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Rule of thumb for drying lumber....for firewood.

Started by Piston, November 02, 2011, 01:14:53 PM

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Piston

Some people use the rule of the thumb for air drying hardwood lumber, of one year per inch of board thickness to get the lumber properly air dried.  Of course this is a general rule and there are a lot of variables.
Since lumber is stacked a lot more carefully than firewood, and stickered, I would figure it takes less time to air dry lumber than it does firewood. 
Most firewood is stacked with minimal air space, at best, it's loosely stacked for good airflow in a windy, sunny spot.

How come we don't use the same rule of thumb for firewood?  I figure a lot of my wood is 5" thick, which of course doesn't directly correlate to a 5" thick piece of lumber, but still? 
I would think that if my firewood is split 4-5" thick, it should take about 4-5 yrs to properly dry. 

Is it because the splits are a lot shorter than lumber and more drying happens from the ends? 

I also realize that dry lumber is a lot different than seasoned firewood, so maybe that makes up the difference.  Anyways just curious why we don't use the similar rule of thumb for drying firewood? 


Then again, I have a hard time getting one year ahead in the firewood department, let alone 4-5 yrs ahead  :D
-Matt
"What the Lion is to the Cat the Mastiff is to the Dog, the noblest of the family; he stands alone, and all others sink before him. His courage does not exceed his temper and generosity, and in attachment he equals the kindest of his race."

John Mc

Firewood just needs to get down to around 20% (a bit lower is better). That doesn't take to long if properly stored.

Lumber will take longer, since it needs to be drier. Also, the closer you get to equilibrium with the ambient air, the more the drying process slows.

Think of drying wood like car in neutral rolling down hill: A bigger the difference in the moisture in the wood and that of the surrounding air is like a steeper "hill" for the car. When the moisture gets close to ambient, the slope of the hill is almost flat. It can take a lot longer for that last few percent to get pulled out of the wood.

Heat and air flow are sort of like ways of giving that rolling car a push. You can let nature give the push (sunlight, warm weather, wind), or give things a man-made boost (a kiln with heat and fans).
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Ianab

Another factor that helps firewood is that wood dies about 10X faster from the end grain, for about 6" in. This is why boards tend to split at the ends as they dry, and people end coat their lumber with wax or paint to slow the drying and keep it even. So if you cut your firewood to 18" long, 12" of that will dry really fast. An old guy that lived up the road used to cut his wood into about 6" biscuits if he wanted them to dry fast. Line those up in a sunny, breezy spot and they are dry in months, and already split in 1/2 for the fireplace.

But leave the wood in log form, and it takes years to dry properly.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Piston

That makes sense.  A friend of mine has an outdoor wood burner and instead of cutting and splitting his wood, he just cuts 4" discs from the log with his saw, and then stacks them with good airspace. 
-Matt
"What the Lion is to the Cat the Mastiff is to the Dog, the noblest of the family; he stands alone, and all others sink before him. His courage does not exceed his temper and generosity, and in attachment he equals the kindest of his race."

Al_Smith

Well I suppose you could cut cookies instead of splitting it .Makes a lot of sawdust though .You'd either get good at filing chains or the local saw shop is going to make a killing grinding chains.

Much ado about something as basically simple as firewood .It isn't rocket science now people .Green cut oak timely split and stacked takes about two years to season before it burns well .Any other hardwood green cut takes about a year .

You can toss it in a big heap,stack it on pallets or just scatter it willy nilly and given enough time it will dry unless it's in a swamp .Cover it with a tarp ,pile it on concrete ,put it in a wood shed and it will all burn although some perhaps better than others .

WH_Conley

Cut it, stack it in the basement(where the wood stove is), let a fan blow through it. After a few days it burns good. I did play with the moisture meter on some firewood that had been drying outside for awhile. It ranged from 41% to OL, the meter only reads 42%. In 2-3 weeks it read 19%. It finished seasoning in the stove. Burned real good too.
Bill

beenthere

WH
Something doesn't add up there. The surface may have showed 19%, but the wood inside wasn't.
Others might be misled by the results you "found". ;)
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

WH_Conley

I never re split to get to interior moisture. I also did not have any creosote buildup. We had some warm days that the stove was just idling. Regardless of the weather I open the drafts and let the stove burn out once a day.
Bill

martyinmi

Two of my friends have moisture meters, and they rarely read within two points of each other. The best way that I've found to check moisture is with our silage scales and cooker. I take a baggie of sawdust to work, measure out 100 grams, dry it down(usually takes about 25 minutes), and re-weigh it. If I lose 75 grams of moisture, my firewood is 25%, 80 grams =20%. It's the only method I've found that is repeatable within 1 point every time.
No God, No Peace
Know God, Know Peace!

John Mc

Quote from: martyinmi on November 03, 2011, 04:49:17 PM
...I take a baggie of sawdust to work, measure out 100 grams, dry it down(usually takes about 25 minutes), and re-weigh it. If I lose 75 grams of moisture, my firewood is 25%, 80 grams =20%. It's the only method I've found that is repeatable within 1 point every time.

I think you've got some numbers mixed up there...
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

WH_Conley

That sounds like the oven dry method for checking moisture in a kiln. That would be the most accurate method. Are you making sawdust to do that test for checking lumber?
Bill

martyinmi

John Mc-
You are right. When we check silage (hay or corn), the feedstocks are typically in the 55%-70% moisture range. When the rations are calculated they are based on dry matter as opposed to moisture content. I haven't checked any wood since last year when I bought my gasser OWB, so I mixed my numbers up. Thanks for catching that.
WH_Conley-
I did a little research on my own to see what moisture firewood works best in a gasifier. It seems that moisture content under 10%(or over 90% dry matter) smokes badly in a gasifier(Empyre Pro-Series 100), especially if the pieces are split very small. We re-confirmed that theory with another gasser(P&M Optimizer 250) a month or so ago. The too dry wood was kiln dried scrap from a local sawmill. The moisture meters usually read 2-7 points drier than the readings I get from the fresh cut sawdust. We couldn't even get a reading on a few pieces of the wood from the kiln with one of the meters.
No God, No Peace
Know God, Know Peace!

WH_Conley

Ya learn something new everyday. I always thought a gasser worked better the drier the wood was. As a disclaimer I have never used one, just read about them. I use an Ashley coal stove, coal because they don't have to have a catalytic converter. Too green wood they smoke and stop up the pipe if you have them choked down. Too dry and you can't keep wood in them or throttled down, even with good door gaskets. Wood that has been cut and split about 6 months seems to work best for me. Maybe that is just the combination of my flue, the house environment and the stove.

As far as moisture meters, I was just playing around with mine. The wood was cutoffs from logs that had been laying around for a few months. It was freshly split. On green wood it won't give a reading.
Bill

beenthere

Quote from: martyinmi on November 03, 2011, 04:49:17 PM
Two of my friends have moisture meters, and they rarely read within two points of each other. The best way that I've found to check moisture is with our silage scales and cooker. I take a baggie of sawdust to work, measure out 100 grams, dry it down(usually takes about 25 minutes), and re-weigh it. If I lose 75 grams of moisture, my firewood is 25%, 80 grams =20%. It's the only method I've found that is repeatable within 1 point every time.

For wood, to calculate moisture content:
Using your numbers of 100 grams wet weight and 25 grams oven dry weight, that would be (wet - dry) / dry x 100 = 300 %  (As John Mc pointed out, my 3% before editing was not correct. And not a realistic set of number to use for an example as it means 75 grams of water were boiled away in the drying)  edited 11/4/11 - thanks for catching that error John Mc

This is the most accurate way to get moisture content and a good way to check the moisture meter performance (readings).

south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

John Mc

Beenthere -

Your formula actually gives 300% using the numbers given. Are you sure the formula shouldn't be (wet-dry)/wet x100 = mc ?   In this case: (100-25)/100 x 100 = 75%

Choosing 100 gm as the starting weight makes the match easy, since the % number ends up just being  "wet-dry"
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Piston

I know I've heard about the oven dry method and read about it many times, but I've never actually done it.  I'm going to check my firewood using that method and see what I come up with.  I'm not gonna ask about the whole process again because I know it's on here somewhere anyways.  I do believe I'll need to buy a scale though since I can't seem to find my old fish scale and not sure the old bathroom scale is accurate enough. 
I have a feeling I'm not going to like my results  :D
-Matt
"What the Lion is to the Cat the Mastiff is to the Dog, the noblest of the family; he stands alone, and all others sink before him. His courage does not exceed his temper and generosity, and in attachment he equals the kindest of his race."

beenthere

Quote from: John Mc on November 03, 2011, 10:17:15 PM
Beenthere -

Your formula actually gives 300% using the numbers given. Are you sure the formula shouldn't be (wet-dry)/wet x100 = mc ?   In this case: (100-25)/100 x 100 = 75%

Choosing 100 gm as the starting weight makes the match easy, since the % number ends up just being  "wet-dry"

Yes, you are right. Need to multiply the 3 x 100. Thanks for the correction, and sorry to add the confusion.
However, wood mc is always based on dividing by oven dry weight. The example given was not a good one as wood wouldn't be 300% mc. More realistic numbers would be a 100 grams of "wet" sawdust would dry out to 75 grams of oven dry weight and calculate to (100-75) / 75 = .33 x 100 = 33% mc
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

John Mc

Now those numbers make sense. I never had paid much attention to what the formula was, since I've never done the oven-dry method. (In fact, after some initial playing around with a borrowed moisture meter to get an idea of what it took to air-dry firewood in my area, I don't even check any more.)

John Mc
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Piston

Thanks for the clarification on the oven dry formula, makes sense  ;D

I want to check my firewood because when we were out of power for a few days from that last Nor'easter my wife was filling the stove with seasoned oak, then she grabbed some green oak by accident and threw it in the stove, she said she couldn't believe the difference in how hard it was to burn.  I've never checked the moisture content of my 'seasoned' wood but I'm interested to know.....I might just realize it's not as seasoned as I thought  :D
-Matt
"What the Lion is to the Cat the Mastiff is to the Dog, the noblest of the family; he stands alone, and all others sink before him. His courage does not exceed his temper and generosity, and in attachment he equals the kindest of his race."

martyinmi

Ok John Mc and beenthere-
   Now you've got me confused. I've never heard of your formula for figuring mc. I even called our feed guy and explained what you folks were discussing, and he's scratching his head also. He's been helping with feed rations on our farm for more than 20 years now. He said that if you start out with a 100 gram sample, cook it down to boil out all moisture, and you end up with a weight of 75 grams, your beginning moisture content is 25%. His formula is much simpler than yours(25 divided by 100= .25). He illustrated his point even further by using a different starting weight. 60(starting weight) and 40(cooked weight). 60-40=20.Now 20 divided by 60= .3333. That wood sample is 33 and a third percent. We all know that 20 is one third of 60. Right? Or are we missing something?
   Just look at the confusion I started by running my fingers(I use three to type with) across the keypad before putting my brain in gear.

   Marty
No God, No Peace
Know God, Know Peace!

John Mc

His formula actually sounds similar: the 25 he uses in 25/100 is actually what the "wet weight - dry weight" in the formula mentioned earlier is all about. The 100 in the 25/100 is the wet weight. It sounds as though grain MC is measured as a % of total starting weight: the 25 grams of water is 25% of the starting weight (which is water + grain weight).

The formula beenthere posted shows moisture as a percent of dry wood weight. In your example, 25 gmwas the water driven off, and 75 gm was the remaining dry grain. The moisture was 33% of the dry grain weight (25/75).

I can never remember which is the standard for wood: is it expressed as a percent of the starting weight (100 gm in your example) or as a percent of the dry weight (75 gm in your sample). As Beenthere indicates (and wikipedia confirms, see Moisture Content of Wood) the convention for wood is (starting weight - oven dry weight)/oven dry weight  [multiply by 100 if you want that in % rather than as a decimal].

Apparently grain drying uses a different convention.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

martyinmi

Quote from: beenthere on November 04, 2011, 10:19:37 AM
Quote from: John Mc on November 03, 2011, 10:17:15 PM
Beenthere -

Your formula actually gives 300% using the numbers given. Are you sure the formula shouldn't be (wet-dry)/wet x100 = mc ?   In this case: (100-25)/100 x 100 = 75%

Choosing 100 gm as the starting weight makes the match easy, since the % number ends up just being  "wet-dry"

Yes, you are right. Need to multiply the 3 x 100. Thanks for the correction, and sorry to add the confusion.
However, wood mc is always based on dividing by oven dry weight. The example given was not a good one as wood wouldn't be 300% mc. More realistic numbers would be a 100 grams of "wet" sawdust would dry out to 75 grams of oven dry weight and calculate to (100-75) / 75 = .33 x 100 = 33% mc
Now...food for thought, guys.  Using above formula with 100 gram sample "wet" dried out to 40 "grams oven dry weight":  (100-40) /40= 1.5 x 100 = 150% mc? Or... 100 wet dried to 20: (100-20) / 20 = 4 x 100 =400% mc? Or... 100 wet dried to 10:  (100-10) /10 = 9 x 100 = 900% mc? Or even... 100 wet dried to 50:  (100-50) / 50 = 1 x 100 = 100%mc? I think I'll stick to my math. Makes more sense to my feeble old brain!
No God, No Peace
Know God, Know Peace!

beenthere

south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Piston

 ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ???
-Matt
"What the Lion is to the Cat the Mastiff is to the Dog, the noblest of the family; he stands alone, and all others sink before him. His courage does not exceed his temper and generosity, and in attachment he equals the kindest of his race."

beenthere

Piston
For wood mc, it is

(wet - dry)  รท  dry x 100 = MC

The weight of wood substance doesn't change when drying in the oven, but the water evaporates.
So the difference between the original weight of the wood (referred to as "wet" ) and the oven dry weight is the weight of the water lost in drying.

Referred to as the oven dry method, as oven dry is the basis in the forumula.
It is important to use realistic numbers in the examples, as there is a limit as to how much water there can be in a block of wood. ;)  Likely not over 100%

martyinmi can use whatever he wants to calculate mc in his wood. :)
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

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