There is black "goop" which sticks and spots onto clothing. It is probably from setting the iron too high & ironing "permanent press" items. I have tried everything I can think of, but cannot get enough off to risk ironing anything I really like!!
try the liquid cleaner used on ceramic cooktops/stoves. it works, just be sure to clean the surface well prior to your next ironing (maybe iron a towel)
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Try fixfault.com to fix steam irons.