Wood’s metal and the evolution of fusible alloys

I’m pretty squeamish. Not for me the iconic eye scene in Luis Buñuel’s film Un chien Andalou. In hospitals, my imagination can run riot; to my profound embarrassment I once fainted in an intensive care unit at the sight of metal struts supporting shattered bones. So it’s odd that two of my favourite museums are in the Royal Colleges of Surgeons of London and Edinburgh. If you’ve never been to the Hunterian or the Surgeon’s Hall museums drop everything and go! They’re the places to be if you want to know what an aneurism looks like or see polyps of the colon (without having to watch them on a screen as a probe explores your interior).

In one of these collections I once saw a silvery cast of the lungs made by filling them with the low-melting-point alloy Wood’s metal; it is a remarkable 3D visualisation of an organ I’d only seen either pickled in a jar or sliced and sauced in a restaurant.

I first came across Wood’s metal while clearing the lab I’d been assigned when I first started as a lecturer: in a dusty corner, on a tripod, sat a porcelain dish two-thirds filled with a silvery metal, a thermometer embedded into it like Excalibur in the stone in the legends of King Arthur. One of my older colleagues told me it was a useful and non-flammable alternative to an oil bath. Intrigued, I found a Bunsen burner, heated it up, and found, as I removed the teetering mercury thermometer, that the metal melted around 70˚C.

The idea of fusible metals goes back a long way. The Romans knew that tin reduced the melting temperature of lead, making it easier to join pipes by fusion. In the 17th century Robert Boyle mentioned low melting mixtures of bismuth, tin and lead. As Master of the Royal Mint in the early 18th century Isaac Newton used several of these alloys as temperature standards for thermometry. Through the 19th century several compositional variants appeared that are named after their inventors: Rose’s (a German pharmacist) and d’Arcet’s (a French chemist) metals. In 1812 the British engineer, William Onions, patented the composition that bears his name today.

In 1860, by adding cadmium, American dentist Barnabas Wood obtained an alloy that melted at 70˚C. What is known about Wood himself is quite sketchy and confused. Born in the village of Guilderland, halfway between Schenectady and Albany in upper New York State, US, he studied dentistry with his brother at Albany State College. He is said to have had an editorial role at the eclectic American Magazine and Repository of Useful Literature edited in Boston by Nathaniel Hawthorne and his sister Elizabeth, but this seems unlikely. There is no mention of Wood in the magazine and he lived hundreds of miles away. Rather, he seems to have worked as a dentist for a time with his brother.

In the 1840s he moved south to study at the University of Nashville, getting a medical diploma either in 1844 or 1852. After this he ran his own dental clinic, and in his spare time developed the alloy that he termed a ‘plastic metallic filling’ that could be melted in hot water. The warm alloy remained workable enough to be moulded into cavities in teeth with little pain to the patient. Much cheaper than gold, but more durable, Wood argued that it should be used to replace what he saw as the more poisonous mercury amalgams.

The patent filed in 1860 was reported in the local press and in Scientific American and The Chemical News. In Germany, Friedrich Lipowitz reported experiments with it in Dinglers Polytechnisches Journal; you sometimes find it called Lipowitz’s metal. In his patent Wood pointed to other possible applications such as for thermal protection on steam boilers, taking impressions of complex objects, silvering mirrors and more.

But Wood wasn’t just a dentist/metallurgist. He had a wide range of interests from physiognomy and phrenology via slavery and morality, to botany and zoology; his papers at the University of Tennessee include essays titled ‘Why carnivorous animals cannot subsist on vegetables’, ‘Disposition for Marriage at Mature Age dependent upon a natural desire of authority & control,’ and ‘Mind not dependent on its faculties – It could exist without them’. There is plenty of work for someone getting into the archive.

At the outbreak of the American Civil War, Wood’s allegiance to the North led him to move back to Albany. He married, had two children and continued to write and work until his sudden death at the age of 56.

The invention of phase diagrams at the end of the 19th century would lead to the discovery of even lower melting alloys – the alkali metals at the more reactive end, and GalInstan at the more inert. They remain strangely magical. You can still buy Wood’s metal, which is sometimes used for model-making and for practical jokes. Maybe I’ll score myself a few ingots online and a lung from my butcher to make my own fractal sculpture for the mantelpiece.