Good evening,
I have a question regarding air dried lumber versus kiln dried lumber.
If I buy green lumber from a saw mill, sticker it, and let it dry, it will eventually reach the average moisture content of my area (12-14%)
What happens when I buy kiln dried lumber and let it set in my shop for a long period of time? Does it reach average moisture content of my area also?
They say you should make furniture from K.D. Lumber only. If I let my green wood set long enough (2-3 years), can I safely build furniture from it?
If I get a really good deal on some K.D. wood, am I wasting my money because I might not build with it right away?
Thank you in advance for your thoughts.
Ray
Replies
Who is the "they" that say to use KD lumber only?? Kiln drying does not guarantee proper drying -- it can easily be case-hardened, or have other problems that result in scary wood coming off the saw. I'm certain I've read posts from Knotheads that insist on using air-dried only.
forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Ray, Howie's answers pretty much cover the practicalities, but I suspect you are searching for the "why" of it. Essentially, kiln dried lumber differs form air dried in that the higher temperature used in the kiln process causes the natural adhesive in the wood (lignin) to soften, which in turn allows the wood cells to realign themselves in relation to one another as they give up moisture and shrink. Once the wood is dried and cools down to normal temperatures, this ultimately means that the lumber will be under less internal stress at lower ambient moisture contents (EMCs). There are a host of reasons why air dried lumber of certain species is nicer to work with, but the essential, functional difference is that kiln dried stock tends to be more stable (less likely to distort or fail) in arid environments, such as centrally heated homes in cold climates. This of course assumes that it was propertly kiln dried...which isn't the safest bet when buying commercially processed lumber.
Well, in my opinion (obviously), the only major advantage to KD is the speed. Some things can go wrong during KD that you will not see with air dried lumber. Like Case Hardening. Also bleaching of the color if steam is injected during the KD process (in an attempt to minimize case hardening).
Disadvantages with air dried are 1) slow, 2) worms, 3) rain storms, 4) more prone to end checking, and probably a whole lot of others (including theft). If you leave that stuff setting out for a long time, evil eyes are sure to see it.
The way I look at it, there are two reasons to dry lumber - 1) to get the MC down to a level where the wood is workable, and 2) to find the bad lumber! Getting the MC down to 6 or 8% is a pretty good way to find the bad stuff!
I store my KD lumber in a non air conditioned shed here in Houston where the humidity is constantly at the dripping point. I have had no problems (other than glue creep) working with the lumber. I also store air dried there and it behaves just like the KD when working it (including glue creep).
Regardless of which type you use, you MUST allow for contraction and expansion!
PlaneWood by Mike_in_KatyPlaneWood
Curiosity here, related to the direction of this thread: I have noticed when buying 5/8"x3" red oak backsplash material at the local Lumbermen's store (contractor-oriented place) that much of the oak has significant greyish areas in it. Is this likely due to the KD process??forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Forest Girl, I can't think of any process associated with kiln drying that would cause oak to develop gray pigmentation, unless it was exposed to iron somehow. There is such a thing in oak (especially in red oak harvested in the southern Appalachians) called mineral stain...probably resulting from the soil in that region, but I haven't seen any research confirming the exact cause. I know as a hardwood buyer years ago, I always tried to avoid southern red oak, because our contractor-customers got down-right hostile if we shipped them material with these stains...Personally, as a furniture maker, I think the stains can be attractive.
If the grayish hue in the stock you are referring to seems to be isolated to the sapwood, another likely possibility is that it was caused by fungi, probably prior to kiln drying.
I'm siding with you on this one, Jon. That southern red oak has a grayish pallor. I always figured the oak grown south of the Catskills was firewood, flooring or pallets. There really is a big difference in the color and those that have it will generally describe it as "Northern Red Oak".
LeeLee Grindinger
Furniture Carver
Thanks Jon, I'll see if I can get some specific info on where this wood comes from.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Grey stain is due to a bacterial infestation and results from improper storage of wet logs and/or lumber. It is a defect!
I'll just add that there certainly is a difference between air dried or low, low temp dried lumber and conventional kiln dried lumber. I do a bit of carving and the elasticity of the wood itself is greater in air dried wood. This makes carving easier as the fibers can stretch a bit more without tearing as I carve.
I certainly don't need a published source to tell me that.
LeeLee Grindinger
Furniture Carver
Lee:
I pulled my copy of the DKOM. If you are referring to the section on "Effect of Drying Temperature (p 8-2)" let me cite the entire relevant parts:
For clarification purposes, "high temperature" appears to have two connotations. The general usage of "high-temperature" drying relates to temperatures between 225 and 240 degrees F and is primarily limited in use to certain species of softwoods (generally pines). I have never heard of anybody drying hardwoods with high-temperature kiln schedules and in fact most hightemperature kilns are for drying Southern Yellow Pine.
The other connotation of "high temperature" relates to elevated temperatures early in the schedule. In this particular scenario, a temperature >110 degrees may be considered too high -- but the issue here is not the temperature in and of itself but rather it is related to the fact that with an elevated temperature, it is almost impossible to maintain the correct wet-bulb depression (a measure of the kiln relative humidity conditions). If the wet-bulb depression is not correctly maintained the relative humidity level in the kiln is too low and drying (especially surface drying) is too rapid leading to checking, splitting, collapse and honeycomb that becomes manifest later in the schedule.
That is (some of) the process/theory of kiln drying. It is important to recognize that the "strength reduction" of conventional drying is reversible and not permanent (as stated above). Your belief that air-dried material is more elastic has not been substantiated by research (unless you are using material dried by high-temperature schedules). Rather I would speculate that the air-dried material you are carving has a slightly higher moisture content and because of the higher moisture content, it therein has lower strength values (ie greater elasticity). I doubt if you could differentiate between woods (kiln dried vs air-dried) in a blind test if both were at the same moisture content level (especially considering the inherent variability in wood strengths/densities that normally exist).
Stanley, to continue your quote from the text in DKOM...
"Temperatures ranging from 140 to 160F have little effect on mechanical properties."
This implies to me that temps above 160 do affect the mechanical properties and I'm sure I don't need to list the drying schedules that exceed 160 for you.
To clarify for you...I agree with you that kiln drying is preferable to air drying. Were conventionals built and operated properly I'd have little complaint about the conventional kiln process. This is where it all falls apart. Many (most?) conventional kilns are not operated properly and this begins with the loading. I think the conditioning step is compromised by poor air flows and a fundamental flaw in the notion that conditioning somehow penetrates the "shell" to the same depth that the case hardened layer extends. There is no "shell", it's just a handy term and to think that conditioning penetrates to exactly the depth of the case hardening is a bit unrealistic.
I operated two low, low temp dehumidification kilns in New York State for years. The highest I went was 140 and because the chamber was sealed I had near perfect control over the gradients that cause so many defects. I found that lumber dried at 120 or less had the same carving properties as air dried lumber with one benefit not yet discussed, pigments.
Pigments are affected by temps in the 160, 180 range. Walnut pigments change at those temps but I can assure that they stay put at 120. Walnut dried at 120 had the same coloration as air dried walnut. Any walnut run through a conventional kiln will lose many of the highlights that air dried walnut has whether the uniforming dead pile steam cycle was used or not. Temperature alone will affect those pigments. This is true of cherry as well.
As to being able to differentiate between AD and KD wood at the same MC. You bet I could! One chisel stroke is all it would take. I've done it. I now live in Montana and I've actually wetted my floor in the winter to raise the RH because 17% is just too low. I'll save you the trip to the EMC chart. 17% at 70 degrees is about 4% EMC so yes, I can air dry lumber to very low MC's. I moved here with several thousand feet of stock left over from the DH dried stock so I've been able to compare it to KD stock that's been laying around my shop here in Montana for years, same MC, different kiln, and I can assure you there is a difference.
I do agree that control over moisture gradients will produce better lumber if that control is used properly.
LeeLee Grindinger
Furniture Carver
Lee:
You are getting to a subject that is near and dear to my heart -- training of individuals within the entire Forest Products sectors. It is abysmal -- first due to the fact that in many industrial situations, the low wages tend to create a labor pool from individuals with lower levels of education and secondly because the industry does not generally invest in training and staff development programs for the workers.
At OSU, now many years ago, I did a survey of training within Oregon sawmills and 49.9% of the workers from responding mills (responsible for about 80% of the state's production capacity) were getting 1/2 hour or less per worker per month. Hell, that's the monthly safety meeting. And with the advent of more sophisticated machinery and computer aided manufacturing, I feel the industries are falling behind rather than advancing. Even now, the sawmills have not appreciably changed their attitudes regarding training and the possible benefits that might result. I think it is worsening as workers are more and more being considered as some disposable commodity rather than a valued company asset.
Primary mills typically invest more time in training than do secondary operations so that secondary operations tend to tranfer knowledge from one worker to another. This is a sure way to screw things up and to muddy both theory and practice. For example who is training most cabinet makers -- they certainly are not getting anything near what Sgian when through.
Even at the craft level, training opportunities are of pretty low levels. Generally somebody that has attained some level of proficiency AND notariety is the one giving the seminars and workshops. They may be good at what they do but that does not imply in any way they can teach. The other major source of training comes from equipment manufacturers and their sales staff. These programs are often nothing more than info-mercials for a particular product line and therein do not attempt to cover fundamental principles. A perfect example of this is the extremely low level of knowledge existing about basic wood machining principles (as demonstrated in some of the "discussions" I get involved with when it comes to sawing). Go into a lunchroom at a small commercial establishment or get a group of recreational woodworkers together and start a conversation about technical aspects of wood and wood utilization and in my mind, the result is appalling.
Without good training, people just do not know how to differentiate quality (in either work or material). It results in lower levels of production and in ever diminishing levels of quality. It is no surprise to me that the wood products industries are struggling and why consumers are turning to other product choices. Hardwood plywood is a good example -- how good a product is it with the current thinness of the face veneers? And it makes it more difficult for good firms to compete. Consumers cannot differentiate and therein price become the primary determinant.
I got pretty upset with a recent survey of circle saw that was run in FWW. Despite my comments and reassurances that FWW would contact me for support on various technical subjects, I have yet to do any cooperative work with their staff. For me it is disheartening to read published information within the pages of FWW that is incorrect -- largely because so many individuals use the publication as their primary (maybe only) source of information.
Kiln operation does not require a PhD but kiln operators certainly fit a profile of wood industry personnel that are not properly trained or educated in wood properties and drying theory. It is also fact that kilns are often poorly maintained and continue to operate in marginal conditions. Many are also old and beyond their effective service life but as long as the industry can continue to pump product through and the customers don't know the difference -- why should the industry implement change? Why not cut some cut corners in areas like conditioning?
OK so much for my rant!!
But for the sake of continuing the above discussion -- if strength properties are decreased with higher temperatures (as you imply at even levels between 160 to 180 degrees F) wouldn't that mean that kiln dried wood would be easier to carve than air-dried material?????? Where is my logic failing me?
Where is your logic failing you? I doubt that it is, Stanley.
If I were to list a few properties of wood they would be these, compressive strength, stiffness, hardness, shock resisting ability and bending strength. Both strengths, bending and compressive, are affected by temperature but I suspect the effect of 180 F is not of any real consequence to woodworkers.
Of these properties 180 does seem to have a noticable effect on stiffness and hardness. I used the term "elasticity" earlier to describe a characteristic difference between conventional drying temps and air or low drying temps and elasticity seems to be the best word I can come up with.
Let me give this a shot. As I put pressure on wood with a chisel the cell walls stretch. They eventually break and if my chisel is sharp enough they break where the pressure is applied, not two cells over where the bonds are weaker or between cells. The bonds are different in air dried lumber or lumber that's never seen higher temps, they seem stronger so the cuts are cleaner because the breaks occur more readily at the point of pressure. There seems to be less tearing. The entire structure of the wood seems softer. I could equate it to the difference between clothes from the dryer with a fabric softener and without softener at some level. Being able to cut into the grain makes carving much easier and I'm more able to do this with a more elastic wood. It seems the bonds are weaker that hold wood together in lumber exposed to higher temps.
I suppose one could quantify this difference by comparing walnut dried at 220 F to walnut dried at low, low temps. Starting with a huge difference and devising a test to measure the difference would be the way I would approach it. Brittleness comes to mind when I think of overheated wood and lack of brittleness (elasticity?) is the characteristic I'm trying to convey.
Donald, these threads are all too rare. It's a genuine pleasure to talk with Jon and Stanley on this topic. My gray matter needs all the exercise it can get.
Lee
Lee Grindinger
Furniture Carver
Lee, I think it's interesting to note that the fatty acids found in lignin have melting points beginning around 155 degrees F. It might well explain why your low-temp kiln, operating in the 140 F range, produced "KD" lumber very similar to air dried stock, while even the ordinary, conventional kilns operating at only slightly higher temps (in the 180 F to 200 F range) would produce stock that is noticeably different.
...And Stanley, it's good to have you back in action and posting again. You're a wealth of information...but it's going to be awhile before you convince me there's no difference between KD and AD stock once they've both acclimated to the same EMC. As for the ravages committed by misguided kiln operators, though, you have my complete agreement.
To all three of you--Lee, Stanley, and Jon--it's an overwhelming pleasure to see you back posting genuinely useful information. This site has become so oriented to social notes and ill-thought-out political rants that I can usually get more actual woodworking information at Badger Pond, if you can imagine that.
But this thread has taken me back to the days when there were regular contributions by people who really knew something about wood and woodworking.
I am, as always, indebted to the three of you for a big part of my woodworking education.
Don:
You are breaking one of the fundamental rules of this site -- no flattery for me as it might go to my head!!
Thanks anyway.
The reason I have not been posting is that there has really been little to write intelligently about -- unless you consider my political and philosophical opinions intelligent!!??
I am in agreement with Donald Brown. The primary attraction of checking this Forum almost daily is to see if Jon or Stanly have found a post worth responding to.
BJGardening, cooking and woodworking in Southern Maryland
Jon, Stanley, Lee -
What is the history of Kiln drying? When was it first started as a wide spread general practice, and in what country? Was it at first just used for a particular species?
PlaneWood by Mike_in_KatyPlaneWood
Edited 12/18/2002 9:19:40 AM ET by PlaneWood
Mike, I think the general principle of kiln drying, i.e., the fact that heat dehydrates and stiffens wood was first discovered around a Stone Age campfire...when our ancestors came to appreciate how much better a sharpened wooden spear would penetrate, if the tip was first hardened in fire.
As for using heat to assist in seasoning wood, for centuries cabinet makers would often store partially air dried stock in racks around the chimney in their shops...But I believe the advent of modern kiln drying procedures traces back only to the early 20th century...at least in terms of attempting to use an enclosed chamber with controlled heat and atmospheric conditions, coupled with specific "schedules" for various species of wood.
...With that said though, I'll defer to Stanley...Since he probably worked with the guy who gave it the first shot.
Edited 12/18/2002 9:51:00 AM ET by Jon Arno
Jon, that is really interesting. We need a government grant to run this down. Who here can write grants? Let's get on the dole... This is of huge importance to the four of us here...
It does seem more than a coincidence that your fatty acids start dancing in the temp range right between conventional and DH/air dry temps.
Just like volatiles become more volatile at 160 in pine there is something that leaves the woods at these higher temps. It certainly appears to be temperature related and perhaps it is something within the fatty acids.
LeeLee Grindinger
Furniture Carver
Lee:
I have written grants! Successfully too!!! And before we get really diverted into some discussion of governmental spending/waste and being "on the dole", remember that there are other sources for grants than governmental agencies. Private industry also funds them as part of their contribution to overall R&D.
But before I do so I want to get more details about which fatty acids Jon is considering. The more I think about it, the more I believe that any low temperature melting fatty acids are likely to more closely associated with the extractive content instead of the lignin fraction. Fatty acids as I vaguely remember are open chain molecules while lignin is phenol based.
What you are describing (regarding carving) is discussed in Koch's book Wood Machining for cutting orthogonally perpendicular to the grain as a MacKenzie Type 1 (a) failure (of the wood). This is defined as "This failure type is distinguished by constant average cutting forces for successive cuts after the first cut is made. Below the cutting plane, splits occur parallel to the grain . . . these splits may be short and regular . . . and are associated with small wave length and amplitude of recorded cutting force fluctuation. Relatively high surface quality results from this failure type."
I consider wood machining/cutting to be an induced failure of wood (hopefully/ideally) in some controlled format.
It is interesting to note that high moisture contents are generally associated with this machining/cutting failure type.
Four variables were considered significant -- species, moisture content, rake angle and chip thickness.
Stanley, the fatty acid I had in mind is lignoceric acid, which melts at 84.7 degrees C...which I think translates to about 184 F.
Lee, in wood tech jargon, I've always taken the term "elasticity" as being more of a reflection of the wood's "memory"...In other words, the degree to which a wood is "elastic" reflects how far it can be bent and its ability (desire?) to return to the same shape it had prior to being bent. The conditions you describe seem to deal more with what I would call the brittleness of the wood, i.e., how much it can be bent before it fails (breaks.) I think the more "brittle" a wood is, the more likely it would be to fail at a point other than precisely where the pressure is being applied. In other words, it would tend to fracture at the nearest weak point.
...But in my experience, these terms get used rather subjectively...and I certainly don't mean to quibble with your choice of words, since I very much agree with what you are describing as it relates to the difference between KD and AD stock.
Edited 12/21/2002 12:18:59 PM ET by Jon Arno
Jon, in my simple mind I would equate elasticity with brittleness, one being the opposite or absence of the other. AD wood's ability to cut more easily is because it's able to stretch without failing more than KD stuff can. Sheesh, whadd'ya want me to do?...go back to school and become literate?
A rose is a rose is a rose...
I have tables comparing wood properties for these 6 characteristics at hand. Compressive strength, bending strength, stiffness, hardness, shock resisting ability and shrinkage. It seems to me these 6 are quantifiable values but it gets a bit fuzzy around the edges when we start talking about the touchy-feely characteristics, probably because we start describing traits that are a combination of of the quantifiable properties.
LeeLee Grindinger
Furniture Carver
Lee and Jon:
With regard to the concept of elasticity, I like to think of elastic limit -- if you bend a piece of wood and do not exceed its elastic limit it will return to its original shape. If you exceed the elastic limit, permanent and irreparable damage will have occurred. In testing a piece to failure, (if graphed) the overall appearance of stress/strain curve would show a rising straight line that at some point beings to curve until a sharp downward drop in the line occurs. The downward drop is the ultimate failure point (load capacity) but the point at which the line begins to curve is considered the elastic limit.
Strength in wood (in bending) is measured as the Modulus of Rupture (MOR) while stiffness (resistance to bending) is measured by the Modulus of Elasticity (MOE) which is also known as Young's modulus.
And Lee, if you could ever find a copy of Koch's book, Wood Machining Proesses (a highly sought after and rare volume amongst Wood Techies), it would be great for those long cold Montana winters cuz it would probably take you all winter to get through it. Light reading it is NOT! Yes he does discuss all aspects of wood machining in minute detail. He classifies orthogonal cutting (as in hand planing) into two types -- parallel to the grain and perpendicular to the grain (end grain).
Parallel cutting failures are of three types -- a Type 1 failure occurs when cutting against the grain and chipping (chip out) occurs ahead of the tool such that the surface will not necessarily be smooth; a Type 2 failure occurs when planing with the grain where a shaving is produced (the shaving having various breaks in it but the surface is smooth) and a Type 3 which is more akin to scraping that produces a very fractured shaving but a smooth surface. These three types of chip define all aspects of machining either with or against the grain and tooling (rake/hook) angles.
The primary differences between orthogonal cutting and peripheral milling (cutting with power tools following a circular path) relates to tool path travel direction and instantaneous tool angles. Chip formation types are similar. In orthogonal cutting, the tool path follows a straight line with constant tool angles while milling follows a circular path. This means in reality that with circular traveling tooling, the tool angle of attack changes (the rake angle being steeper when it enters the wood than when it leaves; the clearance angle being shallower entering than leaving) and there are the concepts of upmilling and downmilling.
Most of the problems associated with discussing wood is entirely related to vocabulary -- wood is very complex and the complexity of language used to describe it can be exceedingly variable unless you get to the basic and specific terminology that has some scientific basis/foundation. Generally what happens is there are huge discussions that somehow end up in agreement once everybody ends up using the same terminology -- like "ya that's exactly what I was saying" -- unless of course somebody is proposing some theory that has no basis in reality or it is a discussion between Jon and me.
Jon et al.
I have been following this thread with some interest as I am an organic chemist with some very considerable ecperience in that area. Although I can't add useful info firectly related to the differences between KD/AD wood properties, I can definitely say that the melting point of any fatty acids in the wood itself is not an issue. First of all the melting point of any compound is quoted for the pure compound and the temperature at which a solid turns into a liquid changes according to a complicated phase diagram if there are additonal components present. Hence relating dryingt emps to the melting point of wood compenents is not accurate.
My own personal feeling is that since the polymeric components of the wood fibers are susciptble to chemical reactionsof the organic polymers, such as hydrolysis, dehydration and/or elimination at elevated temps especially in the presence of water/steam, that there is definitely some chamical change to the wood itself in the any process that involves heating of the wood to cahnge the moisture content.
I would also anticipate that silimar chemical changes to the wood polymers can be effected at even lower temps in the presence of ammonia which is likely to effect the same types of chemical reactions, i.e. cleavage of polymeric ester linkages, dehydrations of alcohol functional groups and dehydration rezctions on the lignan or other structural wood polymers as water/steam does but at much lower temps.
Am following thus discussion with interest because of I have aninterest in both structure of things at the molecular level and in woodworking.
Brill:
I'm glad somebody can talk intelligently about chem-mystery. Most of these fine points of minute details is beyond my ability to comprehend.
In my opinion, you are probably correct to assume that that there are many changes going on and it is not tied to any specific compound. But two questions do arise -- Aren't these same reactions taking place in air dried material (albeit at a slower rate [reaction time affected by temperature, pressure and concentrations])? AND What is the likelihood that these reactions would have significant (statistically) effects on the relatively stable major chemical compounds (cellulose and lignin) that determine wood strengths?
I would like to get this discussion away from personal observation and to a level where differences can be measured. I don't believe there is a difference between air dried and kiln dried material because there is no measureable way to differentiate the two so that two statistically distinct populations are evident. Jon and Lee believe otherwise -- in both cases as it now stands, it is an issue of faith rather than fact
I'm a scientist by profession and I understand the importance of all of these parameters to persons who work wood as their profession.
However, I cut wood on my land and dry it stickered in the barn. I dries really well here in central California. It's my wood in a way this is almost more important than the workability and tensile strenght, the modulus, the lignin content, etc. It's "wild" wood from orchard trees and hedgerow walnuts and it air dries in 3 -10 years with beautiful color and figure. It's what I grew and cut, and what I made and it has a value (to me) beyond anything from the hardwood store.
Does it work as well as what I buy from the kilns? I don't know. But find kilns that dry olive crotch, eucalyptus burl, black/english walnut graft unions and we can see.
But I don't work wood to eat. Those that do have lot's of concerns that I don't. If they worked wood like I do, they sure wouldn't be overweight! I won't tell you what I do for a living - it'll start a whole new thread that ain't about wood. That's not why I come here.
Things probably look a lot different when you eat what your wood work buys!
"I'm glad somebody can talk intelligently about chem-mystery."
I can certainly talk about organic chemistry with some credibility since I've been teaching it and doing research in the area for about thirty years. However, polymer chemistry and specifically wood structure is NOT my area of speciality.
However, I am interested in a rigouous scientifically based project in the area if I can find the right collaborators and if there is some support for it. From your previous post I take it that you are not completely unfamiliar with the peer reviewed research process.
"In my opinion, you are probably correct to assume that that there are many changes going on and it is not tied to any specific compound."
As you've noted, these are my opinions only. I do not have any scientific basis for my opinions.
"But two questions do arise -- Aren't these same reactions taking place in air dried material (albeit at a slower rate [reaction time affected by temperature, pressure and concentrations])?"
Almost absolutely correct. Tempature, pressure, and catalysts all change the relative rates of chemical reactions, i.e. the kinetics, but they do not fundamentally effect the reactions themselves. Changes in concentration can have a dramatic effect on the equilibrium concentrations of reactants and products, i.e. the thermodynamics of reactions.
"What is the likelihood that these reactions would have significant (statistically) effects on the relatively stable major chemical compounds (cellulose and lignin) that determine wood strengths?"
I would like to get this discussion away from personal observation and to focus on differences that can be measured. If there is a difference, then there has got to be a measureable way to differentiate the two. Jon and Lee believe otherwise -- in both cases as it now stands, it is an issue of experience and opinion. While I certainly differ and respect there experience and opinions greatly, perhaps they can suggest some differences that are susceptable to measurement thereby giving us some hints about what to measure or look for in the way of chemical differences.
Scientific approach?? Lot more fun to argue about it!!
Some would say that the process of 'arguing' is a way to establish lasting friendships.
The ages old struggle between passion and logic never ceases!
PlaneWood by Mike_in_KatyPlaneWood
Brill, for starters, I would love to know if there is any difference in lignin extracted from kiln dried and air dried wood from otherwise identical samples of the same species; both in terms of its exact chemistry and also its subsequent reaction to increased temperature, i.e., its melting point and the temperature (or approximate range) where it begins to develop plasticity.
I appreciate that lignin is extremely difficult to isolate without altering its make up in the process, but if you could accomplish the above, it might allow this question to be quantified. I'm not suggesting that it could only be the lignin that causes the difference in KD and AD stock...and there may well be other extractives that are either altered or lost from the wood that contribute to the subsequent changes in properties...but if we could remove lignin as one of the more complex unknowns, I think we would be well on our way to solving this mystery.
Chemistry at this level is well over my head. I can't begin to guess what changes take place at the molecular level that cause me to notice the differences that I do between woods exposed to different temperatures.
I do know that there is empirical proof that walnut pigment changes at temperatures above 150. Exactly where the start of that change begins on a thermometer I can't say but the pigments in the sapwood change color at the normal steaming temperature. Those pigments don't "run", the pigments already present change and they change because of the heat (and steam?).
As I stated earlier I'd start with an extreme and work from there. Dry some wood at 220 F and compare it to wood that's been air dried. Identify the differences and try to quantify them. Dry some wood at 215, do the same, 210, etc., etc., some difference should present itself.
There is a difference. Wood dried at high temps such as 220 exhibits different properties from wood dried at 180, hence the 180 cutoff point for most hardwood drying schedules. I would be reasonably certain that these changes don't just begin at some magic number, I'd guess the changes to occur incrementally as the temperature increases.
Lee Lee Grindinger
Furniture Carver
Brill, what you are adding to the discussion here is very interest. In your comment about a "complicated phase diagram" you seem to suggest that the bonds within the lignin molecule (or any molecule) alter the required temperature (to something other than the melting points of the constituent compounds) at which there is a chemical change. Can you give us any indication as to actual temperature (or the shift in temperature range) at which the lignin molecule would release or alter its fatty acid components?
Also, you indicate that water/steam would definitely cause chemical change with respect to compounds in the wood. It would be helpful to know what compounds are "giving up the ghost" or being created in that critical 160F to 180F range. I've always assumed that it was the fatty acids, because of their melting points, but what other bonds are broken in that range that could potentially result in more rigid lignin...or drive its future thermoplasticity to higher temperature levels? I know that heat accelerates the formation of some of the polymers that cause pigmentation, but can you think of any polymers that would form or break down in that temperature range which would impact the strength and/or rigidity characteristics of wood?
Edited 12/18/2002 1:47:06 PM ET by Jon Arno
I've never seen that book, Stanley. Does it have a description of how wood cuts (fails?) when planes and chisels are used. I can see similarities in what you quote from the book concerning machine driven cuts. I'm just wondering if Koch sees differences between machines and chisels other than degree.
I'm kidding about the grant. The last thing I want to do is get caught up in some research. I've got furniture to build.
LeeLee Grindinger
Furniture Carver
Jon:
I trust most of what you have to say relative to wood anatomy but when it comes to drying theory -- what you say I have never heard or read one inclusive thing that would support what you have just written. Perhaps you can cite something to support your comments! Hopefully, what I say in following will help you and others better understand wood drying dynamics.
"... kiln dried lumber differs form air dried in that the higher temperature used in the kiln process causes the natural adhesive in the wood (lignin) to soften, which in turn allows the wood cells to realign themselves in relation to one another as they give up moisture and shrink."
The purpose of a kiln is to control relative humidity. This cannot be done in ambient air drying. A gradient is required to dry wood -- if external conditions create a situation where the relative humidity equates to an equilibrium moisture content (EMC) lower than the existing EMC condition of the wood, moisture will move out of the wood and into the air. A kiln is simply a place to control relative humidity and it is done by manipulating temperature and the level of moisture in the air (ie venting moist air to the outside or condensing excess moisture to drain).
Yes lignin is thought to be thermo-plastic but there are numerous kiln schedules that exist where lignin thermoplasiticity is likely non-significant (statistically). With sufficient moisture wood is, in and of itself, elastic. And "realignment", based on lignin plasticity, from everything I have studied, does not take place.
Drying implies removal of water, of which there are two catagories within wood -- free water which is water in the inter-cellular spaces like the pore voids; and bound water which is water chemicially bonded within the ligno-cellulosic mattrix (primarily to the cellulose and non-cellulosic polysaccharides (SP?)). Removal of free water does not effect dimensional change hence the conceptional idea of the Fiber Saturation Point (FSP). FSP is that idealized moisture content level where all the free water has been removed and all the bound water remains. In reality, actual attainment of true FSP moisture content never occurs because drying proceeds from the outside in.
Shrinkage in wood occurs when a water molecule is removed from its bonded site on one cellulose molecule site at an hydrogen/hydroxyl site and this/these site(s) then eventually becomes cross-linked with another cellulose molecule.
"Once the wood is dried and cools down to normal temperatures, this ultimately means that the lumber will be under less internal stress at lower ambient moisture contents (EMCs)."
During the drying process, the external surfaces of the wood dry more quickly than internal parts. If the interior of the wood is above the FSP and the exterior surfaces below, the outside surfaces will shrink and be restrained by the wet core. Two phenomenon result because of this -- checking (the fracturing of the wood surface) and compresion of the interior as well (as a surface in tension). As drying proceeds, the interior will loose moisture and size -- this sometimes results in the check proceeding inwardly (to produce splits or honeycomb), in collapse (if the compressive forces are sufficiently great) and residual stresses in the wood (induced drying stresses rather than inherent growth stresses). Were you to better understand lumber drying theory and practices, you would immediately know that unless wood is conditioned (as a separate and distinct step in drying) drying and growth stresses will remain. The primary drying stress is case-hardening but if conditioning is improperly done, it can create a situation known as reverse case-hardening.
"There are a host of reasons why air dried lumber of certain species is nicer to work with, but the essential, functional difference is that kiln dried stock tends to be more stable (less likely to distort or fail) in arid environments, such as centrally heated homes in cold climates. "
Kiln drying does not change any of the physical properties of wood. As such there is no difference between dimensional change increments in either air dried or kiln dried wood. The coefficients of change are not modified during drying and you should understand that. As far as I know there has been no published results that indicate that for a given moisture content, there is any significant difference (statistically) between air and kiln dried material. Some purists contend that there is a difference in tonal quality (vis a vis instument woods) but even on this, I have not seen anything published to support this contention.
Lumber quality is essentially the most important issue at hand and about the only thing I agree with in your statement is that some drying practices are lousy (be they kiln or air). Dean Huber used to say that a logger cuts down one tree at a time and a sawyer cuts one board at a time but a kiln operator can take an entire charge of lumber and screw up the entire load at one time). This generally occurs when the kiln cannot maintain control conditions (generally in maintaining a close wet-bulb depression during early stages), or when the schedule is improperly established or followed. Give me a good enough kiln and I can adapt a schedule that will insure the wood temperature will not exceed 120 degrees F; and I will produce stress free wood that has any target moisture content you desire.
For those purchasing lumber, it should be recognized that drying degrade is a defect and as such, you have a right to return all defective material. This means that if the MC is incorrect or that residual stresses exist -- take it back.
Stanley Niemiec -- Wood Technologist
Stanley, kiln drying does change the physical properties of wood. I'll simply refer you to Chapter 8 of the Dry Kiln Operator's Manual, a book I know you have. Read the section on "Effects of Drying Temperatures" which begins with ..."High temperatures reduce the strength of wood in two ways."
I realize that they are discussing high temp kilns but in point of fact most conventional kilns operate at 180 or above and 160 appears to be where this damage begins according to this published source.
Stanley, I know your background enough to know that you know your stuff however out here in the real world where kiln operators are pressured to get the load done as quickly as possible drying schedules are compromised and conditioning steps are abbreviated or because of outdated structures are inadequate. Here in the realworld case hardening is a fact of life and higher temps are used in drying operations that do affect the properties of wood. I know that given a near-perfect chamber with near perfect air flows and a near perfect schedule you can produce near perfect lumber but that's rarely the case.
This topic always seems to attract you, Stanley. I know you are a huge proponent of kiln drying and if it were done to your specs I'd buy all my lumber from you. But again, it's a hard world out here.
Lee
Lee Grindinger
Furniture Carver
Stanley, I think Lee's remarks sum it up well from the practical perspective, but you might want to delve into the chemical make up of lignin, especially the fatty acid components of this very complex molecule. These fatty acids liquify at temperatures below those employed in modern kiln operations. I'm not a chemist, but I suspect the difference between air drying and kiln drying as it relates to the physical properties of the resulting wood stem mostly from this feature of lignin.
I agree with you that the hygroscopicity and shrinkage properties of AD and KD wood are not significantly different, but the internal stresses in each at any given EMC is different...and not simply because of gradients set up by the way moisture vacates the wood during the drying process...The wood is changed forever more and in a more profound way.
Otherwise, it should be possible to resaturate (by soaking) KD stock and AD stock so that they would subsequently have the same steam bending properties...and this is clearly not the case. I suspect the reason that it is not is that the lignin in the kiln dried stock has lost some of its plasticity at temperatures at or just below the boiling point of water, because those components of its structure which have lower melting points (the fatty acids?) have been either removed or chemically altered...But I certainly would like to see a chemist such as yourself look into this from a more technical perspective. Like Lee, I just know from abundant personal experience that the two are not the same.
Jon:
Please refer to my comments to Lee where I cited from a section of the DKOM. It is especially important to recognize that below 180 degrees F the changes in strength properties are considered to be reversible. Also recognize that I am confining my discussions to properly operated kilns -- I cannot speak to kiln operations that are out of control (either because of equipment failure or operator error).
I do not have a source in front of me but as I remember the fatty acid component of lignin is a very small part of the overall amount. Without specific details regarding particular fatty acids, it is almost impossible to either differentiate these compounds from the parts of the extractive component (of which some are included) and the complexity within the total lignin fraction and hense their importance in bonding. It is also important to recognize that fatty acids are more prevelant in softwoods than in hardwoods.
It is recognized that there is a thermoplastic characteristic to lignin that I doubt (based on my experience at Masonite R&D) will be permanently affected (at a statistically significant level) by the temperatures used in conventional hardwood drying. At very high temperatures, lignin becomes thermo-setting which is part of the dynamic in hardboard production and bonding.
And while it is known that there is a certain level of volitilization of wood components occuring during kiln drying, these are generally acids (primarily acetic and tannic) and some extractives that are evaporated with the water. I do not recall any studies which attempt to characterize the volatile fraction (as the FP industry probably would prefer to leave this sleeping dog lie rather than have to contend with EPA emission standards). ANd there is certainly no study of evaporated volatiles from air-dried material; but at some level, does exist. I also will not attempt to differentiate material that has been steamed for color uniformity (eg walnut and sometimes cherry).
Based on both what I have learned and experienced, I do not believe that there is a difference between properly kiln-dried and properly air-dried material if both are at equal moisture contents. As I said earlier, there is no study that I am aware of that cites statistically significant differences. The problem is that most air-dried material does in fact have a higher moisture content and as such, you might as well be comparing apples and oranges. There is so much variability within wood that other factors (such as density or grain angle or growth site conditions) will be more important than moisture removal method at an equal MC level. The problem with ancedotal opinion is that it is too often based on incorrect assumptions and leads to a lot of the difference between wood science and wood lore.
And with my experience and knowledge of wood bending, the issue is entirely related to wood moisture content. Most information I read and based on my experience/research indicates that kiln dired material is too dry to properly bend. Sixteen percent MC is optimum; the problem is that to increase moisture content with steam requires more time than most people want to wait and that soaking is slow and difficult to control. Therein it is easier to blame the wood than the processing/processor.
In my mind, air drying is wasteful because degrade is often more excessive than would result from proper kiln drying. As such, you throw away material that be a part of the product mix coming from a kiln. I suppose this is what leads to the notion that air-dried material is better than kiln-dried stock -- it has been "naturally" been sorted and lower propertied stock has been pre-culled. Were you willing to "high-grade" your kiln stock ..........
I cannot address material that is improperly dried, conditioned or equalized, or material that has been improperly stored before or after drying. I do know that there are plenty of businesses that do not take good care of their material and as a result there is a lot of defective material on the market. A significant part of the problem lies with consumers who cannot differentiate between defective and non-defective material, I guess, accepting crap and assuming it is the way wood is normally. If I get stuff that is, in my mind defective, I complain, attempt to return it (or at least get a price adjustment) and if the problem persists, will no longer obtain material from a bad source. Quality is more important to me than price or convenience. It also makes me less than popular with some my area's wood purveyors.
>>>If I buy green lumber from a saw mill, sticker it, and let it dry, it will eventually reach the average moisture content of my area (12-14%)
Yes, but it may take a while depending on thickness, time of year and other environmental factors.
>>>What happens when I buy kiln dried lumber and let it set in my shop for a long period of time? Does it reach average moisture content of my area also?
Yes, wood will always be trying to reach its equibrium moisture content (EMC) based on the environment in which it finds itself.
>>>They say you should make furniture from K.D. Lumber only.
It's generally better because the wood should be more consistant but it is very dependent on how and who did the drying. Some woods like Walnut can look better when air drying rather than kiln dried where steam is used which removes some of the color.
>>>If I let my green wood set long enough(2-3 years), can I safely build furniture from it?
Yes, as long as it has reached its EMC and the place where the furniture is used is not too much different from where is it dried.
>>>If I get a really good deal on some K.D. wood, am I wasting my money because I might not build with it right away?
No, if it is stored properly and protected. I just bought about 200 board feet of walnut and cherry that was dried 6-7 years ago and stored in a protected container since that time.
Don't understand a lot of the science, but I'll throw in my two cents anyway.
When it comes to pine, I wouldn't give you a penny a foot for the air dried stuff - it NEVER stops bleeding pitch. When pine is kiln dried, the kiln is flooded with steam (toward the end of the drying process) which literally boils the pitch out of it (try saying that 3 times) and seals the pores resulting in lumber that is much more stable.
This cannot be accomplished in so-called dehumidification systems which use much lower temperatures. Those kilns produce pine that's just as poor as the air dried.
Jeff
Actually Jeff, all it takes to fix pitch is a temp of 160 F. This drives off most of the volitiles that make pitch bleed. Steam is not necessary, it's just used to keep the moisture gradient low. I had good success with fixing pitch at 140 but it was generally in my kiln for weeks as opposed to conventionals where the wood is in for days.
Lee Lee Grindinger
Furniture Carver
Don't know, but we used to dry a lot of pine for another company which had gas fired kilns. Their customer insisted that all pine be steamed to prevent bleeding.
Jeff
Setting pitch basically means that the kiln temperature is sufficiently great so that the turpentine and other volatiles (within the pitch) are driven off.
As Lee indicated 160 degrees F is generally sufficient (for 4/4 material) but like conditioning, it needs to be maintained for a suitable length of time. For thicker stock, higher temperatures are recommended.
You are not incorrect in including steam as a possible method. The DKOM says:
The problem with using steam is that it can result in brown stain. It also should be noted that with direct fired kilns, it is more difficult to control early stage wet-bulb depressions (that are a measure of kiln RH conditions) so injected steam is a way to compensate for this lack of control.
And on top of all of that, the lumber industry promised the government years ago that they would police them selves. They said that Police monorititng would be un=-needed.
Boheimain
Very punny! Perhaps you could step over to the Cafe where they're discussing patience; or is it patients?
Jeff
I was looking for answers rich
So, all this drivel missed it's mark, eh?
Yes, it's worth it if you get decent lumber. I wrote an article for Fine woodworking on this very topic. The article is in issue #151, December, 2001. You can go through the Fine Woodworking link at the top of this page to order back issues.
Lee
Edited 12/18/2002 12:51:58 AM ET by LeeGrindinger
Do woodworkers pronounce the "n" in "kiln"? Potters don't.
Janet
Yes -- the "N" is not silent as is the "P" is swimming.
Are you sure Stanley?...I thought it was a "B" that was silent in swimming...like it is in plumming.
...And Janet, I've heard "kiln" pronounced both ways in the lumber industry...But then I don't think anyone in this industry ever won a spelling bee.
Jon -- you mean silent as in a nat's eyebrow?
Back to the real world of woodworking, how does the use of a solar kiln affect the end product vis a vis artificially heated/steamed kilns? Isn't there liekly to be more variability in the solar kiln product?
Jim
A properly designed solar kiln can be quite effective for drying lumber though it will never likely get to the temperatures of a steam or direct-fired unit. They may only be usable seasonally however -- one could not expect to use solar radiation in Western Oregon in the winter for example to dry wood. You just have to be able to modify the schedule to fit the capacity (temperature) of the unit and you may not be able to sterilize the wood without a supplimentary heat source. There was a comprehensive book of solar kiln designs published many years ago from VPI by Gene Wengert that has about 60+ designs that is very interesting and useful.
Jon -- the silent "p" in swimming --- think about it huh!
Thanks, Stan. In addition to the silent "p" in swimming, you might consider the lesson from the Gospels -- as in Paul's "epissal", where the t is silent: "On the way to Antioch, he stopped for his epissal."
Edited 12/18/2002 9:05:24 PM ET by JIMMACMAHON
Guess I shoulda gone to church more often...probably woulda become a better speller...Anyway, as for solar kilns, I think they produce results more comparable to air drying in that the wood isn't exposed to temperatures as high as those used in conventional kilns. Also, because the relative humidity is not as controlled, I suspect your suggestion that the end product is more variable is probably true.
Personally, I've had good luck with the limited amount of solar kilned stock I've bought , especially a load of walnut I picked up in Illinois many years ago. I think of solar kilns as just a method for speeding up the air drying process...and because they take advantage of "free" energy, they certainly fit into my concept of practical conservation.
...But I'd agree with Stanley; whether they make sense or not depends a lot on where you live...For example, if you lived out in the Pacific Northwest like Stanley, where the only time you ever get a chance to take off your rain coat is when you take a shower, then they aren't a particularly practical option.
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