When your brick fireplace chimney is being built the mason stacks flue tile on top of each other until it is high enough to exit the chase. What happens is this; some masons don’t pay much attention to this part. A flue tile is laid down and “buttered up” with mortar on the top. A lot of mortar is used to ensure a proper seal. Then the next tile is laid on top of the mortar bed. The weight of the flue tile presses down on the mortar and squeezes the excess out of the mortar joint. Not all comes out but enough that it needs to be scraped off and cleaned up. Some masons neglect to smooth the mortar on the inside of the flue. It is extremely rare for this buildup to cause an obstruction in the chimney but instead of being very smooth and nothing to impede the smoke these places give the creosote a place to hang on to and possibly buildup causing trouble.
What can be done about them? We do not recommend chipping them off for two reasons. First they are very hard sometimes and might not chip off at the surface. Chipping might cause the mortar to break out and in effect leave a gap or hole in the chimney. This will allow gases and fire if it ever were to erupt in the chimney, a place to escape and endanger the house and its occupants. Second, if we were to chip it off there is a great possibility that a flue tile could be cracked and that would require the entire chimney be taken apart to replace that one tile. NFPA code states that the flue tile must be finished smooth on the inside so as not to impede combustion particles exiting the chimney. Is that always done? No. Rare is it that anyone pays attention to this matter….more of the out of sight out of mind…saying.
It is dangerous to try to correct this matter after the mortar has dried. A much easier and safer method is to regularly clean your chimney. This eliminates the hiding places for the creosote and makes for a safer chimney anyhow.