T Landreth, of Cromwell, asks :-

I have seen advertised objects, containing magnets, which are reputed to stop lime scale forming. Can this be true?

Dave Warren, a chemist at the University of Otago, responded.

There have been many suggested remedies for the problem of lime scale going back to the first steam engines and most have proven to be ineffective.

Early last century glass spheres (called scale buoys) with mysterious gas contents were claimed to prevent lime scale forming if they were floated in a water supply. Rutherford brought a set to NZ in 1925, the buoys had gases sealed inside along with a small pool of mercury. When shaken the mercury broke into drops and friction charging caused the gas to glow, all very pretty but ineffective on lime scale.

Now we have magnets! I admit to being skeptical when I first came across the idea. However, after reading around and although I can find no convincing scientific explanation, I have been forced to concede that magnets do seem to prevent lime scale. A possible explanation is suggested by the one consistent fact that appears when reading about this subject; the treated water frequently appears to contain many fine white crystals. In fact some engineering firms don’t recommend the use of magnets on solar heating systems because these crystals are known to block the narrow pipes they use.

So, magnets would appear to work by altering the physical form of the calcium carbonate solid, producing fine crystals that don’t form deposits. Although calcium carbonate is not magnetic and shouldn’t be affected by any magnetic field the calcium and carbonate ions dissolved in the water are charged particles and their behaviour when forming a solid might be influenced by a magnetic field, resulting in the small crystals rather than lime scale.

Whatever the scientific reason, no-one appears to have identified the reported fine crystals and until that is done the answer to this intriguing matter is going to remain unanswered. Science Fair project anyone?