Boron

Category: metalloid Atomic number: 5 Colour: variable Melting point: 2,076°C (3,769°F) Boiling point: 3,927°C (7,101°F) First identified: 1732

As with beryllium, boron was known about for centuries only through one of its compounds, borax (which also answers to the names of sodium borate, sodium tetraborate and disodium tetraborate). Borax is a salt of boric acid, and comes in the form of white, soft crystals that dissolve when you put them in water. Historically, it has been used as a detergent, cosmetic, fire retardant, insect repellent, and by ancient goldsmiths as a flux: a substance added to metal to make it easier to work with. It was mentioned in Chaucer’s Canterbury Tales and was used in Elizabethan England as a cosmetic, mixed with egg shells and oil and smeared on the face to create the white foundation that was popular at the time.

In the medieval period, the only source of borax was crystallized deposits from a lake in Tibet – it was traded along the Silk Road to the Arabian Peninsula and eventually Europe. It became more widely used in the nineteenth century after further deposits were discovered, notably in the California and Nevada deserts – the Pacific Coast Borax Company sold a brand called 20 Mule Team Borax, named after the method they used to transport marketable quantities from the desert.

In 1732, the French chemist, Claude François Geoffroy (also known as Geoffroy the Younger), noticed that a peculiar green flame was produced if he treated borax with sulphuric acid to produce boric acid, then added alcohol to set this on fire. This test showed that boron was present, and it came to be used as the standard way of identifying the presence of borax – it was crucial in the discovery of the Death Valley deposits.

In 1808, the French chemists Joseph Louis Gay-Lussac and Louis-Jacques Thénard, and Sir Humphry Davy in England, managed separately to extract boron after heating borax with potassium metal. This wasn’t pure boron – that would only be isolated in 1909 by Ezekiel Weintraub in the USA – but it proved to be a brown amorphous solid with many useful chemical properties.

Life on Earth

While boron is present in some of the Earth’s oldest rocks, it is only found in its pure metalloid form in meteorites. Nonetheless, it plays a crucial role in the biology of our planet – it stabilizes ribose, which is theorized to have played a key role in the development of DNA. And plants simply don’t grow unless there are trace elements of boron in the soil – it is indispensable in the formation of plant stem cells.