Or copper (II) sulphate, the staple of every child's chemistry set and the source of that most wondrous of junior school experiments, growing deep blue crystals from a saturated solution. This week in south-east London, an artist, Roger Hiorns, has taken those classroom memories to a larger scale by sealing and flooding a derelict flat with thousands of litres of copper salt liquid, leaving walls and fittings encrusted as if they were a box of lapis lazuli jewels and its floors a strange maritime sludge. The effect of the artwork, Seizure, is all the stronger because of the artist's lack of control over the processes, the crystals forming in unpredictable patterns as the solution that carried their ingredients cooled. Copper sulphate is a simple compound - easy to manipulate from a pale powder in its dry form to the familiar blue wet pentahydrate salts - and a useful one too, for more than education and art. It helps wine production, controlling mildew in Bordeaux mixture. It keeps water clear of algae; preserves wood; purifies soil; limits the spread of bilharzia; regulates the setting of concrete; colours fireworks green; and is an ingredient in everything from printing ink to hair dye. It is toxic, too - though copper producers take pride in claiming that their metal is safer than many others. Over a century ago this paper regularly reported the price of copper sulphate at Bradford and Liverpool markets to assist mill owners; this week it was reported as art. Either way CuSO4 remains a prince among compounds.