Anyone who’s ever done campcraft firestarting, in the Boy Scouts or what have you, is aware that dry fall leaves, despite what one might guess by their appearance, are actually very poor fuel. They will just barely burn if tossed into an already blazing fire, making lots of smoke and mostly carbonizing. They’re useless for actually getting a fire going. Why is this? What makes the difference between leaves and the parts of a tree such as the wood that do burn well?
Because most of them aren’t really as dry as they seem. They’re more like green lumber - more or less dry to the touch, but still high in moisture content. If they crumble when you crush them in your hand, they’re really dry. If they just sort of crumple and break, they aren’t.
Our next door neighbor decided to burn his leaves this year, a particularly damp fall with less-dry leaves than usual. He stubbornly stayed with it even after most of the neighborhood was fogged out. It wasn’t a pleasant fall smell (I grew up burning smaller amounts of leaves, but in a dry climate) but that forest-fire choke. Sigh.
Besides the moisture I would think leaves are mostly carbon and not very high in hydrogens.
I think a burning pile of leaves may also have a poor oxygen to burning material ratio.
Haha. I live in the Pacific Northwest. The odds of a finding a dry fallen leaf around here is somewhere between the likelihood of a Florida blizzard and a leprechaun’s pot of gold. They’ll decompose into mulch long before they see the sun again.
But that aside, a truly dry leaf will crackle and crumble to pieces in your hand. If the leaves are that dry, they do burn well, about on par with paper. Like paper, large numbers of them also tend to compact down and restrict air flow, so a pile of them may or may not do more than smolder.
If you want to confirm the difference between a merely fallen leaf and a dry one, take some leaves indoors and see what they’re like after a week or two.
:dubious:
What does this mean anyway?
I thought it meant things like sugars, oils, fats etc. If I am wrong my apologies.
I would have said poor air circulation, if that’s what you mean. If you’ve ever tried to burn a book (we tried to burn a couple of textbooks at end of term one time) they burn lousy. You’d think with that nice dry paper they’d light right up but it’s more like a chunk of softwood on account of the zero air available to each sheet. I’m guessing that’s the same thing occurring with the pile of leaves: decent fuel, though still a little moist, but with no air available to reach anywhere but the surface. As opposed to a pile of wood which has those nice big gaping spaces to form a draft.
It means that nearly-pure cellulose like paper, cotton, etc. will burn but doesn’t have any really flammable oils, resins, et al like wood does. That was my first thought of why leaves seldom burn well, but it’s probably the moisture content as mentioned upthread. Thanks everyone!
That’s why you rip the covers off first… (old classbooks used to be a favorite material for burning in Midsummer Fires - many of my classmates later found out he had to buy it again, as he needed to retake that much-hated course).
I know this: we always burned our leaves when I was a kid. They went up just fine and it was no problem getting them to burn. The main reason why leaf burning was banned was to prevent them from setting the neighborhood on fire, and I’ve seen how easily a pile of leaves can go up.
It didn’t work as well if you threw a handful of leaves on a fire, but take a pile of leaves and touch a match to them and they’d go up like newspaper.
Hydrocarbons! That’s the word you guys are looking for.
And most hardwood isn’t loaded with hydrocarbons; it’s varying proportions of cellulose and lignin, with higher lignin woods burning hotter.
Ever burned mesquite? That’s an example of a higher lignin wood. It burns HOT relative to other kinds.
It’s almost certainly the moisture content of the leaves causing them to smolder as opposed to bursting into flames; really truly dry leaves WILL flame up pretty insanely.
While leaves may be no good, a bunch of pine needles will really flame up. Burning the old Christmas tree has become a bit of a tradition in my family.
Volatile Matter would be the correct word. See table 2 in this publication.
Been many years now that we’ve been able to burn leaves here in the Chicago area, and, boy, do I miss that smell. When we travel and happen to drive through a waft of burning leaves, I’m transported 60 years back. Progress isn’t all it’s cracked up to be.