If the glass turns milky white and will not clean
Generally there has been direct impingement of an intense flame onto one area of the glass causing it to be superheated above its normal tolerances so changing the molecular structure of the glass.
This can often be seen where the primary air, under grate air, has been left open when burning wood or left too wide open when burning smokeless coal or anthracite forcing a flame directly onto the glass. It could also be caused by an ash pan door seal having failed and needing to be replaced.
This milky white area cannot be cleaned away and will gradually get worse.
If the glass has Fine crazing lines
This is caused by sudden thermal shock on the face of the glass panel. It mainly occurs when the door or glass seal starts to leak in air. The seal may look good however they may not be forming a strong airtight seal. As the flue and stove heat up so does the surface of the glass, the flue draught increases and suddenly the seal allows cold air in across the face of the glass causing a sudden and damaging change in temperature.
Ensure that the seals in the door are well maintained to keep a good airtight seal to avoid the air leakage.
If the glass has become pitted and crazed
Burning house coal or smokeless coal with a high petroleum content can damage the glass by eating into it. They contain high levels of sulphur and when sulphur burns, it turns into sulphur dioxide. If this gas mixes with moisture, such as if the coal is damp or is burned with wood, the result is sulphuric acid, a highly corrosive acid as found in car batteries. The higher the sulphur content in coal, the more sulphur dioxide is produced and in turn the more potent this boiling hot acid becomes.
The excess heat produced by smokeless coal with high levels of petroleum coke, a by-product of the petrochemical industry, can also be so hot that it melts the surface of the glass causing it to bubble and pit.
We strongly recommend that you take advice from your smokeless coal supplier or seek advice from the Solid Fuel Association before using an unknown brand of smokeless coal. It may be cheap to buy but the repair bills to the stove may be a lot higher.