Well, maybe I can help explain why the viewing window gets dirty and this is just what I've come up with. When you start a fire, or reload, the moisture in the wood evaporates and turns to steam. At the same time, the wood releases smoke. Some of the smoke & steam will go out your flue, some will be blown down your viewing window by your air wash system (I'm sure you've seen your smoke whip down in front of your window). That's your air wash system trying to create a barrier to prevent this moisture rich air from hitting the glass. It's not perfect, any of that moisture rich air that hits it, the water condenses on it. Your air wash system has another trick up its sleeve, constantly blowing non-moisture rich, heated air over it so any that does condense, does not stay too long before it evaporates. So, why the dirty window. When water condenses on your window, while it's there it's collecting any smoke particles that blow across it until it evaporates. After it evaporates, the particles it's collected in the meantime stay on your window and I don't know if they burn later or not, regardless it turns it black.
So, understanding the cause and reasons there are three things to focus on. Moisture, smoke, and air flow. Let's start with moisture, the drier the wood, the less moisture can condense on your window, the faster it warms up, the cleaner it stays. Simple Enough. Also, you want to try to hold off turning down your air until you're sure your unit is in full swing. I normally wait until all the skins of the wood has turned into a flame ball, afterward I then turn down the air. Being to hasty to turn it down the unit is cooler, the air wash blowing less and cooler air, moisture stays on the window longer, and I create more smoke & particles for it to collect.
Next, is to minimize your smoke. The less smoke, the less particulates the water can collect. If you can learn/do the top down method of lighting a fire, instead of bottom up, it has a double benefit. First, it minimizes unburned smoke & particulates. Second, it gets the air pre-heat channels warmed up faster than the bottom up method so moisture on your glass doesn't stick around as long, nor has as much smoke and particulates to grab. Also, when you reload it helps to empty the ashes just in the front, and then move all your glowing embers and coal to the front and load on top of it. That focuses the heat, and you only have 1 log that's smoking (or just the ends of logs if you load front/back) and in no time almost instantly ignites reducing smoke. If loading side/side, it helps to make it two piles of coals & embers in the front with a space between so the air wash system will blow air through that hole and spread the flames into the middle of your pile.
Lastly, watch your air flow. Moisture predominately comes out the ends of the logs. If you load your logs front/back I've noticed on my unit (some are designed better than others for front/back) the moisture comes out the ends and there's not much distance for it to travel to condense on my viewing window. Having rather wet wood and loading front/back covers my viewing window in moisture which dirties it right up. Loading side/side has really helped, as the moisture coming out the ends of the logs hit the sides of my insert first and condenses there instead of having a direct path to my glass. The problem with loading side/side is, it's for the experienced user as it is not as easy as front/back and takes getting used to. It also extends your burn since the air flow isn't freely blowing across all your logs.
Anyway, that's what I've noticed and the things I've done to reduce it. It's not perfect, I used to have to clean it once a week now I'm about once a month, sometimes even two. But, if a log rolls into the glass (one of the problems loading side/side) and it doesn't happen often anymore, that dirties it right up. Wish my unit had andirons.
