We have a woodstove that sees a lot of use through the winter. We order wood 2 -3 times throughout the season, different kinds, differing degrees of dryness etc. We get our kindling from a friend who works as a woodworker in a shop that produces extremely high end custom cabinets using only the finest materials, quality woods, well dried. We are lucky that way.
We last ordered a cord of wood from a kid who had cut up a 500 yr old oak that had fallen on his grandfather’s farm. It was beautiful looking wood. The grain was so straight and clear it split with almost geometric squareness. It turned out to be a little green however, taking quite a bit of coals and lots of oxygen to keep flaming. It did throw heat though.
Now we’ve switched to a couple of cords of something else that we just had delivered. I don’t know what kind of wood it is, but it’s definitely dryer, takes much less to get going.
But I can’t help but notice that when I clean out the stove, next morning there are way, way more ashes. There isn’t any measurable difference in how much wood we put in, over the course of the night, but the difference in the amount of ashes is truly striking.
I was always struck when burning the oak, over an evening, how little ash there was in the stove in the morning. Much fewer than I’d have expected, some days I didn’t even bother to empty them.
So what’s going on here? Can you explain it to me, please?
My experience over many winters of wood heating is that difference species of wood definitely differ in the amount of ash. Walnut was the biggest ash producer that I’ve burned, by far.
There are folks who will pay you to haul away a 500 year old oak. It’s a shame to see that kind of good wood just go into firewood. That wood could have been used to make some beautiful furniture.
Anyway, hardwoods (oak, maple, ash, hickory, etc) are more dense and tend to have less sap and resins in them. They burn with less ash, produce less creosote in your chimney. They burn longer and give off more heat, but they also tend to be harder to burn. The reason the OP had difficulty getting it to burn wasn’t just because it was green (though that likely factored into it as well).
Softwoods (pine, fir, spruce, etc) are less dense and have more resins and sap in them, which means more ash and more creosote for your chimney. It also means less heat and a shorter burn for the same size log.
I’m surprised. I thought walnut was a fairly hard wood without a lot of resins and sap in it.
It’s hard to say whether hardwood or softwood is denser without knowing the species. Hardwood densities range from 384 oven dry kg/cubic meter for Eastern Cottonwood (Populus deltoides) to 1000 oven dry kg/cubic meter for Shagbark Hickory (Carya ovata).
Softwood ranges from 310 oven dry kg/cubic meter for Engleman Spruce ([Picea engelmannii]*) to 560 oven dry kg/cubic meter for Slash Pine (Pinus elliottii).
So while most hardwoods are denser than most softwoods, YMMV. The largest variation in wood ash is probably coming from the bark. Bark contains a lot of non-wood elements, including non-flammable elements. After all, bark is among other thing, the tree’s defense against forest fires.
Even the pitch content between softwoods and hardwoods are hard to generalize about. We’ve all seen the pitch running down the outside of a pine tree. The pitch in softwoods is in resin canals which can be readily seen. The pitch in hardwoods are encapsulated in pitch cells, called parenchyma. This pitch is not released until burned or pulped or otherwise extracted.
My experience with wood, and believe me, I have some, is that softwoods contain less ash than hardwoods. The ash in your stove is probably primarily calcium carbonate coming minerals in the original wood.
All trees contain minerals in different proportions, and although the total quantity is small, the differential amounts can end up having a wide range. I have hundreds, maybe thousands of samples of wood ash, but I’ll give some typical values from one handy source for some woods (note I’m only taking the wood ash, not the leaves or bark, which are all different):
And Maple on an oven-dried basis:
Woody parts: 1.2%
Bark: 9.7%
Leaf: 10.3%
Root: 10.0%
So you can see that not only can your choice of wood change the proportion of ash by a factor of up to 5:1 in just this small sample, but how much of the bark, leaf, and root included can change the ash production (in addition, small stems have more ash than trunk wood, typically on a 2:1 - 4:1 ratio).