Nothing can make firewood sales go up quite like temperatures going down. But University of Georgia scientists say buying firewood to burn right away can lead to problems if you don't get seasoned wood.

It isn't always easy to tell if firewood is dry enough to burn well, said Julian Beckwith, an Extension Service wood products specialist with the D.B. Warnell School of Forest Resources at UGA.

The best indicator, he said, is weight.

"When firewood is cut, it holds a lot of water -- up to 50 percent of its weight," he said. "In fact, one fresh-cut cord of oak firewood can contain nearly enough water to fill six 55-gallon drums."

In a wood-burning stove or fireplace, that wood has to dry out before it will burn, he said. And boiling off the water steals a lot of heat away from the house.

"The critical word when buying firewood is 'seasoned,'" Beckwith said. "Seasoned means the wood has been dried to a level that will allow it to burn easily, and to give up a high proportion of its heat value."

Because of the water in it, unseasoned wood is heavier than dry wood. If you don't know whether your firewood is seasoned, Beckwith suggests comparing its weight to seasoned wood of the same type. Use a bathroom scale to weigh a fixed volume, such as a cardboard boxful, of each.

There are other signs of wet, fresh-cut wood.

"Split a fireplace log and look at the split surfaces," Beckwith said. "Recently cut wood will have a darker, wet-looking center with lighter, drier-looking wood near the edges or ends that have been exposed since cutting."

Wet wood will be easier to split than dry wood, too. And when firewood is very fresh, he said, the bark will be tightly attached. Bark on very dry logs usually can be pulled off easily.

Pound for pound, all seasoned firewood produces about the same heat, Beckwith said, although pine may yield slightly more heat per pound because of natural resins in the wood.

But woods vary greatly in density. Oak and hickory logs weigh more than sweet gum or pine logs of the same size. So it takes more pine or sweet gum logs to produce the same heat as oak or hickory.

Beckwith said the gum-like resins in pine wood lead people to think pine produces more residue or buildup, called creosote, than hardwood. But it doesn't. Burning any seasoned wood in full, hot fires will avoid creosote buildup.

"Creosote buildup on fireplace or wood-heater walls, chimneys and flue pipes," he said, "seems more a result of burning wood at relatively low temperatures."

When wood is heated, he said, some of its chemical ingredients are first changed to gases and then ignited if the fire is hot enough. At temperatures too low for them to burn, though, they become part of the smoke.

"If these gases contact a cool-enough surface, they condense back to a liquid or solid there," he said. "Over time, they form a thick layer of creosote that a hot fire can ignite, causing a dangerous chimney fire."

Filling a wood stove at night and closing the damper to reduce airflow can keep a fire burning slowly until morning. But it can also help creosote to form. So can building little fires just to "knock the chill off."

"Burning wood that hasn't been seasoned long enough favors creosote buildup, too," Beckwith said, "because evaporating water cools the burning process."