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. 
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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. 
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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. 
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There are three main reasons why you will get black marks on your stove glass when burning wood. All of them relate to a lack of heat in the stove at a crucial point in the burn cycle.

The One that happens with older stoves is when the stove door is not sealing against the stove properly, so as well as finding the stove isn't as controllable as it was you may also find the air being pulled in around the door is creating cold spots on the glass which allows sooty deposits to form on the glass. This can lead to more serious damage (see 'Why Can't I clean My Stove Glass?)to the glass and other components in the stove but can usually be overcome by squashing the stove rope between your fingers (when the stove is cold) to fluff it up a pit and create a good seal, or some stoves have an adjustment on the door to tighten the seal, or replace the door seal. Talk to one of our staff and they should be able to give you the best option.

The next is due to driver error. Turning the stove down too low. If all else is working properly, you have dry hardwood, and a good well maintained stove then the only thing left is the person controlling the stove. The thing that keeps the glass clean on any stove is the temperature of the glass. If it is hot enough then no carbon can stick to it because it is burnt before it has a chance. When burning wood and the stove is first lit it needs a minimum of 20 minuets to get up to temperatures, some need up to an hour. The stove should be left on full until all the carbon deposits inside of the lining have burnt off and the inside of the stove is clean. Only at this point should you start turning it down. When fresh wood is added turn the stove up a bit to get a good flame going then turn down. If your glass is going black turn it up a little as there is not enough oxygen in the stove to burn all the gasses so not only will they be sticking to the glass they will also be going up your chimney and potentially causing other problems. Again one of our staff will be able to give you more specific advice for your particular stove.

Now by far the most common reason for the blackening of stove glass is the dreaded 'wet wood'. Many logs when tested may have a moisture level of 15-20% on the outer surface but once split their internal moisture level can be up to 40%+. So the logs burn nicely for the first 10 mins then they get to a temperature that starts to boil the water and the fire is put out by the steam and just smoulders. At this point you will be tempted to open up the Primary Air under the logs, this will get the logs going!!! but don't , it will rip the carbon away from the log making them burn away too fast, it will fill your flue with sooty tarry deposits, it will burn out your grate, baffle, and firebricks, and can cause very expensive damage to your stove, not just blacken your glass. And to everyone who tells me they only burn  kiln dried wood, one look at the condition of your stove will always tell the truth. Although you may not be to blame as most wood suppliers have started marketing their wood as kiln dried wood, but from my own experience testing logs that my stove customers have had from other suppliers, the wood may have stood near a kiln or even been shown the inside of it but very few seem to actually get dried to a usable level in one. 

So if you are having problems with your stove glass please feel free to talk to one of our staff who will do their best to work out what the solution is.