OK, I have a good discussion topic.
We all know that Kiln drying can produce a color change and other working changes in wood when compared to Air drying. I know that one reason for the higher temp and lower humidity is to speed the drying process and therefore turnaround and finally increase profitibility. What I am wondering, is if kiln drying were done at a lower temp and gradual lowering of humidity would it still occur?
As to Air drying, when fully air dry, a piece of wood is typically in ther 12-15% MC range. What if it is then moved into a room that is kept in the 65 – 75 degree temp range but has a dehumidifier in there to drop the Relative Humidity down to say 30% and left there to finish drying – would that wood be considered air dried or kiln dried? and would it suffer any of the effects of Kiln drying duringthat last period of time. And what if just left in a room that was kept at the same temp and humidity as a room in a typical house – would that take it down to about 7% MC% and would it still be considered air dried?
What about Kiln Dried wood kept outside at a lumber yard? It absorbs moisture and comes into equilbrium at about the same 12-15% as air dried.
What then are the benefits of Air dried wood or Kiln Dried? I see some wood touted as Air Dried being a LOT more expensive than the same wood Kiln dried.
1 – measure the board twice, 2 – cut it once, 3 – measure the space where it is supposed to go 4 – get a new board and go back to step 1
Edited 7/31/2005 3:09 pm ET by Rick503
Replies
hoo boy..... there's a BIG THICK book that covers all this and a little information is a dangerous thing. It's really hard to respond briefly. But here goes.
I run a sawmill, build furniture and process wood too. I have a DH kiln, solar kiln, and also air dry.
My preference by far is to use properly air dried wood as it is relatively stress free. It's great to resaw a 2 x 10 and have both pieces simply fall away from the blade with no distortion.
There are two reasons to KD. One is the time factor. To do this properly without ruining the wood is serious science and I don't have the time to run it down here. This often has more to do with a desire to see a quicker return on investment than the desire to produce great wood. The other reason has to do with wanting to collapse the cell structure of the wood so that it will never again be as reactive as air dried wood.,IE it won't move as much. this is very important for manufacturing flooring for instance. Raising temperature also kills bacteria and bugs.
Air dried wood requires time and proper storage, hence the greater expense. I often compare the process to making good wine or whiskey.... It is an art. Ambient moisture content varies depending on location, but you will not bring the wood lower without some effort. I find that if I allow wood to reach ambient slowly before I try to reduce it further, I get vastly superior working properties from my wood. You need to be purchasing way out in front of your needs. ( depending on species)
What I have then is wood with UNIFORM ambient moisture content, that has not been case hardened ( Picture shrink wrapped) by over dry wood on the surface. ( early or improper DH kilning or higher temps can effectively seal in moisture that is deeper in the wood)
At this point ( reaching ambient MC gently) you can bring the wood down to 6-10% simply by raising the temp about 20 degrees and passing air over it. IE build a box, put in some light bulbs and a fan. Vent the air daily. It may be as simple as finding a metal storage building that you can put the wood up in the rafters, or know somebody with a warehouse that can store it up high where it is hotter. The main point being that you want to bring the wood down slowly initially. So to answer your question,,, you don't need a DH at all. If you still think you need one, then yes, you can proceed very slowly, Low temp ( 90-105) while running the DH a lot less than you think . Proper air flow through the stack is crucial.
Your idea of placing the wood in a room with a Dh will indeed dry it out further, but uniform cell collapse occurs with the higher temperatures under VERY controlled circumstances. What you will get is very dry AD lumber, without the stress associated with KD . It will be more reactive than KD seasonally. but will be fine to work with.
The KD lumber in the yards that you mentioned will indeed come back up to whatever ambient MC is, but it will not have expanded as much due to its cell structure having been collapsed.
IF you are in an area that is relatively stable year round, and do not expect to ship your work, air dried wood has worked for millenia. If you have gas heat and humid summers you can expect quite a lot of movement. Traditional woodworking joinery and techniques all evolved from the use of air dried lumber. You must always allow for seasonal movement, regardless of whether wood is AD or KD. If you are expecting to ship your pieces to parts unknown, or intend to show in climate controlled galleries, you had better drop that MC below 10% at some point and allow for movement as you plan your joinery.
A note of caution. I live in a relatively humid area, and the majority of failures that I see stem from the expansion of KD imported lumber. Acclimation to local conditions is huge. Floors can rip themselves right up, and it's always fun to see a whole set of cabinets blow up a month after installation. I've also heard of table tops going off like a rifle shot when someone glues them to an undercarriage.
I've got to go,,,, there's a LOT of knowlege about this and before deciding on a system that would work for you, I would suggest looking very thoroughly at your options.
Hope this helps!
Aloha Tai
I guess we all have our preferences, and that's OK, but I like air dried better. Of course we are taking for granted that our moisture is where it is safe. The reason for this is air dried is not so prone to turn into a potatoe chip after it goes through the saw. Maybe if a person buys all his lumber from the same supplier and you never get burned from lumber that has been dried too fast, kiln dried is not so bad. I was recently working on a small job and time was against me. I had some walnut in the shop just enough for the job with the right dimensions. I make a cut on the chop saw and find out it has radial cracks inside the board that can't be seen until you cut the board. Also on one occasion, I was making a bulletin board for a customer roughly 2x5 foot in size and the wood I was using was kiln dried. When making the frame the wood was flat and straight till it went through the saw, then it went crazy on me. Lots of fun right?
Good luck to all,
Morris
In my opinion, kiln drying absoluely "kills" the look of wood in several ways: dulls the color, "homogenizes" the color so it all looks like a uniform "plastic" products, and obliterates the natural luster of wood. I believe the primary motivation to kiln drying is to increase production, but it also makes it easier for the retailer in that every piece looks the same - no problems with people picking and choosing.
But I'm a "purist"...I don't allow stain in my shop either.
You are one of the people I have been wanting to ask a question of.
Do you notice any differance in finishing wood from the various drying options?
I would be curious what the results would be on 3 similar boards from the same tree, each dried with a differant method and then planed, scraped and finished. Would/is there a difference in the absorbtion of the finish and if the piece is stained, is there any difference in uniformity of color (blotching) and so on.1 - measure the board twice, 2 - cut it once, 3 - measure the space where it is supposed to go 4 - get a new board and go back to step 1
I really can't help much on that because I've never done a test such as you describe, and since I don't stain anything. What I can say is that BLO on air dried will produce far more "depth" in the appearance and will show more "transluent" spots - as I call them. For instance, air dried cherry will show translucent hints similar to the "curl" appearance in curly maple. Same goes for yellow birch. Air dried walnut will show purple sections. In all cases, there are far more color variations within one piee of lumber that will affect any finish technique-same variations will be apparent.
I haven't noticed any blotching in cherry using BLO and oil based poly, Deft, laquer, etc., or skipping the BLO and going to the final finish product.
I had no experience with finishing AD lumber until a current project that incorproates AD cherry about 20 years old. Unformately I had to use it in areas normal reserved for secondary wood since the lumber was originally milled to a thickness that I can not use for tops and sides. The way the wood accepts the BLO/Tung oil/varnish mix, the way the finish builds and the clarity of the grain simply can't be compared to the new lumber. I regret I couln't use the lumber for the entire project but the drawers really look great, worm holes and all.
In theory, yes, there would be mechanical differences in the actual structure of the wood that would affect absorption and colors might vary as well. AD would be less dense and therefore absorb more. ( drying time would then be quicker on KD?) I do know that if you have blotch prone wood, you will still have to deal with that either way. I don't know that most differences would be pronounced enough to notice. It would also depend a lot on species, time of year harvested, etc. I personally find that the workability of AD lumber is much more enjoyable. Not as much chatter, shatter, slivers, tearout, etc. and way less stress.
One very noticeable difference is that high temperatures will "set" sap and eliminate bleed. Air dried lumber that has not been around for quite a while may have this problem.
Other notes on posts: honeycombing ( interior collapse) is a serious kilning defect and supplier must replace wood. Excessive interior stresses are also a defect which should have been mitigated by "conditioning " the lumber to relieve stress. This is done either by steam injection during final phase or simply waiting a while if lumber has been correctly dried. This lumber should also be replaced. This may not be worth pursuing for a board or two, but in the case of badly dried units, this feedback goes back to the source.
I buy my lumber from a yard that sells KD wood.
They process a lot of wood for high end users. Because they process a lot of wood, they know how to KD wood.
The wood they sell me is flat, uniformly dry, and stress free. I can plane 8/4 (2-1/6" dry) to 2.00" and have no saw marks. I can then resaw and have no distortion.
GeorgeR... DAMN.. You died and went to Heaven and never knew it!
Tai has some great info from a sawyers perspective. As a woodworker, I've found the air dried to retain more of the natural oils. I can't say that I've noticed any significant color difference between air and kiln dried in the species I've worked with. There is a big difference in the smell. I did a job recently with air dried red oak and it had a strong odor and a completely different feel, compared to the KD I normally use. I have also found the air dried will move much more than the KD with seasonal changes.
One of my local yards specializes in air dried white pine. They own the trees and do a great job seasoning it properly. Their prices were so inexpensive at one time, that a competing yard was buying their pine at retail and reselling it at their ordinary higher price. The problem was that they were storing it under a building with a dirt floor. Once that wood hit the heat, any work you put into making tight joints was for nothing. I would prefer to use KD when possible. Wood movement is an enemy for me.
Beat it to fit / Paint it to match
Rick, I have never seen "air dried" wood at an MC below 22%. Kiln dried wood quite often is stopped at 8% but then stabilizes at 4-5% higher when it is removed from the kiln. I believe it is impossible to get air drying to take the MC down as low as you are talking about. A kiln circulates warmed air rapidly over and thru a stack of lumber and dehumidifies the air as it is processing with a refrigerant coil. I have seen 55 gallon drums fill up in 24 hours with the exudate from this process. The warmth and air movement break the cell walls and force moisture out generally disallowing it to migrate back in. Most kilns use probes to record core moisture of boards stopping at 140 degrees. Higher temps can cause case hardening and honey combing of the wood, ruining it. Color loss is often the effect of "in a hurry" processing. Buy a kiln or make one and learn to use it properly and you can be custom drying your own wood without the quality loss you describe. aloha, mike
mike
I have been working almost exclusively with air dried wood for about 10 years now. I mill all my own logs, and have a large wood shed with several species always on hand. With cherry, maple, and especially walnut, it is quite easy to get excellent results air drying. I've got a stack of 5/4 cherry (about 750 bf) that I milled last August. It is between 10-12 percent as of Saturday. That's about as low as it will get in the shed. When I need stock for a project, it goes inside the shop, and stickered, of course. I plan about 2 months ahead, and I consistently get the stock under 8 %.
I do have 2 dehumidifiers running constantly in my shop, and as the stock reaches equilibrium with the dry air in there, the MC drops.
I love working with the air dried wood because of the resiliant colors, as well as the extremely wonderful stability, compared to some KD stock that maybe wasn't dried properly.
Just my .02, but whoever told you it couldn't be done is mistaken.
Jeff
Jeff, I find it technically impossible to bring wood to an airdried 8% MC unless your ambient moisture content in the air around you is near zero or 2%. I personally love air dried material and for use in Hawaii its great. When I make a piece for shipment to the mainland its gotta be KD and then it never stabilizes at less than 12% no matter how low they got it to in the kiln. Its just the nature of it and wood is pretty stable in the teens MCwise. Color is not always affected by kiln drying but the brittleness of the material is a dead give away. We work what we got when it comes along in the end and that decides a lot of things for us. aloha, mike
I buy some lumber from a guy who air drys here in WI. Recently a guy came up from Chicago to buy a load of cherry and his meter was reading 6-10%. I didn't actually see it happen, but I know the guy well enough to know he wasn't fibbing.
All good info. Now lets confuse the issue a bit more.
If I Air Dry the lumber outside to as low as it will go and then move it to a closed shed that is kept at 85-90degrees and has a box fan or two to cirulate the air and maybe a humidifier/dehumidifer in there as well, when I finally use or sell it is it AIr Dried or Kiln Dried?1 - measure the board twice, 2 - cut it once, 3 - measure the space where it is supposed to go 4 - get a new board and go back to step 1
You could try marketing it like fine whisky, "Air Dried, Kiln Finished. Enjoy the best of both."Andy"It seemed like a good idea at the time"
Rick, what you describe is the best of kiln drying. aloha, mike
Sorry you think I'm lying to you. I've nothing else to add.
I didnt say you were lying at all.......I said it was not possible to hit that low MC with air drying, thats ll. Its not possible. I operated a kiln for 4 years on Maui with an average relative ambient humidity of 60% and we could only air dry to the 20% mark after 2 years stickered. Add high temp moving air, dehumidify, or put in a vacuum and you greatly affect the equation. But ambient air drying wont get you there...
According to Bruce Hoadley's book, Understanding Wood, an 8% MC corresponds to RH in the 35% to 50% range, depending on the species. There are lots of places in the US where the average indoor RH is in that range or lower, even in the summer.
Wow, those are pretty remarkable percentages for air dried lumber.
Kiln drying speeds up the process and makes it more commercially feasible for the big operators.
The color change only comes about if the kiln operator injects steam into the stack to minimize the possibility of case hardening when drying to fast.
Drying lumber down to the 6-8% range just allows them to find the bad lumber easier. I don't think it causes any chemical or moleculer change to the wood to make it better later.
I keep all my lumber, kiln dried and air dried, out in my non air conditioned shed here near Houston. Doesn't affect the workability of it one bit. But, you have to plan on expansion and contraction when using it. When storing, just keep it dry and stacked and stickered and stay away from 'highly figured' lumber!! The straighter the grain, the better.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)
PlaneWood
Aloha,
This thread has had a lot of anecdotal evidence related to wood/moisture relationships and kiln dried versus air dried. I thought I would add my two cents worth of empirical evidence on Koa. I don't have any kiln dried wood, but here are the results of tests I conducted on some scrap AD koa that has been lying around my shop for some time. I live on the Big Island of Hawaii at about the 1000' elevation. Our average relative humidity runs about 60% to 65%. I took four pieces, total weight 20.5 ounces and put them in my smoker for 10 hours at 220 degrees per procedure from Hoadley's book. I weighed them about every two hours and stopped when the decrease in weight leveled out. At the end they weighed 18 oz. this calculates out to an original moisture content of about 12 %. This corresponds well with the graph in Hoadley's book.
I also measured them with a micrometer before and after; tangential shrinkage ranged from 6.1% to 6.8%, radial shrinkage ranged from 5.3% to 5.4%. According to a study done by The Hawaii Ag Research Center, koa shrinkage green to dry averages 6.2% Tangential and 5.5% Radial.
Although this wasn't done under laboratory conditions I'm satisfied with the results. About a year ago I did the same test with some KD Brazilian Cherry that had been at my location for about 3 months I also came up with 12% MC. At that time I didn't have Hoadley's book but it sounded about right to me. This method is a lot cheaper than a moisture meter (assuming you don't have to use the wife's oven, of course).
Everyone knows that inside heated air in the winter has a lower RH than outside. A question I have, however, is what is the relationship of air conditioned inside air to outside air? Does the condensate on the coils take out enough moisture to compensate for the drop in temperature?
AC varies depending the part of the country. When I lived in New Mexico, we had "swamp coolers" a box with pads on the outside that wer kept soaked with water and had a squirrel cage blower inside that pulled the air thru the pads and then blew it down into the house. That type would increase the RH. What refrigerated AC does would I THINK (but am not sure) would be to lower the RH.
Any AC people out there with more info?1 - measure the board twice, 2 - cut it once, 3 - measure the space where it is supposed to go 4 - get a new board and go back to step 1
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