Kiln drying question

[Chuck Wintle]

I know the point of kild drying is to reduce the moisture content to a workable level so do most kilns just heat the wood to drive out the moisture? Or do some actually introduce steam to replace the sap in the wood somehow? a woodworker told me the most stable wood is that which has had all the sap water driven out, replaced with ordinary water and reduced in the wood to a certain percentage.

[Ed Peters]

Proper "KILN" (not kild) drying will dry the wood and periodically introduce low level moisture to allow the wood to release any tension introduced as a result of the removal of moisture. Some of the information regarding kilns that I have read place solar kilns such as the one designed by the University of Virginia in Blacksburg at the top of the heap. They work at removing moisture all day and then allow the wood to relax during the evening hours. Supposedly produces some very high quality wood with very low reaction wood content.

Ed

[James Ashburn]

The sap is not replaced. Steam is sometimes used to help relieve internal stresses in the drying wood. Sometimes softwoods are heated to relatively high temperatures (between about 150 F and 180 F I think) to set or cure the sap by driving out the volitile compounds that can bleed through a finish. If you have ever painted knotty pine you probably know what I'm talking about with bleed through. It can happen years later after painting.

Turpentine can be derived from the sap and resins in softwoods. This is the kind of stuff that is getting set or cured by the high temps. The presence of the (dried/water removed) sap does not change the stability of the wood, just water moisture content and temperature. This is the same reason that alcohol based stains/dyes will not raise wood grain. Resinous sap is not present in hardwoods.

James

[Chuck Wintle]

The sap is not replaced. Steam is sometimes used to help relieve internal stresses in the drying wood. Sometimes softwoods are heated to relatively high temperatures (between about 150 F and 180 F I think) to set or cure the sap by driving out the volitile compounds that can bleed through a finish. If you have ever painted knotty pine you probably know what I'm talking about with bleed through. It can happen years later after painting.
James

This may sound stupid but has anyone ever heard of taking planks or freshly cut boards and subjecting them to high pressure steam and water in a pressure vessel and then kiln dry them afterwards?

[Jim Becker]

Steam is also sometimes used to increase yield on woods like walnut...it darkens the sapwood and lightens the heartwood to the same muddy brown color...but there is more wood to sell that way.

[Frank Drew]

Charles,

Here's a link that provides a very good primer on the subject:

http://en.wikipedia.org/wiki/Seasoning_(wood)

Steam is introduced during the kiln-drying process so that the outside of the boards doesn't dry radically faster than the inside, which could lead to checking and case hardening.

I haven't heard about pressure drying, but some years ago there was a bit of buzz about vacuum drying; I don't know if anyone's still doing it. Lately I've been hearing about microwave drying -- high start up costs balanced by good results in very quick turn around times.

[Rick Fisher]

I walked through a large kiln in about 1988. It was an old kiln at that time. They had huge fans, Dehumidifiers and heaters. Probably primitive by todays standards.

I remember the kiln manager saying that they turned the fans and Dehumid's off at night and let the moisture in the wood "cook" or something. The idea was to let the moisture even out or move from the wet center to the dryer outside edges. ???

What I really remember is my clothes sticking to me within a few seconds of entering the kiln. The smell of "wood water" and the brownish green water coming out of the Dehumidifier spigots.

They measured the gallons per hour of water being collected to judge progress. They where drying Hemlock for Mouldings. This was prior to the F/J pine hitting the scene..

I am old. lol..

[Tom Sontag]

Rick,

I don't think that DH kiln technology has changed that much, except in the case of monitoring and computer control. They eased back the drying at night to let the moisture in the center of a board work its way outward. Equalization, as you wrote. If you dry too fast, you can ruin wood in a hurry. My solar kiln does the same thing naturally when the sun goes to sleep.

[Dennis Peacock]

Two words.....Air Dry :)

It's cheaper, wood color is better, and you don't need any power to dry the wood. Just stack, sticker, and let set for 1 year per inch of thickness.

I just put up some dry Aeromatic Cedar (red cedar) that was drying in my shop for 13 months. Nice stuff.!!!

Solar kiln is a nice way to go if you want to save some money on a kiln and be able to dry your wood at the same time. :)

[Tom Sontag]

For the home hobbyist, air dry is often the only real alternative. Unfortunately, the wood needs to finish up indoors or in a hot attic or something if it has any chance of getting below 10%MC in most parts of the country. You do not want the first time it gets there to be as a table top in your dining room in the winter's dry heat. Dry your wood all the way before use. Most movement and degrade occur in the very first days of drying AND when removing that last 3%.

[Matt Day]

Proper "KILN" (not kild) drying will dry the wood and periodically introduce low level moisture to allow the wood to release any tension introduced as a result of the removal of moisture. Some of the information regarding kilns that I have read place solar kilns such as the one designed by the University of Virginia in Blacksburg at the top of the heap. They work at removing moisture all day and then allow the wood to relax during the evening hours. Supposedly produces some very high quality wood with very low reaction wood content.

Ed

Not to knitpick but I think you mean Virginia Tech in Blacksburg.
http://www.ext.vt.edu/pubs/forestry/420-030/420-030.html

[Jim Becker]

Tom, I'm not sure I agree fully that your lumber must get below 10% MC if that's in concert with the relative local humidity. What's most important is to build with wood movement in mind so that the "actual" MC isn't really important anymore. 10-12% MC is pretty typical of air dried material in my experience (including my own drying) and I've had zero issues with that in the furniture I build.

One must also be careful with putting the lumber in very hot environments with little or no air movement. That can cause other problems with stability, depending on the species. I'm actually planning on moving the majority of my lumber storage to the second floor of my shop building this fall, but before I do so, I need to get more ventilation up there to be comfortable with that situation.

[Loren Bengtson]

Or do some actually introduce steam to replace the sap in the wood somehow?

Charles,

Moisture, usually in the form of steam, is introduced to kiln dried lumber at the end of the drying cycle to avoid "case hardening", not to replace any sap that was removed.

Case hardening is the term given to describe the fact that the outer shell of recently dried lumber is much dryer than the center of the board. For example, the kiln operator may be looking for a final MC (moisture content) of 6%. At the end of a drying cycle a given board may have a MC of 7% at the core and a MC of 3% on the outer surface where all the drying takes place. If you rip this board down the center, it will bend as the exposed core dries out.

If the outer surface is re-wetted, its MC rises to, say, 5%, giving the entire board an average MC of 6%. At the same time, the differential MC through the board is reduced dramatically. When this board is ripped (for example) it will not bend much as one with a larger MC variation.

According to Don Lewis of Nyle Kilns, you can accomplish the same thing by storing the lumber in a controlled environment (where it will not pick up new moisture) for a sufficient time for the MC throughout the board to stabilize. This defeats one of the purposes of kiln drying, however -- faster drying. Another purpose of kiln drying is to get moisture contents lower than can be achieved by air drying. And another purpose is the production of lumber with less "degrade" than from air drying.

Phew! What a long-winded explanation. I hope it was clear.

Loren

[Loren Bengtson]

This may sound stupid but has anyone ever heard of taking planks or freshly cut boards and subjecting them to high pressure steam and water in a pressure vessel and then kiln dry them afterwards?

Jim is correct. Walnut, especially, is treated to steam before drying to darken the sapwood, hence increasing its value.

Loren

[Loren Bengtson]

I haven't heard about pressure drying, but some years ago there was a bit of buzz about vacuum drying; I don't know if anyone's still doing it.

Frank,

Yes, vacuum drying is alive and well. It produces excellent results in a hurry. Vacuum drying can result in kiln dried lumber in seven days instead of a month or more.

The issue is the cost of the equipment. A tiny vacuum kiln, that will dry perhaps 1000 board feet at a time, is over $100,000. This is why I will be installing a dehumidification kiln instead.

If you can keep a vacuum kiln busy, you can make it pay for itself, but how many non-commercial operators will have lumber standing by to get into the kiln?

Loren

[Loren Bengtson]

For all who are interested, the USDA Forest Products Laboratory has a web site with lots of information. It's here:
www.
fpl.fs.fed.us/


You can download the entire publication, "Drying Hardwood Lumber" on their site. Here:
www.
fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf

Loren