Either it doesn’t have a decent airwash system, or maybe you aren’t using it properly? Or, more likely, it could just be down to the quality of the wood. If wood has a high moisture content (ie. not properly seasoned), when you burn it in a stove the moisture will leak out of the wood, in the form of a tar like substance. This ‘tar’ is carried by the flue gases, solidifying as soon as it makes contact with a hard surface .
This means that not only is your glass blackening up, but also ‘tar’ is being deposited on the inside of your flues and liner. Not a good move. If you are having a problem with blackened glass on your stove, give us a call – we’ll help you ascertain what the problem is and talk you through the solutions.