Chromium

Category: transition metal Atomic number: 24 Colour: silver with a blue tinge Melting point: 1,907°C (3,465°F) Boiling point: 2,671°C (4,480°F) First identified: 1798

Siberian red lead (or crocoite) is an orangey-red mineral that was discovered in the eighteenth century. In 1798, the French chemist Louis Vauquelin showed that it contained a previously unknown element, which he would name ‘chromium’ (from the Greek word for ‘colour’), because of the beautiful and varied colours of its compounds.

Vauquelin did not expect his discovery to be of much use other than for decorative purposes, and to a degree he was right. Only a small proportion of the chromium that is produced is used in its natural form; chromium (or ‘chrome’) plating is used to give a shiny finish to steel (such as on some classic cars and bikes) and also to plastic household fittings. It is more commonly used in alloys or compounds. Steel and chromium are alloyed to form stainless steel, which develops a thin protective oxide layer rather than rusting as unalloyed steel would.

Chromium compounds create a remarkable variety of colours and shades for paint pigments: various types of chromium oxide, lead chromate, sodium chromate, chromium chloride and anhydrous chromium chloride produce shades of dark red, orange-red, bright yellow, light blue, and pale or dark green. Chrome yellow is a colour that was especially well known to generations of American children, as traditional school buses were painted with it so that they were easy to spot in gloomy conditions. (The traditional paint has since been replaced, as it contained lead and other toxic substances.)

Chromium also plays a rather beautiful part in the colouring of gemstones. Corundum and beryl are naturally colourless oxides, but if there is a tiny trace of chromium in the mix you get rubies and emeralds. The transformation of chrysoberyl is even more extraordinary: it is a colourless aluminate of beryllium but, if it contains a dash of chromium, it becomes alexandrite – a highly pleochroic gemstone (meaning that it absorbs different wavelengths depending on the direction of the light). The colour of a good-quality alexandrite specimen can change from orangey red to yellow to emerald green in different positions and lighting conditions.