Descaling your kettle without ruining it

Scale is calcium carbonate, and it comes off with any weak acid. The mistake is reaching for something far too strong and taking the finish off with it.

The dilution

Ten grams of food-grade citric acid per litre of water. Fill, bring to 80C, leave for twenty minutes, pour away. Vinegar works too, but the smell lingers for days and the result is no better.

The parts everyone forgets

• The spout interior, where scale collects into a ring that flakes into the cup.
• The temperature probe, if your kettle has one - scale on the probe makes it read low.
• The lid seal, which holds acid and will perish if you leave it there.

Rinse three times with fresh water. If the fourth boil still smells of anything, rinse again; nothing in this process is improved by hurrying it.

How often

In hard-water areas, monthly. Soft water, twice a year. If you can see it, you left it too long.