Science corner

3rd February 2006 at 00:00
Ray Oliver explores a petrifying experience

Legends surround the petrifying springs that can turn everyday objects into stone. The truth is more prosaic. In hard-water areas, when water evaporates it leaves behind limescale - the material found in stalactites and furred-up kettles.

In Matlock, Derbyshire, there was a petrifying spring where hats, shoes and even birds' nests that were left for months would become covered with crystalline calcium carbonate - the chemical that forms limestone and chalk.

The same crystal magic can be produced in a few days, by placing absorbent materials in solutions of salts, such as alum (potassium aluminium sulphate). Victorian school children suspended spiders, beetles and grasshoppers in alum until they were covered in crystals. Nowadays, we might opt for flowers or corn instead.

For all of the crystal experiments you need a saturated solution of the material chosen. Stir the solid with 100ml of hot water, adding more powder until nothing further will dissolve. Try alum, sugar or magnesium sulphate (sold as Epsom salts). The saturated solution should be decanted or filtered while warm to separate any undissolved material.

In each case, immerse the absorbent material and leave in a cool place for several days. As the water evaporates, crystals will grow on the surface of the sample. You can cover metal objects with crystals if you first roughen the metal surface using emery paper. Try letters and numbers, or wire "people", fashioned from old coat hangers.

To produce crystals in a few hours, use a glass of warm saturated solution and a pencil. Stand the pencil, pointed end downwards, in the solution, and leave undisturbed. Both the pencil and the glass should grow crystalline coatings.

Modify this experiment by adding food dye or water-soluble ink to give coloured crystals, for example with sugar solution. You can use an overhead projector and a shallow dish to show crystals developing. Pour a layer of hot saturated solution into the cold dish, then add a "seed" crystal of the same material. Crystals start to grow around the seed and spread across the dish.

Ray Oliver teaches science at St Albans Girls' School, Hertfordshire

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