Replies

1. 
   Danford_C_ | Apr 23, 2002 05:09pm | #1

   Dunno, Neal.  How does one give "110%"?

   Dano

   "Form and Function are One" - Frank L. Wright
2. 
   StanleyNiemiec | Apr 23, 2002 05:54pm | #2

   I am not going to spend hours looking for a 200% species but a quick look in the Wood Handbook (USDA Forest Service) indicates that the moisture content (oven dry) for the heartwood of black cottonwood (Populus trichocarpa) is 162%.

   It is quite possible to have moisture contents of percentages greater than 100%.  All one has to do is actually know how an oven dry moisture content is calculated, which, in fact, represents a comparison between the amount of dry wood mass to the mass of water in the freshly cut wood.  The oven dry moisture content is calculated as follows:

   Moisture Content = [{weight of wood wet - weight of wood dry}/weight of wood dry] x 100

   There is also a green moisture content calculation that is used in certain segments of the wood products industries, notably the pulp and paper areas.  This green moisture content is calculated as follows:

   Moisture Content = [{weight of wood wet - weight of wood dry}/weight of wood wet] x 100

   The primary reason that the lumber part of the wood products industries uses the oven dry moisture content is because it is reproducable.  The calculation is based on the mass of wood, not on the initial combined amount of wood and water.  That means that if the wood is completely dried (bone dry) any amount of water that is adsorbed/absorbed can then be assessed.  If you are cycling wood to determine its Equilibrium Moisture Content (EMC) for various temperature and relative humidity conditions, you will always have a fixed weight to return to.  You cannot do this with a green moisture content because that value is dependent entirely on its initial mass content of water. 

1. 
   PlaneWood | Apr 24, 2002 01:35am | #5

   So, your saying that if you extract 2 lbs of water out of a piece of wood that weighs 1lb afterwards, then the moisture content was 200%??

   PlaneWood by Mike_in_KatyPlaneWood

1. 
   StanleyNiemiec | Apr 24, 2002 02:46am | #7

   I didn't get my calculator out on this one to check, and if I read you correctly, your answer is correct.  Another way of expressing the oven dry moisture content is:

   MC% = (weight of water in the wood/bone dry weight of the wood) x 100

   Because of the open spaces in the cell lumens (ie the void volume) it is possible to have more water than wood.  When this occurs, the end result is an oven dry moisture content of > 100%.

   If you want to get some level of understand of how much void volume actually exists in wood, you need to know that the basic density of the actual ligno-cellulosic chemical components of wood is about 1.38.  If you consider this and begin to extrapolate with a species like Balsa (sp. gr. = 0.17 @ 12% MC) you end up with a whole lot of open space (void volume).  Comparatively, with lignum vitae (sp. gr. = 1.09 @ 12% MC) you don't have much open cellular spaces.

   With extremely dense woods, like lignum vitae, it is obvious that this wood will not float (sp gr > 1.0).  The reason you have sinker stock (ie logs that do not float) is due to the fact that the combined ligno-cellulosic matter and the mass of water in the lumens, without sufficient trapped air, is also > 1.0 sp. gr..

   Collapse that occurs in drying is almost entirely due to wood that has very high moisture contents such that there is little air/gas in the open cellular spaces (ie the cell lumens).  Collapse occurs before normal shrinkage begins.  One theory talks about the surface tension of the water, such that when there is no air/gas in the cellular structure, the cell walls (wet and plastic) are literally pulled together.  Imagine (as with a half-filled straw) trying to suck the water out with and without your finger blocking the opposite end/opening.  As long as the opposite end is open, the water can be evacuated readily; when the opposite end is closed, most likely the straw will collapse. 

   The most susceptible species (to collapse) are co-incedently those with very high moisture contents (eg. redwood, western redcedar, cottonwood, et. al).  Collapse sometimes occurs when the wood has wet pockets (where the moisture content in one area of a board is much greater than in others and significantly higher than the over-all board moisture content).   The heartwood/sapwood variation in some species can really drive some kiln operators crazy.   If there is any advantage to air drying relative to providing an quality improvement, it is entirely related to the fact that with air drying, collapse is generally eliminated/prevented.  Not entirely though; I have seen collapse in air-dried tanoak; it was supposedly milled in July and stacked out in the open without any shade cloth (to restrict air flow).

3. 
   jonsherryl | Apr 23, 2002 06:47pm | #3

   Neal, as Stanley's post suggests, the green wood moisture content is a function of the density of the wood as well as the actual amount of water present. The woods which typically have the highest green moisture content are the least dense species. An MC in the range of 200% is common in freshly cut balsa.

1. 
   StanleyNiemiec | Apr 23, 2002 11:22pm | #4

   I suppose that there might be some general trend that correlates green moisture content with density, but I doubt if it is an absolute or something that I would risk any money on (were I a betting man).  It would seem logical to infer that a lower density wood would have a greater void volume to wood mass ratio than more dense woods but that doesn't necessarily mean that those void volumes are going to be filled with water.  Sometimes the void volume will include both air and water --  darn tricky things these trees.

   The first is the apparent disparity between softwoods (gymnosperms) and hardwoods (angiosperms).  The highest MC listed for hardwoods was the black cottonwood heartwood I cited above.  The Wood Handbook lists the sapwood moisture content for Western Redcedar (WRC) as 249%, Sugar pine as 219% and old growth Redwood as 210%.  In my quick visual analysis of the numbers, these softwood values are greater than any of the hardwoods.  The green density for both Black Cottonwood and WRC is exactly the same  --  0.31.

   There is a real problem with the MC/density correlation if you consider sapwood to heartwood variation.  The density variation of these wood classifications is generally statistically non-significant especially considering overall within species density variability.  Yet for WRC the heartwood average moisture content is only 58% (compared to 249% sapwood MC).

   It doesn't help matters for easy classification either that there is no apparent hard and fast rule about sapwood moisture contents.  Ideally one would like to assume that the sapwood should have a higher MC than the heartwood but then there are species like Paper birch (Sap MC = 72%; Heart MC = 89%).  Hickory (pignut) is another example of this atypical trend (Sap MC = 49%; Heart MC = 71%).   Even Black walnut has a sap MC of 73% while the heart MC is 90%. 

   Even when one considers familial comparisions you would expect some consistency but alas with wood nothing is easy.  Sugar pine and Western White pine are both soft pines of about the same densities (Sugar 0.34/0.36; Western White 0.35/0.38).  They are really pretty close anatomically and if my memory serves me correctly, Sugar pine can be included in with Western White pine according to WWPA grading rules.  For all the similarities between the two species, you end up with two very different moisture contents (Sugar - 98/219%MC; Western White - 62/148%MC [both listed in heartwood/sapwood order]).

   So I guess that this all means that I disagree with you Jon.  Based on my "learning" and expereince, there are no hard and fast rules relative to MC.  I have learned to look it up in a good and reliable reference book, and to especially appreciate the wonderful variability of wood.  It may be possible to more correctly infer, relative to some aspects of wood properties, that there are no consistent rules.  But that's what makes it so much fun and so challenging!

   Edited 4/23/2002 4:27:56 PM ET by NIEMIEC1

1. 
   bckelly01 | Apr 24, 2002 02:45am | #6

   A pleasure to read your posts. Gotta love those facts and figures! I'm with you on turning to reference material frequently. With 6 billion people around, it's usually true that someone has dealt with and documented the same issue that's currently puzzling me.

   BCK

1. 
   StanleyNiemiec | Apr 24, 2002 03:25am | #8

   Thanks for the comments of positive affirmation.

   I also envy your apparent positive regard for general humanity.  I am slightly more cynical or at least have a slightly bleaker attitude about the 6 billion.  I can see no apparent correlation with sheer numbers and intelligence.  Likewise I see no correlation or trends regarding intelligence when it comes to race, gender, age, ethnicity, religion or geographic locale.  It seems to me that, as with the organic variablity of wood, there is considerable variability within humans especially related to their densities and the relative amounts of their void volume.  (a double entendre especially when it applies to individuals like Sgian when he is imbibing in his pleasures).

   And if there are all of these 6 billion, how come I never can seem to find answers for some of my questions/problems? 

1. 
   jonsherryl | Apr 24, 2002 06:44am | #9

   Stanley, you're a great detail man...Now go back and review the stats mentioned in your earlier post and I'm hopeful you will note a correlation between the specific gravities and the green moisture contents of all of the woods on the list you've cited. If not, let me know and I'll put you in touch with a good optometrist.

   The point is, the denser a wood is, the more its wood tissue displaces its capacity to contain moisture within a given amount of volume. There are variences between sapwood and heartwood and between species resulting in part from the composition of each species extractives...and also to some degree its anatomical structure, since all trees must maintain a vertical continuum of free moisture (sap...even during dormancy) in order to sustain life and the diameter of the vascular cells influence the amount required...but there is still a strong correlation between a wood's density and the amount of moisture it can contain.

2. 
   StanleyNiemiec | Apr 24, 2002 09:36am | #10

   Ok Jon --  who's the optomotrist you recommend?  I am not sure what I am supposed to be seeing?

   It is a very fine point of contention, I think, between us.  If I understand your premise, you are saying that with increasing density, there is proportionally less void volume in the total combined wood/void volume mattrix.  Ok I agree so far; but I cannot agree the next assumption you add on to that.  Correct me if I am wrong but I seem to hear you saying, secondarily, that with a greater void volume (as with lower density woods), there is a resultant higher moisture content.  Post hoc, ergo procter hoc.

   The difference I am trying to make is that just because there is a greater void volume, it does not necessarily mean that that volume is completely, totally or even nearly full of water/sap/whatever liquid.  There may exist a possible maximum moisture content (a saturation level) for the different wood species (ie when the cell lumens are totally full) but according to Panshin and DeZeeuw, "Normally wood in living trees will not contain more than half to two-thirds as much moisture as is theoretically possible". (p. 204 [1970 ed]).

   I am looking at Table 3-3 in my Wood Handbook.  This table is labeled Average Moisture Content of green wood, by species.  Needless to say, I have some added notations for other domestic woods, most likely from the Markwardt Forest Service Publication from the '30's, the DKOM, or from Panshin and DeZeeuw.  If there is supposed to be some consistent trend, I'll be damned if I can see it.  It may be that your experience is with tropical species but when it comes to domestic ones, I cannot conclude that lower density absolutely correlates with higher moisture contents.

   Here are some more numbers to support what I am saying:

   Both Beech and Northern Red Oak have the same specific gravity (0.56), they are both in the same family (####) yet for heartwood Beech MC = 55% while the red oak MC is 80%.  Why does the red oak contain approximately 50% more water than the beech?  If their densities are the same, then their void volumes are the same.

   Black cherry sp. gr. = 0.47; Black walnut sp. gr. = 0.51.  By your reasoning the cherry should have a higher MC  -- guess what? It doesn't!  Heartwood Cherry vs. Heartwood black walnut is 58% to 90%.

   How 'bout silver maple vs. sycamore.  Sp. gr. ratio is 0.44 to 0.46 respectively yet the heartwood moisture contents are vastly different -- 58% to 114%.  How 'bout tanoak vs black locust?  Both have a sp. gr. of 0.66; tanoak MC is 115% while the locust is listed at 40%.

   I will concede that among the softwoods, the extractive content might be a factor especially with highly resinous woods.  These components are notably hydrophillic, but not all the softwoods contain pitch.  The heartwood moisture contents for WRC and W. White pine are virtually the same as is their density.  WRC contains no pitch and the Thuja plicene extractives in WRC are partially water soluble.  When it comes to the hardwoods, it's more or less a toss up as to whether the heartwood MC is great than, equal to, or less than the sapwood MC.  Black walnut with a very pronounced extractive content has a heart/sap MC ratio of 90/73.

   I suppose that if I was really concerned about a density/mc correlation, I could enter all the native species data into a spreadsheet, graph it and do a regression analysis.  My guess is that I would get a flat line because there is just too much overall variability.  If there were some trend, it would likely be more or less worthless because the confidence interval would likely be huge. 

   I tend to remember two quotations that have applicability to a density/MC correlation:  "All generalizations aren't worth a damn including this one" and "There are exceptions to every rule". 

   There is another quotation that you should consider that comes from the DKOM -- " ... at the same moisture content, high specific gravity species contain more water than low specific gravity species". (p1-9)  Water in wood is probably one of the most complex aspects of wood physical properties; nothing about it is simple and straight forward!!!!  Even to the point that everyone, more or less, is taught that liquids are non-compressable.  Yet in wood, water can attain a specific gravity of 1.1 because with the bound water within the ligno-cellulosic mattrix is compressed.

   So where's this eye doctor? 

   Edited 4/24/2002 2:41:41 AM ET by NIEMIEC1

3. 
   jonsherryl | Apr 24, 2002 04:39pm | #13

   Stanley, I still think you are cherry picking the stats. The woods with comparable specific gravities that you are citing here have vastly different textures, such as beech vs red oak and cherry vs walnut...but in looking at some of the same references, I must admit there seems to be greater MC variation resulting from texture than I would have expected. However, I don't think a thorough correlation would flat line. Also, a wood's density would impact MC readings as the wood approaches or is below the Fiber Saturation Point, because of differentials in bound vs free MC. A denser wood would bind more moisture relative to its volume. As the woods approach complete saturation though, the denser species would contain proportionately less moisture. It would be physically impossible for it to be otherwise.

4. 
   StanleyNiemiec | Apr 25, 2002 12:07am | #16

   Jon:

   Stand back 'cuz I'm about to plunge off the deep end.  You can watch me make a fool of myself, take the plunge with me, or remain on the sidelines.

   Somewhere I heard that there are roughly 25,000 species of plants that have sufficient volume in the woody stems to produce lumber/wood.  I have heard that there are approximately 2,500 commercially viable species worldwide.

   Given that (here comes the plunge off the deep end), you somehow have to recognize and accept the fact that each species has some sort of inate expression of life that drives it to grow and reproduce.  I accept that each species has a god given right of self-determinism to "choose" what adaptive strategy is best.  Call it evolution, call it a collective-consciousness, call it life-force, call it whatever, but you have to recognize that trees have been around for millions (hundreds of millions?) years and are incredibly complex and sophisticated.  Within the plant kingdom, angiosperms are at the highest level (potentially as complex and developed as humans if you assume that we are at the highest level in the animal kingdom).

   I gave up long ago trying to impose some all inclusive "system" for understanding wood.  Anything you or I try to devise to "explain" wood is likely to have variations and contradictions.  Are diffuse porous woods lower in free water moisture content than those that are ring porous?  Who knows?  Try and develop some sort of all inclusive rule and you might just go mad in the process.

   Maybe it is my wacko environmental spiruality, maybe it is that Zen state I get into when I'm working with wood (especially with hand tools), maybe I'm just plain crazy; but I more or less accept and appreciate each wood for its uniqueness and beauty.  My working premise is that god didn't make any bad trees!  I might be cocky but I will never be so arrogant such that I will try to fully comprehend or attempt to impose "order" on the Creator's plans.

5. 
   jonsherryl | Apr 25, 2002 01:27am | #17

   Stanley. I'd agree with your philosophy...and in fact pretty much share your reverence for wood. Never met a species I didn't like (but hemlock comes close.)

   As for the number of arboreal species world wide that attain adequate size for producing some useable lumber, it seems I recall seeing Dr. Regis Miller of the FPL quoted as estimating the total at something less than 20,000 and I think he's one of the best minds in the field on this subject. As for commercial species, I think the 2000 figure is pretty close, but many of the individual species come to market in groups of woods like; the oaks, the lauans, the pines, etc. The number of commercially important woods could be counted in the hundreds.

   With respect to the origins of woody plants, the first protogymnosperms date from the Coal Age, more than 300 million years BP. Cycads and conifers began to dominate the world's flora about 50 million years later, with the conifers pretty much running the show by about 200 million BP. The earliest fossil record of angiosperms are assumed to be pollen of an angiosperm type dating from about 130 million years ago. Leaf fossils in excess of 100 million years old suggest the early magnolias and sycamores were among the first angiosperms. By the time of the extinction of the dinosaurs about 65 million BP the angiosperms were well on their way to dominance. One of the greatest mysteries remaining in botany is the genesis of the angiosperms. I'm hoping recent progress in DNA analysis will eventually solve this puzzle...and also that I live long enough to see it.

   ...And by the way Stanley, the best optometrist I know is Dr. Sklar...but he's here in suburban Detroit and I'm not convinced you could find the airport...(just kidding, my friend)...Some of the info in your posts here has set me straight on several misconceptions I've held for many years.

6. 
   ndavis | Apr 24, 2002 09:54am | #11

   Wow! Thanks for all the detailed responses! I have learned more about the chemistry of moisture content than I ever knew.

   But one comment about collapsed wood cells makes me wonder: how would one tell if wood cells collapsed? Does a plank look different where cells have collapsed? Does it warp or distort? Or is this something that could only be determined under a microscope? Could one intentionally dry wood so that the wood cells would collapse?

   Also, is this "Wood Handbook" the one subtitled "Wood as an engineering material"? It seems cited as "the" reference. Perhaps it would be a good book to have around.

   Thanks again!

   Neal

7. 
   SgianDubh | Apr 24, 2002 02:07pm | #12

   Stanley, Thanks for explaining why I sink in water. I've always wondered why. I'm dense, full of hot air, and saturated. Just needed the science behind it. <g> Slainte, RJ.http://www.RichardJonesFurniture.com

8. 
   StanleyNiemiec | Apr 24, 2002 06:22pm | #14

   Nay to those traits, my friend.  The double entrendre I was making was a bit obscure -- it comes from my ex-wife's insistence on using the medical term void.  One never pissed, piddled, went wee-wee, condensed the cloud, or took a leak; you voided.  Thus void volume in that context is the amount you "discharge".  I have to assume that the more you imbibe, the greater is your void volume.

9. 
   SgianDubh | Apr 25, 2002 02:09am | #18

   I do 'void' a lot Stanley. My original interpretation of your post was spot on, but you rounded it out a bo'hair. I'm full of it, and I always sink under the weight my own verbal diaerrheoa. Slainte, RJ.http://www.RichardJonesFurniture.com

4. 
   fdampier | Apr 24, 2002 08:43pm | #15

   Willow