Editor’s note: We’ve long been fans of the designer and entrepreneur Peter Buchanan-Smith’s work, including Best Made Company, where we were honored to sell our skillets. Peter’s excellent new book, Buchanan-Smith's Axe Handbook, is the result of his obsession with “objects that are simultaneously ordinary and extraordinary.” Below, we excerpt Peter’s deep-dive explanation on the dynamics of firewood, and how to select the best fuel for your grill, firepit, campfire, or fireplace. For more from Peter’s book, check out this article on what makes “good tools.”
Hardwood vs Softwood
Wood is commonly distinguished as hardwood or softwood. But these designations are misleading. The definition of these categories lies not in the actual hardness of the wood but in the trees’ places in evolution. The hardest softwood is actually harder than the softest hardwood. Hardwoods are mostly deciduous. And softwoods are mostly coniferous; they are typically used for economical applications, especially construction, due to the rapid growth of softwood trees and the ease of milling.
The structure of hardwood and softwood is inherently different, resulting in distinct behavior when chopped or burned. Softwood, being the more ancient of the two, has a simpler construction with long, thin, tubular cells. The hardwoods are much more complex, with less-fibrous, less-uniform cell arrangements, and a denser construction. Softwoods are typically lighter and less dense than hardwoods. They split very easily given their simple construction, and handle easily with their lighter weight. Hardwoods are much tougher to split, given the disruptions in their cellular structure caused by vessel elements, and a much more tightly packed cellular arrangement. The effort is well worth the payoff, though, because hardwoods are vastly superior for burning.
Softwoods also contain large resin canals that hardwoods lack. While pine or cedar is fairly easy to chop and ignite, they don’t make a good fire. The lower density and high resin content of the wood means it burns hotter and faster, with lots of crackles and pops. The burnt resin will lead to a buildup of creosote in a chimney, which can have potentially dangerous consequences, or at the very least lead to the need for more frequent chimney cleanings.
Due to their ease of ignition, softwoods can be chopped into kindling and kept by the stove or fireplace to help get a fire going quickly. As long as softwoods are not the bulk of the wood fuel, they’re excellent for starting or reanimating a fire. Hardwoods, the denser of the two, take longer to get up to the temperature necessary for ignition, but once lit will burn longer with steadier heat. Whether burning wood for heat or cooking, the hardwoods will pay off considerably when it comes to heat produced per cubic foot.
Heat produced is typically measured in British Thermal Units (BTUs), which is the heat required to raise BTUs 1 pound (455 g) of water by 1°F (0.56°C). For a quick com- parison, applewood produces about 26 million BTUs per cord, while white pine produces about 14 million BTUs.
Green wood, or wood that’s been freshly chopped, is the most difficult to burn, and will give off loads of steam and smoke if added to a fire. This is why firewood is seasoned, or dried over the course of at least six months.
If only green wood is available, the wood can be “feathered” by making a series of slashes along the length of the wood. This maximizes surface area, to speed the process of evaporation. As a general rule, the denser the wood, the longer it takes to dry, so cedar kindling will be ready much faster than ash firewood. Drying occurs fastest at the ends of the logs, which is why firewood is stacked to allow airflow across the cut ends. The majority of the water bulk will evaporate quickly, but the water trapped in the wood cells takes time to escape. The firewood pile should be covered as protection from rain and snow to prevent the logs from reabsorbing moisture. Drying time will vary depending on climate, but even in northern regions, wood cut in spring should be dry and ready to burn that winter. If a moisture probe is used, firewood is ready to burn when the moisture content drops to 20 percent.
The Chemistry of Fire
Wood is comprised of primarily cellulose and lignin. Cellulose is basically the molecular fiber of wood, while lignin is the natural “glue.” Also present are various other chemicals such as oils or tannins that give each wood species its distinctive color and aroma. When wood burns, it’s not the wood matter that’s going up in flames but the gases released by the wood. This is why we observe the shape of a campfire with flames licking upwards, rather than wood brightly radiating in the shape of the log. Wood begins burning when it reaches the temperature of ignition, around 572°F (300°C). The cellulose begins to decompose under heat, or pyrolyze (pyro = fire, lyse = cut or break). As the chemical bonds of cellulose break, flammable gases are expelled from the wood. These gases spontaneously combust in the presence of heat and oxygen, releasing more heat, causing more wood to burn and more gases to combust.
The process is a feedback loop, which is why fires persist as long as there’s fuel—campfires don’t suddenly cut out unless deprived of fuel, air, or heat. As the wood pyrolyzes, it’s reduced to char, which is essentially pure carbon, and how we get charcoal. Minerals present in the wood, such as calcium and potassium carbonate, that do not burn materialize as ash. Molecules that are released as gases but don’t combust form a cloud of particulates we call smoke. Charcoal, which is mostly lacking in secondary compounds, does not smoke for this reason. The chemical compounds in smoke are often toxic, which is why wood- stoves that encourage complete combustion or open-air campfires are preferred to fireplaces.
Hardwoods as Fuel
Choosing the right wood to burn is as important as being able to start a fire at all. Most folks know it’s best to reach for hardwood over softwood, which is a good general rule, but exceptions exist. For the most part, hardwoods (from deciduous, broad-leaved trees) are denser woods without the resin content of softwoods (from evergreen conifers). As a material, wood generally burns at the same heat pound for pound, but not all woods have the same burn quality. The denser hardwoods are harder to light, can be harder to split, and, of course, weigh more, but burn with more heat per log. This makes them more ideal for the bulk of the fuel, as wood is usually measured by the cord (volume), not weight. Get a fire going, and hardwoods such as oak and ash will sustain you through the winter.
Softwoods as Fuel
Softwoods, on the other hand, are much less dense, and thus require more logs to produce the same amount of heat. They burn quickly and tend to be smokier and spark more often, leading to more air pollution in your home (if burning in a fireplace) and more buildup in your chimney. If softwoods such as cedar or pine are harvested, they make for great fire starters. Split up into kindling and keep a bundle by the stove for a lively, crackling start to every fire, or to renew the coals from the night before. However, there are some deciduous hardwoods such as cottonwood or quaking aspen that don’t have the density or clean-burning properties shared by most hardwoods. Conversely, Douglas fir is an abundant softwood in the American West that burns quite well. As species vary across the world, there will be optimal choices for every county. When I bought my property in the Catskill mountains, one of the first things I did was hire a local forester to walk the property with me, and this small investment was one of my best. He taught me almost everything I needed to know about the trees on my property, including the best practices for not just harvesting them, but for being a steward of the land.
Best Practices for Burning
Keep an eye on the chimney while the fire roars. Until it’s really going, almost any fire will smoke, but once it’s caught a good hot fire burns cleanly, without heavy smoke billowing out the chimney. The hardwoods in the “Best” and “Good” categories will be less prone to smoky fires, but any large logs left smoldering in the stove will smoke profusely. Get the firebox up to temperature, and the fire will be easier to maintain and will burn cleaner. Be sure to burn dry wood only. With excessive moisture in the logs, heat will be absorbed in the process of evaporating the water trapped inside, rather than being converted to combustion heat. The resulting fire will be smoky as well. Make sure the wood has had sufficient time to season. When purchasing firewood, handle the wood to be sure it isn’t excessively heavy, and look for cracks at the ends of logs that form as wood dries. In time, you’ll find what woods are abundant in your area, and what splits and burns to your liking.