In the U.S., the most common variety of cedar is red cedar, commonly found throughout the Midwest. This is the wood used in cedar chests, because it is very aromatic and has a strong oil.
The strong concentration of oil in young cedar wood, especially the heartwood of the common red cedar, means that the wood will burn very quickly. Often this will result in loud pops and sparks erupting from the fireplace as the oils burn faster than the wood itself. These sparks are extremely dangerous and can fly great distances, starting fires or burning your house. If you must burn cedar, use a fireplace screen and monitor the wood constantly to stamp out any sparks.
Old cedar wood must not be burnt in any situation, if possible. There is a possible risk of explosion. Although there are few incidents recorded of cedar logs exploding in fireplaces, in areas such as Oklahoma, dry cedar trees have been seen to explode when wildfires sweep through the area. Burning dry cedar wood is very risky and should only be done in areas where the sparks will not ignite any fuel.
Using Cedar in Your Fireplace
Despite the risks of using cedar logs, sticks of cedar have almost no risk if used as kindling. All the problems that make cedar dangerous to burn in large amounts make it highly effective for starting fires in your fireplace. Cedar twigs catch fire quickly due to the high concentration of cedar oil, and burn cleanly compared to other common woods such as pine. Finally, you get a hint of that cedar smell as the wood burns. It is worth keeping a few sticks of cedar around for kindling.