Extremely stubborn dirt you may need to repeatedly treat succession, possibly soak even up to an hour.
Have a lot cleaner tried - this is the most affordable and best!
Addendum 31.3.2013: I actually still found a better. The Hagos fireplace glass cleaner is a gel that better solves the pollution and does not drip and without exposure. But is also a bit more expensive, but still economical.
I switched to this product and believe that it is much better.
