Softwoods like hemlock, spruce, pine and cedar contain large amounts of resins and pockets of moisture. When the wood burns, the heat causes the moisture to vaporize and expand, which cracks open the wood, causing the popping and crackling that is heard in campfires and wood burning fireplaces. Some hardwoods such as black locust, sassafras and sumac will also pop and crackle when burned for the same reason.
Season Firewood for Home Use
Large pops and cracks that occur when a fire is burning create a fire hazard as sparks jump out of the fireplace. When choosing wood to burn at home, firewood should be seasoned, or dried outdoors, for at least one year to reduce the moisture content to less than 20 percent. This reduces the amount of popping and flying sparks. A metal or glass fire screen is also recommended to increase safety.