In my recent work I’ve been making a carbon nitride material that produces mercury sulphide (HgS) nanoparticles as a side product. Although this ‘metacinnabar’ HgS is certainly toxic and definitely needs to be handled (and disposed of) with care, I was really surprised at the general nervousness of many trained scientists about going anywhere near it. For example, during our investigation we were looking to measure the surface area of the carbon nitride foam which supported the HgS particles, but no company or analysis lab would touch it with a ten foot pole. Eventually we gave up on performing the measurement.

This apprehension about the use of mercury and its compounds is entirely understandable. Although mercury is a material that is both incredibly useful and quite magical, it is extremely toxic and hazardous to the environment; this means that we do need to avoid it where possible. Nonetheless, this is quite a recent viewpoint. For example, it was only 2013 when the global Minamata convention on mercury (named after a Japanese town infamous for an enormous case of mercury poisoning and intended to protect human health and the environment from releases of mercury) was signed.

Although this seems surprisingly recent, in some ways a more startling example of our altered perspective is that within my lifetime you could still buy a toy for your kids that contained ~1 ml of pure mercury. The toy in question is called ‘the mercury maze’ and, as you can guess, it was a puzzle where you guided a sizeable blob of mercury to the centre of a plastic maze. I wasn’t aware of this toy until recently, but I was really shocked when I found out about it, and even more so when I found out that a friend had three! You can see pictures below.

The example in the black packaging was produced in around 1978 and if you look at the back of the packet (below) you can see that there is plenty of waffle about the history of mercury and labyrinths, but no mention that mercury is toxic and that it should be kept locked up in its container. Later, in ~1989 you could still buy the toy, but safety standards had moved on. Now you had to be at least 7 years old to play with the maze and they suggested that you don’t swallow the mercury they’re selling you! Sadly, I couldn’t find out when these toys disappeared from the market but you have to assume (or hope) that it wasn’t much later. One side note: this puzzle is really hard! If you’ve ever interacted with mercury you’ll know that one of its most enchanting properties is the way that it moves; its high density means it is incredibly fast, as you’ll know if you’ve ever dropped an old thermometer! You can see the mercury move in a video here.

If you read the back of the black packaging carefully you learn that mercury can dissolve gold, which means it has been used in gold mining. Unfortunately the type of mining most commonly associated with mercury use is the ‘artisanal’ kind, common in developing countries where the miners aren’t working for a big company, but independently, often in hugely unsafe conditions. After the gold-containing-ore is ground, it is mixed with the mercury, which is then boiled off (boiling point 356.73 °C), leaving pure gold. This process is most often performed in open air, releasing huge quantities of mercury into the environment and seriously poisoning the miners. This contributes to numerous deaths every year.

These two examples above highlight that although mercury is both fun and useful in applications including science and technology, the risks mean that we simply must move away from, or even entirely eliminate, its use. Despite this, we do need to be careful to maintain the practical skills and knowledge in our technical workforce to handle mercury and its compounds, especially because it seems clear that we’ll be cleaning up mercury-containing-waste well into the future.

TSM