Originally posted at: http://www.structuretech1.com/2012/07/dirty-glass-at-gas-fireplaces/
After years of use, gas fireplaces often develop a cloudy white haze on the glass. This white haze makes the flames difficult to see, ruins any potential illusion that you're looking at a real fireplace, and really kills the mood. Ok, maybe that's a little dramatic, but it is annoying.
The good news is that you might be able to clean this off. To clean the white haze off of your gas fireplace glass, you can start by purchasing a product that's made specifically for this, such as White Off. I picked some up at the Minnetonka Fireside Hearth & Home store for about $8. It's a small bottle, but for how often you'll use this stuff, it's a lifetime supply.
The next step is to remove the glass on your gas fireplace. This is probably the most difficult part, and every gas fireplace is different, so I can't offer much advice here. You can try looking up instructions online by searching with your model number - just pop open the bottom access panel, clear away some of the dust, and you should be able to locate the model number. You'll obviously want to turn the gas off to your fireplace first, and allow the glass to cool down before removing it, if necessary.
After you have removed the glass, place it on a flat surface, such as the floor or a table. You might want to lay down a towel underneath for good measure, but I chose to live life on the edge while cleaning mine. Next, follow the directions on the bottle; shake well, apply liberally, clean with paper towel, etc... repeat as necessary.
Once you're done, the glass should look much better and all of the white stuff should be gone. If the glass still looks dirty once you're done with this step, there's probably no cure. Have you ever noticed how the glass on your gas fireplace will get a bunch of condensation on it when the pilot is lit after having been turned off?
This is happening because the warm, moist combustion gases are coming in contact with the relatively cool glass. As soon as the glass warms up, the condensate disappears... but over time, this repeated exposure to condensation can be enough to permanently etch the glass. Once this happens, the only repair is to have the glass replaced. Replacement glass panels for gas fireplaces range from $200 - $400, according to Ken at Glowhearth.
Reuben Saltzman, Structure Tech Home Inspections