Roly Hayes of Dunedin and T Landreth of Cromwell asks :-

What is the best way to stop crystals forming in urinals? They are hard to remove, is an acidic or alkaline solution best, and do magnetic preventers work?

Alan Happer, a chemist at the University of Canterbury with an interest in industrial chemistry, responded.

The ‘crystals’ that are causing the trouble are probably what is commonly known as “lime scale.” This is not usually a major problem except in areas where the water is hard, and even then is most commonly encountered in cases where the water is heated. The scale probably consists mostly of calcium carbonate.

In other words what you have actually been doing is creating limestone stalactites and stalagmites on a small scale. Calcium carbonate is soluble in acid, so treatment with a weak acid such as white vinegar should solve the problem, although it can still be hard to shift. There are commercial products available, but most are also acids. The problem is that extended contact of a solution of the acid with the scale is needed and urinals are not well designed for this purpose.

In sports clubs one way of minimising the problem that I suspect has not been considered, is not to open the bar.

I would be wary of advertisements for a little magnetic device which is claimed to prevent the formation of lime scale by placing it in the cistern of a flush toilet.

I had not heard of this method, but a search of the web under “lime scale” turned up claims of success using a device such as you have described. The mechanism by which it is claimed to operate may impress a layman with no scientific background, but unlike the cistern doesn’t hold water. I seem to recall that a similar device was marketed in the past to help your car use less petrol. It could be the same thing being recycled. And in the days of steam engines all sorts of shonky devices were promoted as a cure for lime scale.