Crazed or cloudy stove glass
You may have noticed that your stove glass is not quite as clear as it was when it was new, particularly towards the bottom of the frame. This 'milky' effect, which usually starts with white spots, may appear to vanish when the glass is washed but it will quickly reappear when the glass has dried out. Unfortunately it is virtually impossible to remove this as the glass surface has actually been etched by an acidic residue produced by the incomplete combustion of fuel, either coal or wood. This residue will tend to settle at the bottom of the glass where the stove's airwash system, which is designed to keep the glass clean, is least effective – especially during long periods of slumber burning.
Incomplete combustion, which creates the acid (and which is actually sulphuric acid) is caused by deliberately starving the fuel of combustion air, either by closing down the stove's controls, for overnight burning for example, and thus effectively turning off the airwash; or from having an insufficient supply of air in the room as determined by current building regulations for the heat output of the stove.
There are many reasons, apart from fuel saving, why stove owners regularly turn down their stoves for long periods. The most common is that the stove produces far too much heat for the size of the room and therefore it is often turned down or alternatively it is operated with a fuel load much smaller than that recommended by the manufacturer in order to control the heat output. Whatever the fuel choice, both will result in incomplete and inefficient combustion of the fuel.
In addition in such circumstances, fuel with a high sulphur content, such as petcoke (petroleum coke) or manufactured products which contain it, will cause the most damage to the glass, as well as to other stove components, and therefore it is not surprising that petcoke is not recommended by virtually all stove manufacturers. Even wood logs contain some sulphur, and burning wet or unseasoned wood logs will always result in incomplete combustion, as well as provide the all-important water required to turn the sulphur into sulphuric acid.
Regularly cleaning the inside of your glass before you light your stove, avoiding fuels with a high sulphur content (and wet wood), as well as severely limiting how long you slumber burn without also burning on a high output with full airwash will all help to prevent crazed glass. However, if your stove glass is so cloudy that it is stopping you from enjoying your stove then perhaps it is time to replace it.
NB If acidic condensates are damaging your glass then it is highly likely that these are also damaging your flue liner, particularly at the terminal where the flue gas is at its coolest and therefore more likely to produce damaging condensation.