Blue Glass, Vitamin B12, and Invisible Ink

Among the treasures discovered in the tomb of Pharaoh Tutankhamen, who ruled Egypt from 1361–1352 BC, was a small glass object coloured deep blue with cobalt. Synthetic gems dating from 1400 BC have been found at Nippur, the religious centre of the Sumerians, and these too were coloured with it. Cobalt glass has also been recovered from a shipwreck of around this time. Cobalt blue was known in China long before then, however, and used for pottery glazes, but it was always a rare pigment because cobalt minerals were rare.

Cobalt is now more abundant and still used to colour porcelain, pottery, glass, tiles, and enamel jewellery. Its rich blue colour is also known as Sèvres blue and Thénard blue. Cobalt was particularly popular with glass bottle makers in the 1800s.

Curiously this element got its name by being cursed. When silver miners in Saxony in the 1500s tried to smelt what they believed was a silver-bearing ore they were disappointed and cursed the mineral, saying it had been bewitched by goblins and referred to it by the German name for these evil spirits, kobald. (It was in fact smaltite, CoAs2.)

Kobald was eventually to give rise to the name of the metal itself. In 1730, the Swedish chemist Georg Brandt (1694–1768) began to investigate a similar ore found in a copper mine at Vestmanland and showed it was a hitherto unknown metal which he christened cobalt.

Today cobalt comes mainly as a by-product of nickel production, and it is used in alloys for magnets, in ceramics, in catalysts, and in paints. Certain types of stainless steel contain it, including that used to make razor blades. One alloy of cobalt, chromium, and tungsten is known as Stellite and is used for heavy-duty high-temperature cutting tools. Some cobalt alloys are used in body parts such as knee replacements and in dentistry.

Cobalt can be magnetised, like iron, although it is less powerful. However, it has the advantage of maintaining its magnetism to much higher temperatures; iron demagnetises at 770°C whereas for cobalt the temperature of its so-called Curie point is 1130°C. Alnico, an alloy of aluminium, nickel and cobalt, is used for permanent magnets that are very powerful. However, in small appliances it has been superseded by neodymium-based magnets. Cobalt catalysts are used in the chemical industry to convert syngas, which is a mixture of carbon monoxide and hydrogen, into liquid fuels.

Cobalt is at the heart of vitamin B12 and in this molecule it is directly bonded to a methyl group, one of the rare examples of a metal-to-carbon bond in nature. Lack of this vitamin causes pernicious anaemia, a condition in which the body cannot produce enough red blood cells to transport all the oxygen that it needs.

Most animal species can make the B12 they require, but not humans. We get this partly from our food - sardines, salmon, liver, and eggs are good sources - and partly from the bacteria living in our gut. They produce it, but not quite enough for our needs. However, the average person can store 1–2 milligrams of B12 in their body and this is enough to last for more than a year, because the daily requirement is only around 2 micrograms.

In the years after World War II, when nuclear weapons were tested above ground, cobalt-60 contaminated the planet. Its half-life is 5.27 years. Then it was an environmental pollutant to avoid, but now this isotope purposely produced by bombarding cobalt-59 with neutrons in a nuclear reactor. Cobalt-60 emits intense gamma-radiation and is used in medical treatment and also to irradiate food to kill mites and microbes and to sterilise medical supplies. Irradiating diamonds with cobalt-60 colours them blue.

Cobalt chloride was once popular as invisible ink, and was first used in the 1700s. It consists of an aqueous solution of CoCl2 to which glycerol (then known as glycerine) had been added. Dissolved in water this salt produces an almost colourless solution (very pale pink) and was used to write the secret message. Once this ink had dried another innocent letter in ordinary ink was written at right angles across the paper. When the letter reached its destination, the recipient simply heated the paper and the hidden writing appeared. The heating drove off the water and glycerol molecules that surround the cobalt, and chloride ions moved in to take their place, giving dark blue CoCl42- ions.