If the temperature of your fire is too low, then you might experience a build-up of tarry soot in your chimney. The combustion temperature will be too low if there is not enough air supply for your fire compared to the amount of wood. Tarry soot is easily recognised by its brown sticky coat. You can supply more secondary air to increase the temperature of your fire.
You can also place a stove thermometer on the lower part of your flue pipe, which will indicate if there is an optimum air supply for your fire. The flue gas temperature should be approximately 250°C. If the flue gas temperature is considerably lower, then tarry soot may be created in the chimney. If the flue gas temperature exceeds 250 – 300°C, the inside parts might be superheated.