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Object lesson No. 12

No Seventies curriculum was complete without a few sessions of chalk and talk - your back to the class, notes on the board, copy this down and give them a good talking-to style of teaching.

Such dusty didactics died out with photocopiers, whiteboards and pupil-centred teaching methods. Now a chalked-up blackboard is more likely to be found spelling out multi-coloured menus in pubs and restaurants, and special offers outside off licences.

Blackboard chalk is only a modern manufactured version of the naturally occuring mineral. So many calcium carbonate deposits - the sedimented remains of marine animals - were laid down between 135 and 65 million years ago, that we call it the Cretaceous period, after the Latin word for chalk.

Most of it settled in Europe and North America, and as a result southern Britain is especially well endowed with striking geographical features, from chalk hill drawings such as the 3,000-year-old "white horse" at Uffington, to the Seven Sisters and the White Cliffs of Dover.

Pollution from shipping traffic and spoil from the Channel Tunnel excavations have been blamed for turning he subject of Vera Lynn's swansong a paler shade of white, but chalk's porous qualities - which gives small plants a foothold, further discolouring the famous cliffs - have also been its downfall. Rainwater seeping into the strata can cause chunks of the cliff to break off, and was the reason for last year's huge rockfall at Beachy Head. Chalk's soft, grainy qualities, as well as its whiteness, marked it out as a handy drawing material long before it became the chosen medium of pavement artists. But why is chalk white? Because calcium carbonate's atomic structure reflects all visible frequencies of light from red to violet, combining in the eye to produce white light.

In its refined state, chalk is used as a filler and pigment in ceramics, cosmetics, rubber, paper, paints and putty. As lime, it is loved by gardeners, and chalkstreams are a mecca for anglers. It's the chief ingredient in indigestion tablets, and pregnant women craving a calcium fix have been known to gnaw their way through a few sticks of the white stuff. Which makes it one of the most useful minerals known to man. Or woman. By a long chalk.

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