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Is platinum more ductile than gold?

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Category: Metallurgy

Subject: Is platinum more ductile than gold?

Question

Currently the Guinness World Record database has listed gold as “Most ductile element”. However, somebody brought to their attention that it is platinum that should have this record and not gold. 

The record states: “One gram of gold (Au) can be drawn to 2.4 km, or 1 oz to 43 miles.” 

In the article “The Platinum Decathlon” the section on jewellery states that a platinum rod 10 cm long and 1 cm in diameter can be drawn into a wire approximately 2777 km long. 

I think this translates into about 16.5 km per gram. If so, platinum is far more ductile than gold at 2.4 km per gram. 

Which element is actually the most ductile and should have the record?

Answer

The wire length was based on calculating a constant volume from the starting rod, based on the final wire thickness of 0.0006 mm. Having redone the calculation the final figure came out a factor of 10 higher at 27,777 km (see calculation below).

In order to draw the platinum down that far some tricks have to be used. The main one is that the platinum is drawn down most of the way (0.01 mm), then embedded into silver and the combination is drawn down again (the Wollaston process). The silver is then dissolved off the platinum leaving the 0.0006 mm wire behind. Given that, the length calculated above is correct, but to our knowledge no one has ever tried to draw a bar of platinum fully down to that length. Additionally, in practice a slightly longer bar would be needed to account for the small amounts lost in the process (such as the ends to thread the die with).

The Wollaston process was initially attempted on gold, but the only method available for ‘coating’ the gold wire was to drill a hole along the silver wire and thread the gold along it. This was difficult, and the higher melting point of platinum offered much easier coating methods, so Wollaston switched to using platinum. With today’s more advanced coating technologies, it may now be possible to coat the gold wire and draw down using the same method. Therefore it is not possible to say whether this proves a higher ductility for platinum under those conditions.

Without using this technique, both gold and platinum are sold as 0.01 mm diameter wire, which does not prove that either is more ductile.

Also, as a metallurgist, it is questionable whether comparing ductility in terms of length per gram is meaningful. Ductility is a measure of possible strain under applied stress, neither of which is measured in terms of weight. This becomes important when considering two metals, one of which has half the density of the other. In a typical tensile test to ASTM standards, the size of the sample is set, not the weight. If these two hypothetical alloys fail after the same extension, they should be the same ductility. However, if they were compared by weight, one would be considered twice as ‘ductile’ as the other, despite the fact that both failed under equal strain.

Specific ductility would be the metal’s ductility divided by its density, and might be used for materials selection to pick the lightest metal with the correct ductility. However this not the same as the above described situation.

In summary, the length is theoretically correct, but it may be possible to achieve the same length for gold using the same method. Therefore it is not possible at this time to say whether gold or platinum is the more ductile.

Calculation:

Volume of a cylinder = πr2l

where r = radius and l = length.

Therefore, volume of a cylinder 1 cm diameter by 10 cm length is π × 0.52 × 10 = 7.853 cm3.

For thin wire, diameter = 0.0006 mm or 0.00006 cm.

Therefore, l = 7.853 / (π × 0.000032)

l = 2.778 × 109 cm

l = 2.778 × 107 m

l = 27778 km.

Answer posted April 2013

Answered by: Andrew Fones

Affiliation: Johnson Matthey Technology Centre, Sonning Common, UK