Americium

“Proclaim Liberty Throughout All the Land Unto All the Inhabitants thereof”

This quotation from Leviticus 25:10 is cast into the Liberty Bell, an iconic symbol of American independence. The words resonate today, the day that Joe Biden was inaugurated as the 46th President of the United States of America.

I stitched this bell pattern while watching the inauguration to represent Americium, element 95, named for its country of discovery. It is a synthetic element, first made in 1944 by Glenn Seaborg and his team of whom you will read more in later posts. Americium is the only radioactive man-made element that you will almost certainly find in your home – in your smoke alarms. A tiny amount is used as an alpha-particle emitter to ionise the air in the detector and allow a current to flow. If smoke particles are present they absorb the alpha-radiation, stop the current flowing and cause the alarm to sound. But don’t let the presence of a radioactive isotope on your ceiling worry you – the amount is truly tiny, and alpha-radiation can be stopped by a piece of paper so is not going to escape the casing of your detector.

To divert from the element in question, the chemistry/metallurgy of the Liberty Bell is also interesting. It was cast in 1752 in London by the firm that later became the Whitechapel Bell Foundry, and shipped to Philadelphia. It cracked when it was first rung. The foundry blamed this on it being struck too hard and too close to the rim, but local founders John Pass and John Stow who were commissioned to recast the bell decided the metal was too brittle and added extra copper. As noted in my earlier post on tin, bell metal is a particular mix of copper and tin with a higher tin content than standard bronze to make it more sonorous. However, too much tin can make the metal too brittle, particularly for larger bells. Sadly the newly-cast bell sounded terrible and had to be recast a second time, this time with added tin to attempt to redress the balance. Even so it still didn’t sound good – perhaps because Pass and Stow had never before cast a bell.

Analysis in 1975 by the Winterthur Museum suggests that the original alloy may indeed have been at fault, but that the two re-castings compounded the issues. It seems that the first recasting introduced unusually high amounts of zinc, probably from an accidental use of brass instead of some of the copper, and the second re-casting used a poor-quality pewter, with significant lead content, rather than pure tin. The analysis also suggested that the melt was incompletely mixed, most likely because Pass and Stow did not normally cast such large items and so wouldn’t have had a furnace or crucible of sufficient size to melt the whole quantity together. This third bell did stay intact for eighty years before the now-familiar crack appeared, but this is not a ringing endorsement of its quality: many other bells cast by the Whitechapel foundry at that time are still in service.

To find out more about the chemical and X-ray fluorescence analyses of the Liberty Bell, see Hanson, Victor F., et al. American Scientist, vol. 64, no. 6, 1976, pp. 614–619.