Keeping stove windows clean is a complex subject and varies for each appliance, type of fuel and other operational differences such as overnight burning. All of which can then be affected by chimney performance.
Most modern stoves have an "air-wash" built in. This is basically a curtain of air that is desigend to flow across the surface of the window and stop any carbon being deposited onto the glass.
Air-wash works best when the appliance is fully up to temperature, with the heat from the fire being the main driving force of the air across the glass. However when the stove is first lit or when all the air controls are shut for overnight burning the airwash is significantly reduced. It is at these times when the deposits of combustion materials, condensation or sooting most often occur on the glass.
On start-up, the window is cold. Water vapour escaping from the burning fuel (even the most seasoned wood will still probably contain 15-25% water) reaches the cold surface of the glass and condenses, this vapour normally carries with it a waxy like substance which once deposited on the glass acts like a magnet and deposits more and more onto the glass.
However once the stove has heated up these deposits burn away relatively quickly, with more stubborn carbon deposits normally burnt off at a higher temperature.
But inorganic oxides produced are often left behind and then burnt onto the glass, the process repeats itself with more area for further deposits and this can quickly escalate once the process has begun.
Prevention Not Cure
On shut-down, or for overnight burning the air-wash control is normally fully closed. The fuel burn rate drops and inefficient and incomplete combustion can occur. The speed of the air in the air-wash is now too low to carry away any of the heavy combustion products which will lead to them being deposited on the glass.
Unfortunately it's not a case of stopping it, as once the process has started it becomes a landing spot for more and more deposits to land on the glass, its more a case of preventing it. .
To put it simply, the cleaner the glass is kept and the more air-wash present during operation, the cleaner the glass will remain.
Other helpful precautions include:
Not burning wet or unseasoned wood.
Ensuring the air-wash is open at all times
Not overfilling the grate
Allowing the last refuelling of the day to burn on high for half an hour or so before turning the air wash down.
In short, it's the common-sense factors that can ofter prolong the clear view.
Should the window become more heavily marked then a specialist ceramic glass cleaner is available from most local hardware or DIY shops. Followed by a water wash and thorough drying.
It is best to avoid traditional acid based window cleaners or abrasive oven cleaners, as well as the traditional meothod of newspaper dipped in spent ash, as fine abrasive ash can damage the surface of the window. Actually making it easier for deposits to build up in future.