finish drying - first time dryer

[sunriseseamless]

I built a dehumidifier kiln and loaded it with oak and walnut - a mix of 5/4 and 9/4.  The wood has been air dried for over a year.  Things are going nicely and I want to know how to finish and what to shoot for.  The air temperature is between 90-100 degrees and the rooms humidity is down around 35%.
From charts and reading, I am thinking around 7%.  I was getting erratic readings from my moisture reader so I invested in a Delmhorst J-2000 - especially to get deeper into the 9/4.  Plus it has adjustments for species and temperature.
I thought it would take 4-6 weeks to get to this point but it has only been 11 days.
Once I hit my target % then I'll stick a heater in there and cook the bugs.
I read in one article that its good to introduce some moisture at the end but I'm confused why one would do that - won't the wood absorb moisture by itself.
Thanks

[scsmith42]

5/4 oak should be fully air dried in a year and around 10 - 15% MC - depending upon your location.  So it should only take a week or less to finish off and sterilize in a DH kiln.

Typically for furniture grade hardwoods to be used inside a modern climate controlled house, most folks shoot for 6% - 8% MC.

Adding moisture back into the kiln at the end of the sterilization run is for "equalizing" the MC% in the lumber.  Frequently a delta may develop between the core MC% and the shell MC% of the lumber (say 6% measured at the shell and 8% measured in the core) and the addition of humidity into the kiln is so the shell can regain enough moisture to be equal to the core.  Otherwise the lumber may twist and distort more when millwork is done post kiln drying.

There are some great publications that you can review to learn more about kiln drying.  The USDA FPL has a number of free publications that you can download, and the Forestry Forum archives had a plethora of great information.

For sterilizing, you want to heat all portions of the lumber until it has reached 133F, as measure in the core.  BTW, your Delmhorst J2000 is an excellent choice.

Best of success to you.

Scott

[YellowHammer]

If you dry to 8% and then sterilize, you will drop to a desired 7% when one.  That's what I do.  Try to not overshoot, over drying will actually make the wood start to get less flat, and curl up like pieces of bacon in a frying pan - not that much, but you get the idea.

Air drying then kiln drying will generally keep you from having to equalize.  In the future, put some significant weight on top of the stack of lumber to keep the lumber pressed down flat.  

[jimF]

Adding moisture into the kiln is for conditioning the lumber, removing drying stresses. This addition of moisture must be quick to accomplish the purpose. Slow gain from it sitting around "outside" or in a moderate RH kiln settings is not fast enough.
Equalization comes before conditioning. To do so you you increase the RH and initially the lumber will gain a small amount of moisture( in the surface and the moisture in the center continues to also go to the surface layers) but then continue to loose moisture.
You can over dry if you go below ~7% and the wood strength is reduced and gets brittle. And as mentioned above may distort more if it is already distorting.
I don't recommend much weight on top; only to keep the boards from moving from the "wind" in the kiln. Al sorts of attempts  have been tried to reduce distortion of the boards on the tops layers with no success: large concrete blocks, cinder blocks banding, spring loaded straps. The drying stresses are greater than any weight you can add to the load.  What causes the distortions are nonuniform or equal air flow across the two surfaces of the top boards. In most kilns there is little or no airflow across the top surface with increasing rates as you go down the stack. This causes nonuniform drying between the two surfaces resulting in nonuniform shrinkage.

[sunriseseamless]

What are some typical ways to add moisture?  I have a concrete floor.  Could I just spray some water on the floor and let it evaporate?

[YellowHammer]

Not my experience at all, especially in 4/4 lumber, been doing for years and even briefly shown the technique in some of my Youtube videos, but have discussed it at length here in years past.  Significant weight, properly applied, on top of a stack plays a huge factor in keeping the first several layers flat when drying for small scale kiln operations, to the extent that I will not dry without putting at least 100 lbs per square foot on every pack.  I use cracked granite slabs on a structurally sound and flat pallet, basically a constant force, constant shape (flat) press.  I have tried it through the years with and without top weights, even recently when I had a weight pack fail and dried a pack without it, and the noticeably increased distortion on the top three or so layers devalued the boards to such an extent (I lost a lot of money) convinced me to never do it again. 

This is caused by multiple things, two of which are boards that are not drying flat due to their own internal stress, and also boards that are not drying flat due to other nearby in the pack,  forcing them to distort.  The effect can be prevented, but enough weight must be applied to overcome the strength of the distorting boards and prevent them from jacking and distorting the boards above.  It's a simple force vs force equation.  In many ways it's the same effect as flattening a rose in a thick book.  It is true that sometime the bad board will still have internal stresses that will cause it to spring back into a distorted shape when the weight pack is removed, yes, that's a dead board, but that just proves the weight packs work to protect the otherwise flat boards in the layers from being distorted by the bad apple.  That's also the reason layers will be flatter further down in the stacks, the accumulated weight of the layers above overcome the distortion force of individual boards below. 

Anyway, you may ask what does a few layers on top of a pack matter? At approximately 30 bdft per layer, protecting the first 3 or 4 layer per pack in my kiln that dries 4 packs per load, about 400 bdft per load top layers, and at $20 per bdft for our 4/4 ultra flat, zero knot, zero sapwood walnut, that's potentially $8,000 of exceptionally flat wood per kiln load that may otherwise be defected.  That's big money for us.  Then assume we dry a load through each of two kilns, every 7 to 9 days throughout the years, of many kinds of mixed but high quality hardwood, from oak to Kentucky coffee, to persimmon to walnut, protecting the top layers is big money for us.

Anyway, don't take my word for it, anybody can run the experiment.  Put two packs of 4/4 green lumber, preferably high value sap free walnut so the effect is financially painful, next to each other, one with no weight on top, one with at least 3,500 lbs of flat weigh on top, and see which has the flattest top 3 or 4 layers after the boards are dry.  There will be an obvious difference.   


 
     

[sunriseseamless]

one with at least 3,500 lbs of flat weigh on top

Have you tried strapping?  I see that reccomended often.

[beenthere]

Strapping requires continued attention to re-tightening the straps, as the wood shrinks when it dries. 

[YellowHammer]

Yes, BT is correct, straps require constant tightening, and keeping the force load perpendicular to the face plane of the wood stack is paramount and almost impossible long term with straps.  When straps are wrapped around a bundle, they will apply a circumferential force to the entire pack, basically trying to make the pack round.  Take a handful of soda straws, put a rubber band around it, and it will try to settle in the shape of a cylinder.  That is not desired.

[scsmith42]

I've had good success with using weight on 4/4 and 5/4 lumber. Can't say the same for 16/4 oak though. I've followed Gene Wengerts advice re 150psf.

What are some typical ways to add moisture?  I have a concrete floor.  Could I just spray some water on the floor and let it evaporate?

Lots of folks pour water directly onto the concrete floor, but I've had my best results with a high pressure fogging system. It really works great.

[YellowHammer]

 
Here is an old but useful document from the University of California Agricultural and Natural Resources and it specifically discusses weight stacks.

https://ucanr.edu/sites/woodybiomass/newsletters/hardwood_handbook36231.pdf

As Scott mentions, no less of an authority of Dr. Gene Wengert, Forum Member and also known as the "Wood Doc" has addressed this in the past.

  

[Larry]

Some of my experiments with weights on the top layer. I'm going back 30 years to my solar kiln time. First I stacked concrete blocks on top. Lots of hand labor and the weight wasn't enough to accomplish anything.

Next I came across some 5' long all thread rod. Fabricated cauls for the top and bottom similar to the strap idea. I also tried the straps. Seemed to work ok but I think my cauls helped a lot. I made them out of heavy oak and they had a camber on the bottom. In other words the caul had a flat in the center of about 12" than a taper of 1/8" to the end. The cauls were also about 4-6" longer than the stack so no side pressure is exerted.

The downside of straps in the solar kiln is the kiln was hard to access to check tension everyday.

Than the water bed trick. Came across a twin size water bed. Put it empty on top of the stack and fill with water. In my solar kiln the water also held the heat overnight. Worked great until the sun UV or heat deteriorated the plastic.......drip, drip, drip.

Finally some smart guy with the handle of ElectricAl suggested using a concrete slab. By this time I was running a DH kiln and had something big enough to lift and place the slab. Also worked great.

The last time I moved I left the concrete slab and back to using straps with my cambered cauls in the Nyle. Checking tension everyday is not a problem as I'm going in daily to weigh sample boards and check everything out. Getting a concrete slab again is on my to do list, just hasn't made its way to the top yet.

[jpassardi]

Just FYI: If you plan to pour slabs to place on top, in order to meet the 150 PSF you'll need 12" thick concrete, 100 PSF would require 9" thick.

[K-Guy]

For weight on top of the stack, you need enough weight to keep the first 3 courses straight from there the accumulated weight of the wood will keep the rest straight.