Guest post by Jen Dougan
I attended five weddings this year, between February and October, making 2014 most definitely the Year of the Wedding. Each day was unique and reflected closely the couple, their relationship, vision and hopes for the future. And while there was a broad range of traditions at each, whether religious or humanist approaches, one tradition – the exchange of rings – provided a common element throughout.
The element is, of course, gold. But what is the chemistry of this symbol that is ritually shared as a sign of commitment and lasting love?
Gold is the most noble of metals, it doesn’t corrode or oxidise in the Earth’s moist, oxygen-rich atmosphere. Ductile and malleable, it can be fashioned at will and, since it doesn’t tarnish, is an ideal material for producing jewellery. The decorative attributes of gold have long been coveted – examples of gold jewellery have been found in the tombs of the Queens of Egypt and Sumeria, 3000 years BC.
Gold is one of only three coloured elemental metals (the others being copper and caesium). Most metals appear whitish-grey as they reflect all wavelengths of visible light. Gold, on the other hand, due to its electronic configuration exhibits a reduction in reflectivity efficiency in the blue region of the visible spectrum. It has, in the pure form, a striking yellow colour and brilliant lustre for which it became known by the latin Aurum, meaning ‘shining dawn’ (depicted by artist Murray Robertson in the image above).
Today the purity of gold jewellery is expressed in terms of ‘karat’. Pure gold is 24 karat, where, K = 24(Masspure/Masstotal), meaning 24/24 parts of the material are gold, by weight. In practice, even 24K gold is often stated as 99.99% pure due to residual impurities from the extraction process. Although 24K gold can be used for jewellery, the softness of pure gold means that alloying the metal to improve its physical properties or to design colour variants is a popular approach. The European standard, for instance, is 18 karat, meaning 18/24 parts are gold (75% pure by weight). The other 25 % of the weight is made up of another material (in the case of jewellery, a second metal) to make an alloy, which is typically stronger than the pure metal.
The improved strength of gold alloys occurs when, having melted two or more metals together, a new structural phase is formed on cooling. The precipitation of the added metal atoms reduces slippage of the original atoms across one another.
Common alloys of gold for jewellery fall into the yellow gold or white gold categories. Most jewellery alloys of yellow gold, including that of rose and green-shade gold, fall within the Au-Ag-Cu system (see image above). Zn is sometimes added to further increase the alloy strength and to change the red colour of high Cu alloys to a paler rose.
For white gold, Au-Pd-Ag is a common alloy system, where Pd is used as a bleaching agent. Nickel, another metal capable of producing the desired bleaching effect to produce white gold, is a known skin sensitiser and has become less popular in jewellery making.
The bleaching effect occurs due to the added material changing the reflectivity of the alloy in the low energy part of the visible spectrum. In all cases, the relative ratio of the alloy’s components dictate the resulting hue and mechanical characteristics. The alloy systems allow the characteristics to be achieved such that the design lasts the length of a marriage and beyond.
A word of warning/advice to the newlyweds: although your gold is stable, it is not chemically inert. Avoid exposure to mercury, with which the gold will form an amalgam; cyanide salts, which will react with your band; and aqua regia* in which it will dissolve. Of course, if you encounter any of these three materials by accident, you have greater worries than the integrity of your wedding ring!
The chemical susceptibility of gold for these reagents has proven useful. Mercury and cyanide salts have been used to extract gold from its ore. In fact, gold cyanidation, developed industrially in the 1880s by Glaswegian chemists John Stewart McArthur and Robert and William Forrest is still the main method of gold extraction – and the biggest use of sodium cyanide – globally. The ability of aqua regia to dissolve gold helped foil the Nazis during World War II. Two German Nobel laureates had placed their medals (23K gold) at the Niels Bohr Institute in Copenhagen for safe-keeping. But removing gold from Nazi Germany was a grievous offense and with the laureates names inscribed on the medal, this was an incriminating piece of evidence. ‘While the invading forces marched in the streets’, George de Hevesy – a (future) Nobel laureate himself – dissolved the medals in aqua regia. While the laboratories were laboriously searched by the Nazis, the medals remained dissolved, but undiscovered, on the bench. The metal was recovered after the war and the medals recast and returned to the laureates.
Whatever gold you choose for your wedding band, you add to a rich history of human civilisation fascinated by and adorned with gold. And to the newlywed class of 2014, congratulations!
* Note: Aqua regia is latin for ‘King’s water’, a 1:3 nitric:hydrochloric acid solution, it is known in Russia as Tsar’s vodka and its ability to dissolve gold is shown wonderfully in this Periodic Table of Videos.
- The Chemistry of Gold, M. Concepción Gimeno, Modern Supramolecular Gold Chemistry: Gold-Metal Interactions and Applications, 2009, Ed. Antonio Laguna http://onlinelibrary.wiley.com/doi/10.1002/9783527623778.ch1/summary
- Precipitation hardening and ordering of carat gold jewellery alloys, Gold Bulletin, 1978, 11, 4, p 116, W.S.R. (William S. Rapson)
- Phase Transformations in 18-Carat Gold Alloys Studied by Mechanical Spectroscopy, PhD Thesis, John Hennig, 2010, ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE, http://infoscience.epfl.ch/record/144083/files/EPFL_TH4635.pdf
- Coloured Gold Alloys, Gold Bulletin, 1999, 32, 4, 115, Cristian Cretu & Elma van der Lingen http://link.springer.com/article/10.1007%2FBF03214796
- Alkali Metal Cyanides, Ullmann’s Encyclopedia of Industrial Chemistry, 2006, DOI: 10.1002/14356007.i01_i01, Andreas Rubo, Hanau-Wolfgang, Raf Kellens, Jay Reddy, Norbert Steier, Wolfgang Hasenpusch
- Nobel medals in aqua regia: http://www.nobelprize.org/nobel_prizes/about/medals/
Guest post by JessTheChemist
‘Where the telescope ends, the microscope begins. Which of the two has a grander view?’ – Victor Hugo
In 1873, German physicist Ernst Abbe reported that the resolution limit of the optical microscope was 0.2 micrometres. Although this still remains true, recent work in the field of microscopy – specifically Stimulated Emission Depletion (STED) microscopy and single-molecule microscopy – has allowed scientists to visualise molecules smaller than this limit. This is accomplished by tagging molecules with fluorescent labels, which allows a more detailed picture to be visualised. On Wednesday 8th October 2014 Eric Betzig, Stefan Hell and William Moerner were awarded the Nobel prize in chemistry for their ground-breaking work in ‘the development of super-resolved fluorescence microscopy’. You can learn more about the ins and outs of the Nobel prize winners’ work by reading the recent Chemistry World article.
I am interested in finding out how chemists are connected to each other, and in particular, investigating whether your likelihood of winning a Nobel prize is increased by having a high number of laureates in your family tree. It is also interesting to see how closely related, if at all, are the scientists that share a prize.
If we consider his academic pedigree, one might say that Eric Betzig was destined to become a Nobel prize winner. He is connected to a number of notable laureates, including the father of nuclear physics, Ernest Rutherford. Rutherford won the Nobel prize for chemistry in 1908 ‘for his investigations into the disintegration of the elements, and the chemistry of radioactive substances’. Through Rutherford, Betzig is also connected to Niels Bohr, who won the Nobel prize for physics in 1922 for ‘his services in the investigation of the structure of atoms and of the radiation emanating from them’.
Additionally, Betzig is academically related to John William Strutt (Lord Rayleigh) who won the prize for physics for the discovery of argon in 1904, along with his collaborator Sir William Ramsay, who won the 1904 chemistry prize for the same discovery. With ancestry like that, Betzig was always destined for greatness.
Alternatively, William Moerner is closely linked to Dudley Herschbach and Yuan T. Lee, who won the 1986 Nobel prize in chemistry ‘for their contributions to the dynamics of chemical elementary processes’. To find out more about Herschbach and Lee’s academic family connections, check out last month’s blog post about one of their connections, Sir Harry Kroto.
Although Eric Betzig and William Moerner worked independently from one another, they both developed single-molecule microscopy, so it is not a surprise that their lineage is intertwined. As you can see from the tree, they are connected via some of the science greats such as Linus Pauling.
Stefan Hell worked in a slightly different field to Betzig and Moerner and, amongst others, is connected to biochemists Robert Huber and Johann Deisenhofer, who won the 1988 Nobel prize for chemistry ‘for the determination of the three-dimensional structure of a photosynthetic reaction centre’. As predicted, however, all three of the prize winners can be connected through their academic relations – via Deisenhofer, Hell can be connected to Moerner and, therefore, Betzig.
As you can see, all three winners have a rich Nobel history but what I found particularly interesting about this academic family tree is that it contains scientists from all sorts of backgrounds – from nuclear physics to biochemistry to organic chemistry. This led me to think, what kind of scientists am I connected to? To find out what kind of scientists you are connected to, head to academictree.org, where you can add yourself to the website and start creating your very own tree. You never know who you may be connected to.
And don’t forget to tweet me (@Jessthechemist) with suggestions for the focus of next month’s blog post!