Thursday, January 20, 2022

Black Locust as firewood


Sprite and I share a fence line and we have been working together to clear it of Black Locust (Robinia pseudoacacia) trees.

I have been keeping the fence-post size pieces and cutting up the other into stove-wood for Sprite.

I was curious about how much moisture was in the wood. Consequently, I asked Sprite if she would mind if I kept a couple of 7" diameter chunks. She was fine with that. The outside temperature when we cut the wood today was 20F so most of the sap had been driven back down to the roots.

I split the rounds into quarters, then I cut one of the more typical looking quarters into a 5.25" long sample. The triangle measured out as 0.0185 cubic feet and weighed 636 grams (1.40 pounds) or 75 pounds per cubic foot.

The reason I cut the sample to 5.25 inches of length was so I could fit it into the microwave for drying. The protocol is to put it into the microwave for a minute or two, then weigh it. As long as it continues to lose weight it is still giving up moisture. If the weight loss diminishes or you smell scorched are done.

The final weight was 415 grams (0.91 pounds) or 49 pounds/cubic foot. It indicated a sample moisture content of approximately 35%.

The problem with moisture

If the 0.91 pounds of dried wood were perfectly combusted it would release about 7500 BTU.

Heating up the 0.50 pounds of moisture from 60F to 212F would take 75 BTU and boiling off the 0.50 pounds of water (liquid-to-vapor) would consume another 480 BTU for a net gain of about 7000 BTU for the original 1.4 pound chunk.

It gets worse if the chunk is frozen and immediately pitched into the woodstove.

Let's say the wood is 20 degrees F. It would take 96 BTU to raise the water content from 20-to-212 degrees, another 70 to thaw the half pound of water and an additional 480 to boil off the water. That leaves a net gain of about 6850 BTU for the chunk of wood that originally weighed 1.4 pounds.

If the wood was 50% moisture by weight then a 1.4 pound chunk has a gross BTU potential of 5740 BTUs and it would take 134 BTU to raise the temp from 20F, 100 BTU to melt the ice and another 660 BTU to boil off the moisture. The 1.4 pound chunk would net 4800 BTU or only 70% as much as the 35% moisture wood.

Bottom line

Even at 35% moisture it burns well when placed on a bed of coals. It burns with very little flame. Without the bed of coals it seems as flammable as concrete. Six hours ago the wood in this fire was still in a live tree.
If things got tight, I could burn freshly cut Black Locust and it would generate net-heat but it would be at the expense of more creosote in the chimney.


  1. In my experience,locust doesn't smell all that great when it's burning either. Props for the mental gymnastics for a chunk of firewood. I just cut it, thank the tree and into the stove it goes.

  2. Great science. And props for "showing your work."

  3. Would it dry much if you brought it in the house for a couple of days before burning, which is pretty dry during winter months?

    1. No, it'll take weeks stacked inside to season. I tried it one winter. If your wood is green, your best option is to drive down the back roads after a windy storm and pick up deadfall. It's usually soft and punky (dody is what the old timers call it) but it will burn. Or scrounge some pallets. They burn really good. Gotta pick the nails out of the ah pile or they wind up in tires.

  4. Properly cured and kept dry, you'll find that it burn hot a cleanly. Very hot though. BTU level higher than oak.

    Fresh and uncured and wet, it's like trying to burn an old tire....takes forever and smells bad. Plus it will fill your chimney with creosote.

    1. Locust was what my wife used for the really cold days & nights back when she lived in Tennessee. Very high heat output once properly dried.

  5. Interesting ERJ. I have burning through the older pine and oak and getting into the much better cured materials. Quite a difference in the heat and the length of burn.

  6. Agree with Toird. The well-dried apple and ash I have takes a bit to get going, but burns very clean and long.

  7. Here in Missouri, honey locust is the only wood that I know of that will burn reasonably well with no seasoning. It saved my sorry rear one winter when I ran out of anything else.

  8. Our rule is that the wood must have been dead for at least a year before it goes into the pile. We still have a lot of dead ash trees around the farm. Dry ash burns very well. I cut several black locusts away from an electric fence last year. The sheep loved the leaves and even stripped off the bark.

  9. Here in central california we have a lot of almond wood available. Freshly cut almond will hardly burn, 2 year old almond burns well and long, almost no soot.

  10. How your brain works is amazing. I'm thankful for this and all the peeks behind the grey matter curtain!
    This post makes a good case for girdling trees for firewood. Gravity does the drying! Of course bucking a tree of opportunity demands the dry time the old fashion way.

    When you are picking fence posts, what size is ideal?