldering pad contraption and set the oven to 1400° C to see if I could re melt it and maybe the cavity would fuse closed.

The cavity did not disappear.
All that happened is some aluminum oozed out and formed some nodules.
The cavity can still be seen at 12 o clock in the picture.
Probably the fact that it was not physically stirred and so not homogeneously mixed during the first melt had also something to do with the oozing.

Even though the piece of metal had been heated up twice and some aluminum had oozed out, the color, when I filed the bottom flat, was still purple, which led me to suspect that not all of the aluminum is 'used' up in the melt.
Nevertheless, this material was heated up 200°C higher than the original melt and remained stable, showing the aforementioned thermodynamic stability.

Under 60 x magnification, a triangular crystal lattice can be seen.
My metallurgy knowledge is not broad enough to know the mechanics for this.
However, I suspect that the molten aluminium acts as a carrier or solvent and so facilitates the crystallization of the inter metallic compound as it cools.
Or it could be that at a certain temperature, an eutectic solution forms between the gold and aluminium and provides the mechanics for crystal formation.
Or it could just be plain magic.

When the material in made in a inert atmosphere induction smelter, clear slump lines can be seen
I suspect that in between the slump ridges, an aluminium rich line forms, which also has not been 'used up' in the melt.

Faceting Purple Gold

I have extensive gem cutting and gem carving experience, and faceting or carving purple gold was not difficult at all.
The raw material was cut on a standard diamond saw using water as a lubricant.

I use a two component epoxy to dop all the material I cut.

I use a Imahashi gem cutting machine.
I first used a 260 diamond lap for rough forming and then a 1200 diamond lap for fine cutting with water as a lubricant.
With a normal gemstone such as tourmaline for example, I would use a 14,000 grit diamond pre polish before finishing with a 50,000 grit final polish.
With purple gold it was quite possible to skip the 14,000 stage and go through straight to 50,000.

For polishing, I used a Batt Lap charged with 50,000 diamond powder and a cutting oil as a lubricant.

The purple gold feels just like opal when it cuts and polishes.

The resultant gemstone, and I think I can call it a gemstone, is quite pretty
There are of course only external and no internal reflections.
A bit like a small, expensive disco ball.

I have enlarged a section of this table and I suspect that the different colors ( pale and darker purple) in the material are because of different ratios of gold and aluminium.
Under high magnification, the material shows a high degree of porosity.
Also, the polish has a reasonable amount of cats whiskers ( fine scratches) that I was unable to remove.
This is nothing to do with the material as such, but rather my polishing technique.
Unfortunately, the limited amount of material I have available, makes extensive experimentation unfeasible.

Speaking of experimentation, I took one of the faceted stones and gave it a light rouge on my jewellery polishing machine.( it is a metal, after all)
The results were that the stone became more 'smooth' looking and the cats whiskers were effectively gone.
But so was the 'crisp' look. The facets have clearly rounded edges.
This made it clear to me that were one to set purple gold into jewellery, rings, bangles and bracelets would be unsuitable for day to day wear.
Only special occasions, for when you meet the queen, for instance.

I include this picture to illustrate the purple 'chrome' look that the gemstone has.
It is particularly striking in sunlight and flashes brightly, just like the fore mentioned disco ball.

One of the derivative hobbies that I have form gem cutting is to laminate different types of gemstone together and then facet them as if they are one piece of material.
By way of illustration, the gemstone in the picture is a spesartite that is laminated to 18 kt yellow gold . I set small diamonds in the gold.

I use Hxtal resin to bond the components together.
This resin takes five days to harden and once it is set, will break the parent material before the bond breaks.

I cut some tourmaline and purple gold in preparation.
I used some low quality tourmaline, because I did not know whether it would be possible to achieve a high polish on the tourmaline with the purple gold inlay.
I was worried that the aluminium in the purple gold would ball up and scratch the gemstones.

All contact surfaces are polished and then I hold the three pieces in a small G clamp, very gently.
The masking tape is there because if the gemstone is bonded directly to the metal of the clamp, it will be impossible to remove it without breaking the stone.

I preform the stone as it were a normal tourmaline.

Here I have cut and polished the pavilion.
The purple gold gave absolutely no trouble during the polishing phase.
Again, I used the same sequence that I used in the cutting of the purple gold, except that I also cut with 14,000 grit for the pre polish.

The finished stone. Mistakes were made.
Firstly I used low grade material, which does not justify the effort put into it.
Secondly I used a bi-color stone, so both ends are not the same color.
And because tourmaline is like a skittish cat, the right hand side of the stone cracked something fierce two days after the stone was finished. That's tourmaline for you.
Anyway, the principle was proved to me, which was that the purple gold does not affect the polishing process.

Another picture showing those tension cracks.

Final Experimentation and Pictures.

I am an experienced spin caster and so naturally I wondered what would happen if I cast some sterling silver around a piece of purple gold.
Since I had proved to myself that the purple gold was stable under high temperatures I wondered if it would be possible to melt the purple gold into the wax and then cast it in situ. With titanium this technique works like a charm, and I have often done so successfully. With purple gold, not so much.

I suspect that the aluminum in the purple gold caused an eutectic solution and thus ruined the cast. A eutectic solution is when, in this case, the aluminum and silver melt at a single temperature that is lower than the melting point of either of the component metals.
It is worth noting that the integrity of the center part of the purple gold is still intact.
The hole in the center was drilled before so that the purple gold would be held in place by the plaster during the burnout phase of the cast.

At the same time that I commissioned Mintek to make me some purple gold, I also asked them to make me some 'blue gold'
The following quote is from the World Gold Council paper on colored golds say the following.
“Two other intermetallic compounds that are known to produce colours in gold alloys are AuIn which has a clear blue colour, and AuGa which displays a slight bluish hue.”
“Blue gold' contains about 46% fine gold and 54% indium. Which comes in at about 11kt.
Looking at the image, I think the world gold council is some what enthusiastic in their description of blue gold.
It's a pity, because I could really make some nice jewellery with sky blue gold!

Nevertheless, it also is an inter metallic compound and thus brittle and hard and easy to facet.
So I cut the piece just for fun.
I photograph the two together so there is a color reference.
Unfortunately the blue color is somewhat elusive.
It could be that the ratios that were used are not optimal for color expression and that a different ratio of indium to gold could produce a vastly different intensity of color.
If I had the money and equipment, there would be endless experimentation.

Faceting Purple Gold

Gem Lamination

Final Experimentation and Pictures.

Blue Gold