stickmaker | JOHT: Bismuth!

The Joy of High Tech

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

Bismuth

Bismuth is, in some ways, too innocuous for its own good. It is an element with no known biological function in the human body, yet which is not normally dangerous to it. (Only one person is known to have died from taking too much bismuth, though taking large doses for long periods can damage the liver or cause neurological problems.) While sharing some physical properties with lead, it's toxicity is so low that medicinal bismuth compounds - these days mostly bismuth subsalicylate - have been used as stomach remedies for centuries. (It is from bismuth that Pepto-Bismol gets part of its name.) Which makes it pretty unusual for a heavy metal. In fact, most of the toxicity associated with bismuth compounds comes from what it is compounded with. Though it was not scientifically described until 1753 it was known and used long before this.

Bismuth does have several unusual properties, though. As well as some interestng uses. Among other applications, in antiquity it was sometimes included with copper and tin to make a type of bronze used in knives.

In its pure state bismuth is a silvery grey metal with a slight pinkish tinge. (However, the color of the stomach medicine is due to an additive, since the actual color of bismuth subsalicylate is much less distinct.) Most isotopes of bismuth are unstable, with half lives running from five days to twenty minutes, so they are not found in nature. However, Bi-209 is either stable (which would make it the heaviest stable element known) or has a half-life so long that decay has never been observed. Recent research lends strength to the latter option, with Bi-209 estimated to have a half life of around 20 billion billion years. (Yes, that's many, many times the current age of the universe.)

Because of its low toxicity and optical characteristics bismuth has a number of cosmetic uses. For instance, bismuth oxychloride is a lustrous crystalline powder used in nail polish and lip gloss. Bismuth compounds have been also been used in non-toxic pottery glazes, replacing lead. A number of other uses also involve replacing lead or lead compounds with bismuth compounds.

Bismuth is fairly common in the Earth's crust. It is normally obtained as a byproduct of the refining of lead, copper, tin, silver, and gold ores found in Bolivia, Peru, Japan, Mexico, and Canada. The price is therefore both relatively low and very stable, even though only small amounts of bismuth are obtained from these processes. Fortunately, a little bismuth goes a long way in most uses.

Besides being dense and looking much like lead, bismuth also has a low melting point, actually lower than that of lead. However, where lead is ductile, bismuth is brittle. Bismuth also has unusually low thermal and electrical conductivities for a metal. Wood's Metal, an alloy of bismuth and cadmium, melts below the boiling point of water (note that cadmium is most definitely toxic). Wood's Metal is used in plugs for automatic fire sprinklers, and bismuth itself has been used in electrical fuses.

Another of the unusual properties of bismuth is that - like water - it expands slightly when going from liquid to solid. Although not itself magnetic, it can be alloyed with manganese to produce strong magnets.

Because of its high density (9.79, compared to 11.4 for lead, around 8 for steel and 19.3 for tungsten) low toxicity and low price compared to tungsten, it is a candidate for replacing lead in birdshot. While the risk to waterfowl from ingesting lead and lead alloy shot is as much theoretical as factual, using bismuth would address the concerns quite neatly. It might even make any birds which ingest it healthier! (Or at least give them better digestion.)

In short, bismuth is useful, and has low cost and few side effects however it is used.