how much does pine shrink in a kiln?

[jimF]

Actually, this is related to a topic that is interesting to me!  One can change the apparent color of wood and the dry density of wood by how it is dried.  This is related to drying stresses and voids developing between the constituents of wood as it dries.
1) As Gene mentioned, thin pieces of wood shrink more than thick pieces because the outter shell is stretched as it dries around the unshrunkened wet core.  Then the core is stretched as it dries inside the dry stretched shell.
2)The cellulosic polymere are pulled together as the water is loss as this is what causes shrinkage.  However if the water was replaced by another chemical that forms hydrogen bonds that has a lower bond strength, the force pulling the cellulosic polymeres would be less and more and larger voids would develop.  This can be accomplished by using organic solvent to displace the water, such as alcohol.  Then displace the alcohol with another solvent with even lower hydrogen bond strength.  and finally, let the last solvent to dry off.  This will reduce the density and produce more voids.

Granted, the difference in density will be very difficult to experience, but it will be there.
With the greater number and size of voids in the solvent drying the color will appear to be different than normal drying, lighter in color.  Oak will appear to be almost white.  This is because the light will not be absorbed by the wood material as much but will bounce around in the voids and off the wood.  Similar to polar bear fur is not white but because it is hollow the light is not absorb and bounces around inside and comes out  appearing as white fur.   This is similar to how fiber optics work,  The light reflects off the inside surface of the optic fibers until it comes out the ends.

[doctorb]

Sorry, I just couldn't resist.....

http://www.youtube.com/watch?v=BEnKLhi83J8

[SwampDonkey]

From:  FPL -133 (1999) ed. page 3-8



Further on in the paragraph it is only saying that wood dries quicker on the outside. Further on in that edition it shows linear shrinkage as MC% drops from FSP approaching oven dried.  It is impossible to predict how much voids, and size there of as an individual piece dries.

Also, specific gravity changes in wood as moisture is lost from FSP. Even the tables on shrinkage show this for green and 12 % MC.

There is not only confusion, but conflicts in writing. For a lot of it, one has to have faith in the tabulated numbers, not just the math/models. In fact the SP gravity value given for 12 % MC of red pine is actually considered the OD sp gravity by others as I have a note on it. How many more numbers are questionable? Probably a good many, because as I stated earlier the data does not all line up is nice neat lines and curves. (wide standard deviation) ;D

Anyway, all this voodoo makes no difference to the typical portable/back yard sawyer. And I'll stick with my 1/8" per 2" rule when sawing. ;D

Seinfeld must of solved a view problems didn't he. Love that show. :D

[GeneWengert-WoodDoc]

Thanks to JimF for pointing out another error in Sebastian's theory and that is the density of bound water.  Indeed many people do not realize that bound water is not a liquid or vapor or solid, so those density values for the three states would not be correct for bound water.

Expanding on several recent posts, the Detroit Redwing - Chicago Blackhawks score is not affected by this discussion.

[Den Socling]

Gene, I don't understand why you would say that bound water is not a liquid. In a vacuum kiln, I steadily ramp up the heat to overcome increasingly bound water. Humidity in the kiln does not suddenly drop as if there was no liquid water being vaporized. Humidity decreases linearly until I reach the final temperature.

[beenthere]

 :D :D
http://www.youtube.com/watch?v=airT-m9LcoY

[GeneWengert-WoodDoc]

It is not a liquid because it does not behave as one.  This is why the heat of evaporation with wood is higher than with liquid water.

Water liquid is two water molecules held together by a hydrogen bond.  With bound water, the water molecule is held to a cellulose hydroxyl group, especially as we get drier; it is not bound to an adjacent water molecule.  The drier the wood, the more and the stronger the wood-water bond.

I do believe the transition is not abrupt but somewhat gradual.

JimF can probably explain it better.

Take a piece of wood under 20% MC (all bound water) and cool it to 20 F or other sub-freezing temperature.  Now dry it.  If the water was a liquid, it would take considerable extra energy for the melting or phase change as the water is evaporated.  But it will not, when it only has bound water.  You cannot freeze bound water.  Of course, once it comes out of the wood, it is now free to behave the way it would as a liquid, etc.

[SwampDonkey]

I'm more inclined to think these voids in the cells walls are created when free water (water=water) bonds are broken and the water liberated like water evaporating from a mud puddle you now have a void, and the shrinkage is when the water=cellulose bond in the walls are broken and the water liberated.  That volume of bound water lost is the shrinkage. Water as a liquid has to be able to pass through cell walls for them to survive and carry smaller molecules within it for sustenance and maintenance. There are voids in the walls such as bordered pits. A recess in the secondary wall of the cell, open to the lumen on one side and a membrane closing it on the other. There is quite a wide range of temperatures that the density of water changes insignificantly. I believe also this is accounted for when considering the math, there is always base line temperatures to these models, such as 25 C for instance. What it all boils down to, is the complexity limits any reliable precision.

[jimF]

Let's see now.  Bound water is on First, free-water is on Second, vapor just got out and liquid is running home.

And following that thought, in my last post I was not specific on what density I was talking about.  As Swampdonkey mentioned "actually considered the OD sp gravity by others", the apparent density or specific gravity changes with MC, which is why the MC at which you are talking about should be included in the explanation. "There is not only confusion, but conflicts" ; confusion yes, conflicts no.  If all the articles are read very specifically you can follow at which MC the sentence is referring to and the conflicts disappears.

The quote:"shrinkage.....continues in a fairly linear manner" is a vague description  "Fairly", yes; exactly no and the curve is a reflection of the varying bond strength and development of voids as the MC decreases.  These voids are not microscopic level like pits, lumens but molecular/polymer level( including the crystalline nature as Gene mentioned).  When mud dries the shrinkage is at the molecular level between dirt and water and the large cracks are the accumulation of the molecular level shrinkage. Yes, for liquid water density changes insignificantly over a large temperature range, but when the state changes is when there is a large density change.  For example liquid/ ice, liquid/vapor and liquid/bound.

In a vacuum kiln, both bound and freewater becomes vapor before or as it comes to the surface.  There is no sudden drop in humidity because 1) the bound water becomes increasingly bonded tighter to the wood as the number of molecules become less and 2) the whole board does not suddenly attain the pressure of the chamber.  I know, I know, everyone says it does, but I cannot envision an experimental setup in which one can seal a sensor in the center of a board to replicate an unopened board in a vacuum setup and really prove the internal pressure instantly reflects the chamber pressure.  So actually during drying, one is seeing an averaging of vapor leaving, liquid evaporating and bound water bonds breaking throughout the whole piece of wood.

When one studies to become an engineer there is one year of course work in transfer of energy & matter and year of course work in thermodynamics (study of energy and changing states of matter).  The systems which they study in those classes are quite simple; they don't involve the structure and chemistry of wood and don't come close to the complexity of what occurs in wood during drying. So hang in there, you're in graduate studies here.

In the end, "for typical portable/backyard sawyer", it may not make a big difference but when you are cutting hundreds of thousands of bdft the difference between 1/8 and ¼ does make a big difference.

[Tree Feller]

In the end, "for typical portable/backyard sawyer", it may not make a big difference but when you are cutting hundreds of thousands of bdft the difference between 1/8 and ¼ does make a big difference.

I'm a typical backyard sawyer and while it doesn't make a difference, I find the discussion among you experts absolutely fascinating. As you said, this is graduate studies stuff and best of all, it's free.   8)

[GeneWengert-WoodDoc]

Thanks again Jiim for making clear statements about what I was trying to say.

Your closing statement about graduate level brought back to me the year classes on TRANSPORT PHENOMENA, P-CHEM, and THERMO.  They were tough indeed, especially because all we had was a TI-2500 ($208) which replaced my slide rule.  It could add, subtract, divide and multiply, but did not have a memory.  For big problems, we had punch cards and then stood in line to put them in the computer, only to find out that one card was not right, etc.

[jimF]

ha, slip sticks (slide rules) nothing like them, fond memories, still have one in the basement.  And punch card readers and main frames, the good old days.  I read an article yesterday comparing 1980 computers to computers today. On 50% of the typical office operations the 1980's computer was faster in accomplishing the task then the bloated current computers.  I guess I aut to stop :D

[jimF]

It might help in this discussion to realize that the basic different between the various states of any material is the distance between the molecules or atoms and the distance is a reflection of the bond strength between the molecules.  There is no bond between vapor molecules.  There is moderate strength between the liquid water molecules and the same with the distance.  The distance between molecules in ice is very short and the strength very high.  Bound water has a bond strength between liquid and ice and respectively the same with the distance.  The bond strength determines how much energy it takes to break the bond.  With vapor the bond is already broken.  It takes more to break the bonds in bound water than water and even more for ice.
The high bond strength in liquid water as compared to the bond strength between water and the glass is what causes water drops to form on your windshield.  As mentioned earlier  the bond strength within alcohol is low which is why alcohol does not form drop on glass.  In the same way the bonds between alcohol and wood is weaker than wood and water and is why solvent drying reduces the final density of the wood by causing void space by not pulling the wood constituents together as strongly as water does.

[beenthere]

So...... how does this help answer the question of this thread? Does it change how much pine will shrink in a kiln?

[Paul_H]

 :D Beenthere,this is how analysis paralysis can set in and nothing gets done.Reminds me of the time Lloyd Neuman welded up a cast iron manifold for a D8 with a arc welder.It worked and held because he didn't know it couldn't be done.

[isawlogs]

So...... how does this help answer the question of this thread? Does it change how much pine will shrink in a kiln?

:D  Of course it will help, one needs to find an 1980's computer put in all the data calculus algebra and a little of the unknown facts, and then quicker then a laptop opens the answer will pop up.   : :D :D

;D

[Warbird]

Gene, I don't understand why you would say that bound water is not a liquid.

It is not a liquid because it does not behave as one.

The Forestry Forum - redefining the Laws of Physics, one tree at a time. 

smiley_eek_dropjaw  

[SwampDonkey]

In the end, "for typical portable/backyard sawyer", it may not make a big difference but when you are cutting hundreds of thousands of bdft the difference between 1/8 and ¼ does make a big difference.

It doesn't, in two ways. The small sawyer has small volumes and a large  number of customers to satisfy. The customer is much happier with a board he can dry and finish and be assured the dimension he is targeting can be hit. They're not worried about going broke over losing an 1/8" there are other things to worry about. And two on a commercial mill up here at least, shavings off the planer mill is a value added product destined to the pulp mill. This also means a little less sawdust (which is also salable) because of the extra wasteful ;) 1/8" the saw traveled between cuts. The commercial mill also has customers that are even fussier than the back yard sawyer's customers. The models are just not that reliable to give consistent enough results without adding a margin of error, which is this 1/8" your arguing over. You get this curve or line on a graph for dimensional shrinkage and forget that the data is scattered all over the paper. All it does is predict, sometimes it's close, sometimes it way off.

[WDH]

As far as pine goes, the effort by industry to squeeze out every ounce of wood has led to high variability in product quality.  As much as 25% of boards in a pack can have marginal use or be unusable.  This more a result of lowering the harvest age and plantation wood versus natural wood than manufacturing, but it all goes together.  Pushing down the green target thickness can at some point lead to more wane, and the grade rules do allow a fair bit of wane.

This situation creates an opportunity for the small sawyer where they are not milling hundreds of thousands of board feet per day.  The small producer, like most of the sawyers on here, can produce a product superior in quality by not pushing the limit like the big commercial mills do. 

[GeneWengert-WoodDoc]

WDH...good point indeed.  However bad a piece looks, the mill is trying to make a piece of lumber that will be stiff enough (sometimes strong enough, but usually stiffness is the limiting criteria) to function as a joist, rafter, etc.  The piece must be straight enough and have enough nailing surface on the edge, etc. to do the job.  It is so over-designed in most pieces, that it can tolerate some planer skip, and other defects but still do the job.  That is not to say that a few pieces may indeed be off grade.  However, in construction, there is a principle of load sharing, so that one weaker piece has its neighboring piece to help pick up the slack.  I think the "proof of the pudding" is that we do not see roofs or walls collapse with the present lumber and construction techniques.  However, when building we do order more pieces than necessary as we know a few pieces will be thrown away...mostly due to warping after grading.  In fact, that is why engineered beams (like LVL) were able to compete with solid wood floor joists...more expensive but no waste.

[isawlogs]

 Then should not the grading be looked at differently and make it so that there is not as much waste, seams to me that the stud market grade is full of rejects that twist, bend so much that you basicaly can't use them. It gets veryu frustating to have to buy a pallet load of wood and have a guy sorting the wood needed to build a wall. We cry over made in China, but at times I beleave the big mills are taking a night class on quality from them.

[GeneWengert-WoodDoc]

There are different grades that have different amounts of defects allowed.  The higher the grade, the better the product.

All grading is done at the time of manufacturing.  If a stud is made and graded S-GRN (no drying at all perhaps) then it can be expected to dry and possibly warp after grading.  So, it would be better to get KD19 or KD15 lumber as the maximum MC is 19% or 15%, which minimizes warp after grading because the MC change will be small.

Many of the large stores have their studs inside the building where it is warm and very dry in the wintertime.  So the pieces will dry in the store to a lower MC than they will have in use.  The more we dry, the more warp that we will see.

[SwampDonkey]

Then in rural areas if you go to the lumber yards of the 'Home Hardware' or the 'BMR's, often the storage area is inadequate and often times some of the piles are just out in the open yard. Someone busts open a bundle and it sits for weeks with the weather raining down on it and it's like fishing lumber out of a bath tub, it's sopping wet. It don't happen a whole lot, but sometimes. Myself, being near a local sawmill, and if I needed building material I would be buying direct. But for a few pieces for a tool shed, not much bigger in floor space than a back yard gazebo or for a picnic table we usually head to the BMR. Sometimes if your not checking, you'll be charged for top grade on the receipt but you had to flip 4 pieces of material for every taker and it wasn't top graded according to the stamp. Most people not in 'the loop' wouldn't know one grade from the other, the stamp means nothing. But they do experience that there was a lot of poor pieces they had to separate out and figured the price is the price. Pricing is another game at retail, there's running foot and board foot.

Also I've been in huge warehouses of kilned dried hardwood lumber. None of those warehouses were heated or climate controlled in any way.

I think we've pretty much circumnavigated the globe on this topic. ;D

[jimF]

In the near end it may not make much difference to the small sawyer and to the large producers the waste- sawdust and chips- are of lower value than solid wood, but in the far end it does make a difference.  Gaining knowledge is gaining truth, truth in how God designed the universe.  God created us in His image and with a desire to see Him or know Him.  Moses did not see God's face but he did see his back.  Understanding His work is like seeing Him.  And as the good book says, they will know you are mine by how you love one another.  Well ....they will know you saw knowledge by you being able to predict and explain My created processes.(I know I took liberty here)  I enjoy sharing the knowledge I have gained over the years to those who are interested in gaining understanding - and as Tree feller says: it's free
At least, this is the way I look at life.

[SwampDonkey]

Facts mixed with statistics and conjecture only gives estimates and predictions that are not always favorable.