My Avent steriliser's heating plate shows white/brown spots

Note: In the UK you can find hard water with the equivalent of 200 mg of calcium carbonate (lime scale).

In hard water you also find a high mineral content which could affect its color.

Heating causes the dissolved calcium and magnesium bicarbonates to break down to form a solid carbonate lime scale.

Pure scale from hard water is white in colour, but scale can become coloured by traces of other minerals or metals.

For instance, traces of chopper in plumbing can turn it grey, green or blue while traces of rust from the water pipe network could turn it to brown or black.