‘Tis the season for roasting chestnuts on an open fire and burning the yule log. That cozy blaze in the fireplace will warm little hands and feet that have been nipped by Jack Frost and dry out wet mittens left on the hearth. The flickering flames will fascinate the most doubtful and calm the most harried of us. Yet, as picturesque as all this seems, ’tis also the season for dangerous chimney fires.

The culprit is creosote. Creosote forms from a combination of burning unseasoned wood, incomplete combustion and cool surfaces. As wood burns, its by-products are released and expelled up the chimney. Creosote forms when these substances condense against the cooler inner surfaces of the flue as they travel through the chimney. Smoldering fires form the most creosote. Creosote may be black or brown, tarry and sticky, crusty and flaky or hard and shiny.

“It doesn’t take much creosote,” to cause a problem, said Allen Clough of 20th Century Chimney Sweep. “About one eighth of an inch of buildup.”

The danger of creosote is its flammability. Once an errant spark ignites it, the creosote continues to burn along the length of the chimney causing structural damage to the flue and possibly burning down the house. Of course, that pales in comparison to the injury or death it may bring to occupants of the house.

Depending on the type of wood being burned, that eighth of an inch of creosote might easily happen with as little as one cord of wood. Because the density of wood varies by type, firewood is sold in cords rather than weight. A cord is about 8 feet long by 4 feet wide and 4 feet high – 128 cubic feet. Giving allowance for spaces between the pieces of stacked wood, a cord provides approximately 90 cubic feet of burnable wood or the rough equivalent of two pickup loads.

Clough suggests dry oak, hickory and locust as excellent wood choices for burning in the fireplace. Hedge wood is good option for wood burning stove although it produces too many sparks to be safe for use in a fireplace.

“The two worst woods are walnut and elm,” he warned. The high amount of water in them and the chemical makeup of these woods seem to produce more creosote than other woods when they are burned.

The hotter the fire, the less creosote buildup. Small fires are another good way to minimize problems. Clough says anytime flames get past the damper it’s dangerous.

The only way to remove creosote from the lining of the chimney is by scrubbing it off. Clough attacks the problem from both the inside the house and from the roof. He uses a power brush with three loops of cable on one end and an extension to reach up the chimney. The brush is attached to a device similar to a drill to mechanically remove the creosote. He uses hand brushes from the roof to scrub the inner surfaces of the chimney. The soot that falls into the fireplace is then vacuumed. The entire process usually takes less than one hour.

Although homeowners may opt to clean the chimney themselves, they may be better off letting a professional chimneysweep do the honors. In addition to scrubbing the creosote and vacuuming up all the soot, including the stuff that falls behind the damper, the chimneysweep will also inspect your chimney for cracks in the liner or tiles and displaced or “melted” mortar.

Clough offers one last caution. “A lot of people think that a mild winter causes less problems,” he says. The opposite is true. “Warmer winters means the more creosote you have since the fireplace is not burning as hot.”

– Carol Boncella is education coordinator at Lawrence Memorial Hospital and home and garden writer for the Journal-World.