I think the smoke condenses on the cooler glass. I think some stoves have inner baffles you can put in that deflect the combustion away from the glass. My old Gold Marc "Monticello" has a small glass that has always done the same thing.

my insert does the same thing. i just clean teh glass once a week or so, as needed. i use Greased Lightning and it takes it right off, though make sure the glass is cool first! you can also use a razor blade scraper to remove it quickly.

We just had a Hearthstone wood insert installed, I haven't used it yet but the installer said the glass will get dirty if the fires are not hot enough. He recommended a thermometer ontop of the stove to ensure that the fire is the right temp (400-450 degrees).
I'll let you know how we fare once we get it lit...