Manky iron question

[Philomena]

Dull question to follow:

I'm ironing my sheets and every now and again I get a brown residue on the sheets - sort of sand like - which appears to be coming from the iron steam holes.

What is it and what can I do about it? I keep cleaning it off the iron surface but obviously this does not prevent it happening.

Is there some iron secret I never learnt?

OK - more than one question!

I buy some chalky thing which you rub on it wjhen hot.Gets rid of it.
Hang on,I'll see what it's called....

Ah...Iron Cleaner!

Thanks moondog. What I want though is something to stop the sandy looking bits - I've got something that cleans it off the surface but I want to stop it happening altogether