Drying test results are in...

[Hasufel]

I finally wrapped up the firewood drying test I had been running for a little over a year. I had posted some preliminary results in other threads but brought this to its conclusion by oven-drying the test pieces and replotting the results according to MC.

Just to recap, I used smallish pieces to speed up the process. I used freshly bucked and split red oak heartwood and tried to keep the pieces as uniform as possible. I cut eight pieces altogether, four large (those numbered "1" on the charts, about 2-1/4" X 3" X 4-1/2") and four small (those numbered "2" on the chart, about 1-1/8" X 2" X 4-1/2"). I placed one large and one small piece in each of four locations: A = a wood bin in a heated indoor living space; B = an unheated but closed garage; C = the top of a covered outdoor firewood rack; and D = the top of an uncovered outdoor firewood rack. Starting MC was too high for my meter but I later calculated it as 73%. I normalized the results to account for slight differences in weight between each group of sample pieces.

Bottom line, I found that the indoor pieces dried the fastest overall. The uncovered outdoor pieces dried fastest in the first couple of days but then maintained a higher MC over the long term (and they ended up looking the most weathered, which was deceptive because they were actually the least "seasoned"). The garage pieces dried more quickly than I would have expected. And even thought the pieces were all about the same length, the narrower ones dried faster...demonstrating that a lot of drying takes place across the grain, not just along it (which I think was a topic of discussion a while back).

The results are shown below.The final MC ranged from about 8% indoors to 15% for the uncovered outdoor wood. There's a lot of variation with the uncovered outdoor pieces because they would get rained on, but even the protected pieces fluctuated slightly as the humidity changed. And finally, the lines converge at the end because I brought them all indoors for a month or so before the final oven drying.

 

[Poindexter]

Nice work.

Were all these pieces close enough to each other, say within 100 feet or so, that we can ignore microclimate changes?

I mean like were the uncovered ones 50' higher elevation in full sun while the covered ones were at 50' lower in full shade?

I ass/u/me not, you've clearly put effort into this.

What are your take home conclusions regarding getting red oak heart wood seasoned?

 

[Hasufel]

Nice work.

Were all these pieces close enough to each other, say within 100 feet or so, that we can ignore microclimate changes?

I mean like were the uncovered ones 50' higher elevation in full sun while the covered ones were at 50' lower in full shade?

I ass/u/me not, you've clearly put effort into this.

What are your take home conclusions regarding getting red oak heart wood seasoned?

Good questions! The two outdoor pieces were on different stacks but under generally similar conditions in a wooded (i.e., shaded during leaf season) area. They were at the same elevation and within about 100 feet of each other, but the stack that the covered test piece was on might have gotten a bit more sun in the summer.

My main takeaways were that, if you want to dry oak more quickly, you should split small and get it indoors, or at least top covered. Leaving it uncovered when green, however, helps speed the initial drying. Having a staging area in a climate-controlled space can make a big difference in speeding things along. I see you're in a very different climate but, for the mid-Atlantic at least, I also found that firewood can dry well any time of the year, not just summertime!

 

[Dobish]

that is a pretty cool graph! thanks for the data. that seems to be much more scientific than my method of putting it in a stack, then piling more wood in front of it, and just not being able to get to it for a while!

 

[ValleyCottageSplitter]

I bet temperature was a big factor. Some type of drying time vs temp would be interesting to see, though a lot more data to collect.

I had a few questions. Do you have a picture of the outdoor covered storage? I'm wondering how much it matters if there is hard cover or how the soft cover is positioned.
I saw another study showing similar results. They tested hard cover (shed), soft cover (fully tarped) and open and found the shed a bit above the other two for drying. Maybe I can try digging that up sometime.

Second question is how you measured the MC? Did you have to poke them dozens of times or was there some type of permanent meter set up? If you reinserted into the existing holes there might be some surface drying effect.

Thanks! It looks like a great study. Very interesting.

 

[Hasufel]

I bet temperature was a big factor. Some type of drying time vs temp would be interesting to see, though a lot more data to collect.

I had a few questions. Do you have a picture of the outdoor covered storage? I'm wondering how much it matters if there is hard cover or how the soft cover is positioned.
I saw another study showing similar results. They tested hard cover (shed), soft cover (fully tarped) and open and found the shed a bit above the other two for drying. Maybe I can try digging that up sometime.

Second question is how you measured the MC? Did you have to poke them dozens of times or was there some type of permanent meter set up? If you reinserted into the existing holes there might be some surface drying effect.

Thanks! It looks like a great study. Very interesting.

The covered rack was basically this:


The test piece was at the very top so it was in contact with the tarp and had no direct wind flow over it. I thought about doing covered + exposed sides but figured I'd have trouble repeatedly accessing it for the periodic weighing.

The MC was calculated at the very end, not measured. The only thing I measured throughout was the weight (using a digital balance accurate to 0.1 grams). I finally oven dried the test pieces to determine their dry weight and let Excel do the MC calculations, with MC = (weight - dry weight)/dry weight. I tried my MM at the very beginning but it showed an overload (which means MC >60%).

 

[Hasufel]

I bet temperature was a big factor. Some type of drying time vs temp would be interesting to see, though a lot more data to collect.

Oops, I forgot to address your first point. I had considered tracking the weather but determined that it would be too onerous. If I had more time I would have tested several batches of samples starting at different times of the year to see what a difference that would have made. But because I C/S/S mostly in the winter, I started this test in the winter as well to mimic my normal wood processing cycle.

The test began in early February 2016, and the chart shows that most of the drying had already taken place by the time the weather warmed up. The uncovered samples had a sharp uptick in May followed by a milder jump in August, both of which were wet periods. Even the covered samples regained some moisture at the same time. This suggests that temperature is less important than humidity; we tend to have some pretty sultry summers here and it seems like my firewood dries at least as quickly during cooler, drier periods. Another factor is that my stacks are mostly under the tree canopy, so they get more sun and wind when the leaves are down. So at least for my situation, temperature doesn't seem particularly important in determining the drying rate.

 

[Oww My Back Hurts]

This is awesome, thanks for posting!

 

[Jeffm1]

I finally wrapped up the firewood drying test I had been running for a little over a year. I had posted some preliminary results in other threads but brought this to its conclusion by oven-drying the test pieces and replotting the results according to MC.

Just to recap, I used smallish pieces to speed up the process. I used freshly bucked and split red oak heartwood and tried to keep the pieces as uniform as possible. I cut eight pieces altogether, four large (those numbered "1" on the charts, about 2-1/4" X 3" X 4-1/2") and four small (those numbered "2" on the chart, about 1-1/8" X 2" X 4-1/2"). I placed one large and one small piece in each of four locations: A = a wood bin in a heated indoor living space; B = an unheated but closed garage; C = the top of a covered outdoor firewood rack; and D = the top of an uncovered outdoor firewood rack. Starting MC was too high for my meter but I later calculated it as 73%. I normalized the results to account for slight differences in weight between each group of sample pieces.

Bottom line, I found that the indoor pieces dried the fastest overall. The uncovered outdoor pieces dried fastest in the first couple of days but then maintained a higher MC over the long term (and they ended up looking the most weathered, which was deceptive because they were actually the least "seasoned"). The garage pieces dried more quickly than I would have expected. And even thought the pieces were all about the same length, the narrower ones dried faster...demonstrating that a lot of drying takes place across the grain, not just along it (which I think was a topic of discussion a while back).

The results are shown below.The final MC ranged from about 8% indoors to 15% for the uncovered outdoor wood. There's a lot of variation with the uncovered outdoor pieces because they would get rained on, but even the protected pieces fluctuated slightly as the humidity changed. And finally, the lines converge at the end because I brought them all indoors for a month or so before the final oven drying.

View attachment 197158

So would it be safe to say after a year they are all pretty close to the same, at least for burning purposes? I would conjecture that after two years any differences would be even less...time is the great equalizer.

 

[Hasufel]

So would it be safe to say after a year they are all pretty close to the same, at least for burning purposes? I would conjecture that after two years any differences would be even less...time is the great equalizer.

Yes, after a year the differences were fairly small. Everything was well below 20% MC. The uncovered pieces still had the most moisture but were well within the burnable range. Now keep in mind that the test pieces were much smaller than normal splits so in real life the timeline might be several times longer. Also, the final "convergence point" on the graph is when I brought everything indoors for a month, just to see if the weathering had any effect on wood's ability to dry once brought indoors (it didn't)...so the penultimate point is really the one to focus on for the main results. But it looks like wood that's split and stacked uncovered outdoors will eventually dry below 20% MC, at least in my climate.

FWIW, here are the test pieces right before I dried them for the final dry weight determination. Notice that the uncovered outdoor pieces (far right) look the most weathered by far, while the driest pieces (far left) still look pristine...so the appearance of weathering doesn't necessarily mean that one piece of wood is more ready to burn than another.

 

[Poindexter]

Can eXcel spit out a wet basis graph too?

CW = current weight of sample piece

(CW - dry weight )/ CW = MC sub wet basis

Not sure how many times i have read this thread, quite thought provoking.

I have a sample split in with my other calibration tools. It weighs 4.00 pounds oven dried, but it soaked up 1.00 pounds of water since it came out of the oven and now weighs 5.00 pounds.

 

[Hasufel]

Can eXcel spit out a wet basis graph too?

CW = current weight of sample piece

(CW - dry weight )/ CW = MC sub wet basis

Not sure how many times i have read this thread, quite thought provoking.

I have a sample split in with my other calibration tools. It weighs 4.00 pounds oven dried, but it soaked up 1.00 pounds of water since it came out of the oven and now weighs 5.00 pounds.

Sure, here you go:

 

[Poindexter]

Is there another data set off to the right where everything goes to zero out of the oven?

 

[Hasufel]

Is there another data set off to the right where everything goes to zero out of the oven?

No, I didn't plot the final zero point because it was achieved artificially and it was, you know, zero.

 

[Poindexter]

I am giving this thread a lot of thought. Just asking.

Your curve looks like a good fit with all of my data, and is shaped very much like all the other graphs i have seen published by academics.

The variables seem to be wood species, x axis labels and y axis labels.

 

[Ctwoodtick]

Interesting look at firewood drying. Would not have thought wood would dry as well as it did indoors. I have always thought of firewood drying was like drying laundry with much the same principles in place. As such, I would have thought that outdoors would be the way to go. Wow, who knew?

 

[Poindexter]

I think clothes drying is a very good analogy to seasoning cordwood.

It ties right in with the comment i was going to make about humidity.

Both wood and a pair of jeans dry faster in a less humid environment, yes?

It comes down to both seeking EMC, equillibrium moisture content. I have google images/ emc chart for wood open in another tab.

Today it hit about +50 dF and my local RH was 30%. Real easy to see my cord was , via difusion, trying to get to 6.3% mc. That moisture content is where my stacks would be at equilibrium with the air around them.

I put up all 8 of my cords in March. The nighttime lows have been above freezing for about a week. My stacks are starting to shift, but not sagging yet. I know my stacks are pretty much running 28-35% right now without having to meter any of it - because i know the fiber saturation point for my spruce is right at 30% mc.

Once a split of my spruce gets down to 30% all the sap is out of the tubules, the radial and tangential shrinking of each split starts, and my stacks start moving.

So the splits are at +/- 30%, the emc today is 6.3 %, strong difusion gradient, lots of water loss today likely.

A couple months from now my stacks will likely be about 16% mc, ambients will be about 80dF and RH about 70%, so emc is gonna be about 14%mc. Weak difusion gradient, slower water loss.

This is, i think, why the drying curves all start off steep and flatten out.

Same thing with those jeans in the dryer, getting that last bit of dampness out takes what seems like a really long time.

 

[Hasufel]

Interesting look at firewood drying. Would not have thought wood would dry as well as it did indoors. I have always thought of firewood drying was like drying laundry with much the same principles in place. As such, I would have thought that outdoors would be the way to go. Wow, who knew?

The indoor results were impressive but they didn't surprise me too much because I know it gets super dry in my house during heating season. The itchy skin and the cats sticking to the ceiling from static electricity kinda gave that away. I was more surprised at how well the garaged wood did, considering that it's unheated out there and there's really no air circulation. Maybe it gets enough heat from the house to make a small difference? And I certainly learned the value of top cover for faster drying.

 

[Ashful]

Interesting, we have seen so many here claim that oak takes three years to season, that it has become gospel. Even I began to believe it. But your data shows that within ~125 days (roughly 4 months), they're basically all at or below 20%, and they've all plateaued to their equilibrium state within 9 months of splitting.

 

[Hasufel]

Interesting, we have seen so many here claim that oak takes three years to season, that it has become gospel. Even I began to believe it. But your data shows that within ~125 days (roughly 4 months), they're basically all at or below 20%, and they've all plateaued to their equilibrium state within 9 months of splitting.

Well, keep in mind that the test pieces are smaller than a normal firewood split. But I think the results show that oak can season quickly if you're willing to split it thin enough. There was another discussion (here) that showed the math behind this principle...but it's nice to see it firsthand. I've processed a lot of oak this past season and I've gone through extra effort to split it thinner so that I'll be able to burn it before I retire & move away!

 

[Jeffm1]

Interesting, we have seen so many here claim that oak takes three years to season, that it has become gospel. Even I began to believe it. But your data shows that within ~125 days (roughly 4 months), they're basically all at or below 20%, and they've all plateaued to their equilibrium state within 9 months of splitting.

Speaking of things taken as gospel, ever wonder where the magic number 20% came from? I don't know if it's because I only own an el cheapo moisture meter but 20% doesn't burn as good or as clean as once I get my wood down below 15%. But I rarely use my moisture meter anymore anyhow as I just go by time and the look, sound, and feel of seasoned wood. I can definitely hear the difference between 20% and 15% when I knock two pieces together. So why isn't that magic number 15% (or 16% or 14% or 13% for that matter)? Regarding oak, we do hear it quite frequently that there is a very noticeable difference between 1 and 2 year seasoned oak (and even 3). And that comes from experience which sterile lab tests don't always reflect accurately. So maybe the difference is because, at least with oak, 20% is still too high. I mean, 20% may simply be sufficient but real efficiency comes even lower than that.

 

[Ctwoodtick]

20%, while not anything but a number, I feel does represent a good cutoff when wood stops being a pain to ingnite and at that number, burns very cleanly. I burn wood at times that is a bit higher than this, but at 25% or higher, burning is sluggish and imho, not worth it. 15% is great, of course, too!

 

[sportbikerider78]

I'm going to try the "wrap the stacks in plastic method" and see what happens by fall.

 

[Poindexter]

I discussed the 20% number with @BKVP some time ago.

Not a verbatim quote, but Chris said, more or less that when he meets with epa in a room full of bigs from the other stove manufacturers the concensus of the mfr reps is that most burners have to work to get their wood down to 20-22% mc.

Comment mine not Chris': So the epa backs 20% as the more efficient end of what ordinary burners can get to, and the mfrs build stoves to run on 20-22% because tha is what most folks have to burn.

Chris did go on to comment that those of us on hearth dot com discussing how fast we can get our stacks even drier -during the summer months- are not ordinary wood burners.

 

[Jeffm1]

I discussed the 20% number with @BKVP some time ago.

Not a verbatim quote, but Chris said, more or less that when he meets with epa in a room full of bigs from the other stove manufacturers the concensus of the mfr reps is that most burners have to work to get their wood down to 20-22% mc.

Comment mine not Chris': So the epa backs 20% as the more efficient end of what ordinary burners can get to, and the mfrs build stoves to run on 20-22% because tha is what most folks have to burn.

Chris did go on to comment that those of us on hearth dot com discussing how fast we can get our stacks even drier -during the summer months- are not ordinary wood burners.

Good to know. But forgive me for asking, but who is "Chris"?