Have you changed the source or type of wood being burned? Wood that is not toally cured or stored in area where it can gain moisture content can be the culprit. In addition, try building the fire near to glass. The blackened glass is caused by the smoke in the fire condensing with its moisture content onto the cooler glass.
The warmer you can keep the glass the better. Finally, we have better luck at limiting the carbonization by allowing more air nto the fire through the damper system.. Open the outbound damper (flue) as well as the one letting air into the firebox early on in the life of the fire, only dampening them down somewhat once the fire is well established with some blue coloring along with the yellow on the visible blaze. Best of luck!