Drying Wood Quickly Indoors

I had a cord of freshly cut and split black birch delivered here last week. I stacked up half of it outside and the other half inside (Sunday afternoon, January 2). Monday night I went out and grabbed a fairly dense and sopping wet medium-size piece and brought it in and took a small slice off the end to get rid of any wood that may have dried while it was outside the last couple days. Then I took a 2-3" slice off the end and weighed it on the old hippie herbal remedy weigh station (triple beam scale). Came out to just a hair under a pound - 445.4 grams.

I placed the remainder of the split on my old produce scale, and it weighed 10 pounds, 8 1/2 ounces (10.53 pounds). I will leave it right there and weigh it each day and record the weight to get a drying curve established. I will also take the relative humidity (RH) every day at the same time that I weigh it and record the result. I have a large commercial floor fan blowing on the stack 24/7 to keep air moving through it. It seems to be doing a very good job of moving air through the spaces between the splits so they can dry faster, just like being outside in a constant breeze.

I used the smaller sample to determine the moisture content of the sample split to get a baseline. This stuff is way too wet to even think of using a resistance-type moisture meter on it. I used the much more accurate oven-dry method to get the MC within a couple tenths of a percent. I will use that info to evaluate the moisture content of the larger split as it dries out over the next few weeks.

I thought this was an exceptionally dense piece of wood. I took it into the kitchen, filled the sink and dropped it in. Sank like a rock.

I then took it out of the water and went downstairs to chop it into little pieces with my trusty kindlin' meat cleaver (had to make sure I saved every little piece). That would allow it to dry much quicker. I wanted to make sure I went slow with this one since the last time I did this I just wanted to demonstrate the method to the membership here. This one's for the books. Any unwanted charring of the wood will cause the sample to lose some volatile substances, skewing the result to show a higher initial MC than it really had.

I started out using my kitchen microwave to dry it, but that was taking forever and it was getting late, so I did it old school - overnight in an oven set at 215ºF. Woke up and weighed the pieces on my triple beam - 282.9 grams. There was no evidence of charring anywhere, so I think the stuff is a good.


445.4 - 282.9 = 162.5 grams water lost = 36.5% water by weight (wet-basis), or 57.4% MC (dry-basis). Not as wet as it looked, but certainly twice as high as any moisture meter will allow accurate measurement.


So, the 10.53 pound split sitting on the scale in the basement at 36.5% water contained 3.84 pounds of water when I placed it there and 6.69 pounds of wood fiber. This is the figure that is needed to compute all further moisture loss using the changing weight alone to make the calculations. The ramifications of this are that anyone can accurately determine the MC of there firewood at any time if they take a few representative samples during the season, weight them, determine the MC at that point, compute the dry fiber content, and use this at any point in the future to calculate the MC of their wood merely by weighing it. No meter needed, no re-splitting, no sorting through piles looking for suitable splits. And you don't need to be artificiality drying your wood indoors to take advantage of this technique.

The RH in the shop was determined using a Taylor sling psychrometer. On day one, dry-bulb temp was 83º, wet-bulb temp was 66º. 17º wet-bulb depression at that temp = 40% RH. Not as low as it will get when all the wood starts drying out. The day before I started this I took in 1/2 cord from the same soaking wet load of black birch and stacked it 5' high, 42" in front of the stove (even with soaking wet wood I adhere religiously to clearances) with a large fan blowing against the back of it. On day one it was acting as an gigantic organic humidifier with the wonderful scent of wintergreen emanating from it.


Results so far after four days of drying:


Day # Weight (lbs) Water Wt (lbs) Fiber Wt (lbs) Rel. Humidity MC (dry-basis) MC (wet-basis)

One: 10.53 3.84 6.69 40% 57.4% 36.5%
Two: 10.03 3.34 6.69 35% 49.9% 33.3%
Three: 9.56 2.87 6.69 34% 42.9% 30.0%
Four: 9.38 2.69 6.69 30% 40.2% 28.7%
Five: 9.13 2.44 6.69 28% 36.5% 26.7%


As you can see, the 10 1/2 pound split lost a full 1/2 pound of water in one 24 hour period, 6 ounces the next day, 4 ounces the day after that and 3 1/2 ounces since last night. MC at present is 36.5% (dry-basis), or 26.7% (wet-basis). Just to compare methods, I grabbed a similar size split from the inside stack (it had been inside one extra day), split is and measured the MC. It measured at 34% MC on the meter, which is a dry-basis measurement. Pretty durn close, eh?


As my indoor stockpile of wood dries more and colder weather hits, RH down there should drop to about 20%, at which point all of the wood will be fairly well dried out and ready to burn (25% dry-basis, 20% wet-basis).

I love the taste of black birch....


Matt

And to think he was just a janitor at harvard......how do you like them apples!
Sorry guys. After my head stopped hurting from all the math to this post(all great info by the way!) That movie popped into my head.

BK, you need a vacation.


Seriously.

Wait until after you are done weighing the wood so you don't have to do the experiment again.

Matt

beekay,

sniff. thank you!

geek

As folks say in my neck of the woods would say, Battenkiller is "wicked smott".

But seriously man, your attention to detail and analytical mind never cease to amaze. Keep up the good work.

Al

Interesting thread!

I am a farmer, and make a lot of hay in the summer. To ensure good hay for my customers, I have a moisture meter in the baler, and also a hand held meter which I can use to test hay that has already been baled.

To check the accuracy of the meters, ( especially the hand held one), I occassionally test a hay sample using the microwave.

It's pretty simple really. You start with a sample which weighs approx 50 grams. In the microwave it goes with an 8 oz. glass of water. When the water boils, take it out and put another 8 oz glass in. Do this a 3rd time. Take the water and the sample out and weigh the sample. The difference, as a % of the 50g starting weight is the MC of the hay. eg. 50g(start) - 42g (finish) = 8g.

(8g/50g) x 100 = 16% --the moisture content of the hay. The water, by the way is in there to prevent the hay sample from catching on fire!

I've thought alot about trying this on firewood - so a couple of weeks ago I 'gave it a go"

I re split a split and checked the moisture with my meter - 11%. Then I cut an end off of the split (to get away from the drier end piece) and then cut a small piece for testing. Checked it with my meter and it was also 11%.

It weighed in at about 300 grams--exact #'s are on my desk somewhere -but I can't find them!

The rest of the experiment was carried out as above with the hay - with one exception. Because of the weight of the wood, I microwaved it (with water) probably 5 or 6 times. I weighed the wood after each 'bout' with the microwave and finished when the weight of the wood started going down in weight by only 1 or 2 grams.

Calculated the MC as I did for the hay and, low and behold, it was 11% - exactly the same as the meter.

Just for the heck of it, I put the wood back in the microwave with another glass of cold water. In about 15 or 20 seconds, there was big smoke happening in the microwave! The wood was obviously dry, and probably would have caught fire in another few seconds.

I got the wood out with an oven mitt and took it outside - still smoking like crazy!! The smoke was coming from the center of the wood--c/w good sized black charred area. You couldn't touch it with your bare hand - it was really hot!

I certainly wouldn't do this test on a regular basis, but it was interesting to do it once anyways! BTW, I already knew that my MC was dead on. I took it to a local lumber mill last spring and compared a bunch of readings with their mega $ one. Mine was +/- 1% on every test.

Cheers!!

Analysis like this is woodburner porn....I look forward to the final results.

An observation from a non-technical guy, but your drying will not be linear. It will be a dramatic slope to a very shallow run. As they out parts dry, the inner parts must move the moisture to the air through the already dryer wood. Also, the closer the wood comes to the ambient moisture levels, it will slow also.

Just an aside, I use my microwave from time to time to dry wood turnings. Some are rough bowls of unusual woods I could not help but turn early, and others were contracts for special restorations when the only appropriate woods at hand were too high in moisture content. Like some clock finials in rosewood that came from Liverpool, England. Warehoused two hundred year old wood at 72% moisture. The warehouse was on the waterfront.

I see where this thread is going: stack a cord of wood to dry in your living room and you won't need to put a kettle of water on the stove to increase your humidity (but you will need to scrape the mold off the walls at least once a season).

Not true at all. Even at the highest humidity level, the RH down there never got over 40%. That's still pretty dry. For about three days I got some condensation on the big windows at the other end of the shop, but that stopped pretty quickly as RH dropped to 30%. I do have mold down there, but it's all along the baseboards from the yearly springtime flooding I suffer with every snowmelt. I've been doing this for 20 years now and I have never had to clean mold off of anything. By the time I bring wood in here in that quantity, the wood in my housing structure is so dry that is sucks it right out of the air before it has the slightest chance to condense on surfaces. It become liquid water again, but inside the structural members and sheetrock, etc. by forming hygroscopic bonds with these substrates. And when it does, it releases the heat initially lost through evaporation back into the home.

I never doubted the amount of water they claimed was in the split. The issue was with the faulty way they numerically expressed both moisture content and heat loss. The split they used as an example was 50% water by weight. My birch was only 36.5% water. My wood was so obviously wet that only an idiot or a wet-wood burning genius would try to heat his home with it, so I don't even see the point of comparing a piece of firewood that had much more water than mine has to a well-seasoned split. No one is getting anywhere with wood that is half water by weight (100% MC as a wood technologist would express it). That video would have been supremely unimpressive if they chose instead to demonstrate the difference between a split at 25% water (marginal firewood) and one at 20% (seasoned firewood).

I never thought the drying would be linear, it should resemble an exponential decay function. If you look at the amount of water lost so far each day, it mimics almost exactly a radioactive half-life function where the half-life is one 24-hour day. That will change as the RH in the room drops. In fact, it already did on the last day, where the wood dried more than the trend line would have predicted. That's because the RH in the shop dropped to 28%. At the wood dries and the RH continues to drop, I fully expect that the curve will be become slightly steeper than the original function would have been if that trend continued, although it will always be concave-up in shape. An engineer would tell me I'd have to use differential equations to accurately model the drying, but we don't need to be that precise. We're not trying to land a rover on the Martian landscape, we trying to get a fair idea of how fast wood dries so we can get a rough idea about when we can stick it in the stove.


As I said, you can still take advantage of this methodology without having to use it to speed-dry your firewood. Next time you are bucking wood, cut several cookies from each wood type, weigh them and mark the weight in Magic Marker and toss them out in the sun to dry. This will dry them most of the way in short order. Take some splits from the same wood the cookies came from, weigh them, mark the weight and set them aside. At any time after this, you can bring the weighed cookies inside and dry them out in a 215º oven (must be as close as possible to that temp) until they stop losing weight. Record the final weight and use it to calculate the starting MC. It will be the same starting MC that the splits you set aside had. Now all you have to do is weigh the selected splits to see how much water they lost, giving you a very good idea how the rest of the entire load has lost water weight.

By doing this little procedure at the onset of the season, you can check the MC of your wood at any time thereafter just by weighing the selected splits. If you weigh them once a month, you can get a drying curve established that will allow you to predict about when the wood will be ready. Not perfectly, but infinitely more accurate than saying, "Well, now... oak needs at least two full years... maybe three... in full sun and wind to season properly." That ain't the least bit accurate.



I will be creating a spreadsheet that can be used to do all of the calculations needed to do this, so anyone interested in it can get it from me just by sending me an e-mail. That way, even if you have a hard time understanding (no offense, some folks minds work differently than others) what, and why, and how to use the data, all you'll have to do is weigh, dry, re-weigh and punch in the numbers. The spreadsheet will spit out the MC.

Ahem, not exponential, but should be square root of time....

Interesting study, but if wife had seen me put a split into oven and heat overnight at 215*, I would be in a nice warm, rubber room by now. After further thought, it may not be a bad idea...warm surroundings and 3 meals a day.

I love this type of information. I'm an engineer by trade, a #'s guy by heart, toss in a huge dose of anality and this thread is right up my alley. Besides that it's great info to have. This may lead me to do the same type of testing. I store a full years worth of wood in the pole shed. It's already seasoned when I put it in there but this explains a lot.

A this rate of losing water a guy could cut, split and stack it inside and have ready to use wood in no time. This would explain why the last weeks load or 2 I have brought in the house could practically be lit on fire with a lighter. I need to get a gauge so I can test it. I have a feeling it's too dry.

Holy crap! I think I need to start smoking weed!

Has anyone seen a wood fired firewood drying kiln? Or plans for one? I have a lot of red pine trees available. They are lying on the ground with the bark fallen off. Cured, dead over a year, but full of rainwater and ground moisture. And, of couse, now covered with snow.

Wow. I am always so amazed by the intellegence and experience of the people who post here. This information is so interesting and useful. We do bring questionable wood in about a week early, especially in the fall. It just hangs out in the garage until its time arrives. I just thought I was giving the snow time to dry off, turns out there is science behind it. Thanks BattenKiller.

Wow. I am always so amazed by the intellegence and experience of the people who post here. This information is so interesting and useful. We do bring questionable wood in about a week early, especially in the fall. It just hangs out in the garage until its time arrives. I just thought I was giving the snow time to dry off, turns out there is science behind it. Thanks BattenKiller.

Ah..... yes. :shut:


I was trying to express the disappearance of water over time, but I accidentally used the amount of water lost each day rather than the weight of the water remaining each day. If the amount of water remaining each day was half of what it was the day before, it would look just like a decay function with a half-life of one day. But that's not what is actually happening. The water is disappearing much slower than that, and not in an exponential fashion at all. It's still a concave function, though, as can be seen in the graph below. Thanks for catching that. ;-)


BTW it's down to 25.4% water by weight (% MC wet-basis) after only 5 days drying. Note in the chart how closely the RH in the room matches the percent water weight. The wood is keeping the air humidified less and less each day, which is generating the RH curve seen.