Why Does Wood Make a Snapping Noise?

In the Fire

• Firewood snaps and crackles because of its moisture content. Much of a tree’s living weight stems from plentiful liquid in its tissues and cells, including the saps and resins used to transport nutrients throughout the plant and for other functions. Submitted to the heat of a fire, any moisture remaining in the wood might vaporize. Any trapped vapors might exert enough pressure for an explosive mini-burst. Particularly moisture-rich logs can spit embers and char vigorously during this conversion of liquid to gas.

Seasoning Firewood

• Because wet wood doesn’t burn efficiently, freshly rendered firewood should be seasoned, or dried, for some length of time before use. Moisture content depends partly on the species of tree and that tree’s specific environment, and thus the seasoning period will vary from wood to wood. It typically takes at least six months or a year to properly season firewood. This doesn’t eliminate all the moisture in the logs, of course, so some snapping can still be expected in the flames.

Wood Breaking

• The crack of wood being broken, whether cleaved by an ax or shattered by a large animal, stems from the shredding of its cellulose-based fibers. These fibers, constructed from block-like cell walls, entwine with one another along a common plane. The configuration of the fibers explains the strength of wood in accord with that plane, and its fragility, after a certain point, in the face of pressure against the line of the fibers.

In the House

• Many a homeowner is familiar with another snapping sound: that of wooden house beams settling in response to temperature variations throughout the day, and across seasons. Wood will expand with heat or increased humidity and contract with cold or dryness, emitting creaks and pops that -- particularly in the silence of night -- can conjure supernatural associations. A new house easing into its foundation, or one responding to ground settling, can also be quite noisy, and pop out drywall nails, develop cracks in its walls or get out of plumb in more severe cases.