Blackening glass is a result of the fuel not burning cleanly, see how do I get the best efficiency from my fire. There can be many reasons for inefficient combustion, clearly visible by soot and tars deposited in the appliance, including the following reasons. If a stove is operated with the controls not set as intended, then the fuel will be burning at below optimum. If a stove is “slumbered” for too long, i.e. operated with the controls closed perhaps due to it being too hot, then the fuel will not be able to combust cleanly. If the flue is not creating a good up draught then the same result will happen. Even the weather can have a marked effect on a flue’s propensity to flow hot air upwards, and stoves can react differently day by day depending on wind, temperature and air pressure. If the wood is not seasoned, then the moisture which has to be boiled off before there is any energy will cool the firebox and prevent efficient burning.
Modern solid fuel appliances have been tested and approved as being capable of burning the correct fuel to a high standard of cleanliness. If a stove is failing to achieve this, then the fuel, the usage and the air supply through the appliance need to be investigated.