Different woods and how they burn/what if we are unsure of the wood?

Question

I'm keen on learning more about different woods and how they burn in particular in the context of bushcraft. There is a lot of existing literature on all the different woods out there. I started to read up on the subject and quickly found that it's rather daunting. What I was able to gather was: There are so many wood types and there is a lot of variability in how they burn.

(for example: Hazel burns well, but Alder tends not to burn so well, and it just goes on and on)

In good time, perhaps I will make sense of it all. In the mean time, I hope that I can find some other general rules to follow until I have the proper experience to know for myself. I'm familiar with the hardwood/softwood dichotomy, but I'm hoping there is more to it.

Question

Are there any tells or heuristics I can use when evaluating a wood I am unsure of to gauge how well it will burn? By how well it burns, I mean things like burn-rate, smoke, ect. This way, even if I'm unsure of the exact wood, I can at least have a vague idea of what to expect from it. Or are woods just too heterogenous to generalize their burns?

Further Clarfications

• Let's try to hold environmental factors constant, clearly wood that is wet from a recent rain is different than if the wood is just naturally more moist
• other physical properties of the wood are fair-game, like: water content, hardness, structure, ect

Answer 1

It's hard to be absolutely certain without a specific goal for the firewood, but there are some generic things to look for:

• Sap: Sap-rich woods usually produce a lot of smoke, often black or dark grey in color due to incomplete combustion of the terpenoids in the sap. Classic examples include cedar and pine. Dried twigs from sap-rich woods can make good tinder if prepared properly to increase surface area, and the twigs can make OK (but not great) kindling, but large fires made of sap-rich woods are generally bad for cooking and also bad for your health (the combustion byproducts aren't generally toxic, but the particulates can cause long-term respiratory issues like COPD if you breath in lots). The smoke from some such woods can be very effective at keeping insects away though.
• Density: The denser a wood is, the harder it is to start, but the hotter and slower it will burn. Oak, apple, and cherry are all good examples of woods that burn long and hot, they make good fuel, but are sub-par for kindling and essentially unusable for tinder unless completely dry and started with a something like a magnesium fire starter. Especially dense woods that don't float are a special case here, they're often extremely difficult to start even with a fire that's burning well.
• Water: Wet wood burns with lots of white 'smoke' which is actually steam. New growth often is wet enough to burn like this too. If you can't find dry (seasoned) wood, start a fire using whatever the least wet wood you can find is, and use it to dry out other wood to use as fuel.
• Heartwood versus sapwood: Some species of tree have a marked difference between the inner and outer wood, usually showing as a difference in colors. In such trees, the heartwood (the inner wood) burns a bit differently from the sapwood (the outer wood), but the difference is often not enough to matter for most purposes.
• Things that look like wood but aren't: Bamboo is the easiest example, with palms being another. These aren't technically wood, and they generally don't burn very well, often producing a lot of smoke (just like grass does), but they can be hard to tell from actual wood. They do still burn though, so if you can find lots of bamboo for some reason, don't hesitate to use it.

There's also some more specific advice I can give:

• Certain trees should just be avoided because burning them is a safety hazard. A classic example of this is eucalyptus. The oil in the eucalyptus trees that gives them their distinctive smell has an insanely low autoignition temperature, and may cause eucalyptus logs to literally explode when burned.
• Avoid burning woody plants that aren't obviously trees unless you can reliably identify them and know it's safe to do so. There are a number of plants that when burned produce exceedingly dangerous smoke. Various toxicodendron species (commonly called poison ivy in the US) are a good example of this, the oil they produce that causes dermatitis on contact with human skin can survive the rest of the plant burning and end up in the smoke, which can make inhaling the smoke lethal.
• Always check your wood for plants or fungus growing on it, and don't burn it if you find any. Some fungi will release spores when heated (and the spores can be toxic), and some plants contain toxins that can survive burning and end up in the smoke (for the same reasons mentioned above about burning other plants). Other growth like this also impacts how the wood itself burns, usually resulting in inferior fuel wood.

Answer 2

In my particular area (Alabama, USA), I can mostly tell how well something is going to burn by the hardness of the wood, the density of it, and the general ratio of late to early growth in cut sections. If I cut wood, say a branch, with a saw or something where I can see the rings in the wood, I can look at the amount of late season growth (more solid wood) vs. early season growth (darker, typically less dense, sometime even porous to the eye). You can use a knife or in some woods even your finger nail and tell which rings are soft early season wood vs. harder/denser late season wood. The ratio of late season to early season is a good indicator of burn rate in hardwoods at least, and probably softwoods like pine and spruce and cedar.

Now, I do know how to identify the trees in my area, so that may factor into my general knowledge, but based on that knowledge here are a few examples:

• Oaks are a very hard wood, very dense, and typically heavy for their general size per piece. Red oaks actually have more porous early growth rings than white oaks; white oaks have sealed rings. I looked this up to verify (https://www.wood-database.com/wood-articles/distinguishing-red-oak-from-white-oak/), but have experienced this whilst making selfbows from oak. Red oak isn't anywhere near as strong as white-oak, and it's less dense as well in general. Not to say red oak won't make a good fire, it's what I burn as my predominate firewood at home and in the woods. But, white oak will burn hotter longer in my experience. It's just not quite as easily found around me, and it's harder to process with bushcraft tools. Any smoke from oak is typically white, and is a bit acrid to the pallet if you cook over it.
• Hickory is hard and heavy as well, though not as heavy as Oak typically is. But, it burns very well, and provides a good flavor for meat with a nice amount of white smoke unless it gets real hot, then no smoke. Makes a good selfbow too - but it does absorb water from the atmosphere more readily than other woods.
• Maple burns well, not as hot as oak but it's not nearly as dense either - you can tell just lifting it in hand - it's fairly light. But, it provides a mild white smoke that provides a good flavor to meat if cooked over it.
• We have a lot of Chinese Privet around, and it is very dense for it's relatively small stature. It burns well with little white smoke. I haven't cooked over it.
• Pines burn cooler and a lot smokier, with a thick black smoke, both due to being less dense I think, but also because they have resins in them that smoke a lot. If you mix pines in with oak, you can get a fire hot enough to burn some of that smoke off. I won't cook over pine as it will leave a turpentine taste on your food, and blacken it if you can't get the fire hot enough.

I forgot smell. If I can smell turpentine or rosin or some other kind of chemically smell, I can typically expect black smoke, which isn't good to cook over (see pine above). I don't burn much cedar since only eastern red cedar is native to my AO. But, I suspect at least eastern red cedar would burn poorly with a lot of smoke - although it smells WONDERFUL when cut.

Answer 3

Question said:

Hazel burns well, but Alder tends not to burn so well

Good and well very based on many variables...

As a solid fuel, Alder kiln dried logs are a good choice. However, because the tree grows quickly this means that it’s often not as dense as more popular firewoods, but it provides a pleasant, sweet aroma and burns well nonetheless. Source

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Hazel – good, but hazel has so many other uses hopefully you won’t have to burn it! Source

Question said:

Let's try to hold environmental factors constant, clearly wood that is wet from a recent rain is different than if the wood is just naturally more moist

All wood is naturally moist. With time it loses that moisture, the loss of moisture transitions the firewood from green to seasoned. See What is the energy difference between green and seasoned fire wood?

Firewood choice mostly comes down to two points:

1. Is it seasoned?
2. What is available in my area?

Point 1 = If you want less smoke and high heat; always pick seasoned wood.

Point 2 = It really doesn't matter what the best species of wood is, if it is not available locally for your fire. It is often illegal and generally unsafe to move firewood (dontmovefirewood.org) Kiln dried wood removes many concerns about transportation, but increase the cost and carbon footprint, decreasing it's appeal for many users.

The question does not say what benefit the firewood is intended to provide. That is the final key factor in making a choice: Heat, cost, easy of splitting, ease of starting, flavor (smoking), scent, clean burning. Each type of wood is going to have attributes that make it more appropriate for a specific choice in a particular location.