These black or brown deposits are caused by iron (with a bit of lime). 

It is not dangerous, although it looks a bit dirty, the (metallic) iron is only ingested in very small quantities.

It is difficult to remove. The lime can be removed with a lime remover (available at chemists), the iron with an acid (for example with lemon juice or cleaning vinegar). 

This does require some patience. 

Alternate between a night in the limescale remover and a night in the vinegar. 

Gradually it will become cleaner. You will probably not get it completely clean, because the plastic of the kettle will also discolour over time due to the iron.

If you use tap water, you might want to consult your water supplier, because an increased presence of iron could also be due to the pipes of your own system. 

Try, for example, making hot water with bottled water for a while. 

Then you can draw your conclusions and take the necessary action. 

If it is iron from tap water, it should already be slightly brownish when it comes out of the tap.

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