A glass-covered fireplace adds a high class and quaint accent to the hearth of your home. Whether it's a gas-fired fireplace or an actual wood-burning one, the glass doors can provide a protection from the direct heat and a clear viewing box. Because fires, and especially wood-burning, release carbon, which can blacken the surface, it's important to keep the glass clean as well as make sure it was treated for this high temperature environment.
Fireplace glass is often high temperature treated so that it can withstand the harsh heat environment. During the treating process the glass is layered with a compound that allows it to withstand the heat as well as resist soot.
If your glass is consistently getting dirty to a point that it is difficult to clean you may need a replacement. A fireplace store will be able to guide you through the process of potentially purchasing a new set.
The U.S. Department of Fire Safety recommends that glass fireplaces be kept open during usage. In addition to the safety reasons, it will avoid getting dirty. A mesh metal covering will protect from embers.
The blackened soot on the glass is residual carbon collecting on the door and is the result of a "dirty" burn of excess materials. Using hardwoods such as ash, oak, cherry or some firs (softwood) are the best options especially if they have been seasoned (dried for a year). These kinds of woods are cleaner burning with less moisture and other materials that can cause soot build-up.
If you are getting blackened doors from a gas fireplace, this means that some other substance is burning in there. Turn off the gas and clean out the area thoroughly.
Cleaning the Glass
Using a strong cleaning agent such as oven cleaner to clean the soot should work in most cases. Spray it on and leave it for an hour before rubbing it off. Follow any instructions for the product if they differ.
There may be other products at a fireplace store (likely a mixture of ammonia and other products) that are made for glass cleaning.