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Avoiding case hardening

Started by KenBala, August 07, 2016, 12:14:14 AM

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KenBala

Two quick questions if you don't mind guys...

1.  I think I remember in my research last year that if a load is air-dried to 25%, it can be put in a kiln and dried rather aggressively to the final desired MC without concern of case-hardening. Is that correct?

2.  In looking through Gene's "Drying Hardwood" is states that using fans to accelerate air-drying is acceptable except for refractory hardwoods. It lists oak and beech as refractory. Are there any other hardwoods indigenous in the NE that would be considered refractory and thus not a candidate to use a fan on?

Thank you so much!

GeneWengert-WoodDoc

1.  Yes, thus is true for 4/4 to 8/4 air dried in most of the USA where the morning humidity reaches 100% RH.

2.  Any species or thickness that can be dried quickly is not a refractory wood.  So, maples, ashes, birches, poplar, and aspen can easily benefit from fan air drying.  Even 4/4 a 6/4 walnut and cherry are ok if the fans are not blowing too fast.  Oak, beech, and hickory are refractory.

Any kiln schedule on page 75 in DHL that ends in 1 or 2 is refractory.  A 3 schedule is not, but some care with air fan drying is suggested on very dry days.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

KenBala


KenBala

Follow up question for this...

I cut some beech into 4/4 slabs and stacked for air drying on August 20th of this year.  The stack was roofed, not in direct sunlight, and I did not use fans to accelerate the drying.  I've been monitoring the MC decrease every few days and am a bit surprised that it is already at 21%.  Is this worrisome?  According to what we talked about before, it should be ready to finish off in the kiln.  That's wonderful as far as I'm concerned, but as a novice I am wondering if it might have somehow dried too quickly since it is a refractory wood and now might have some defects in it.  It has been dry here in PA the last month.  I am in no rush and can let it wait until the spring to kiln dry, but could damage already be done?  Any thoughts out there?  Thank you.

Denny

"I've been monitoring the MC decrease every few days"
What are your daily MC% losses ?
What was the starting MC ?
Do you see any checking across the face of the slabs during the dry ? *The checks might be closed up now that you're in the 20% range.
How are you reading MC ?

Its good you have it under roof and no fans, you're probably fine. 
Its been a while since I've dried beech but I remember it being the easiest/most forgiving refractory species, with white oak being the toughest to dry.  I've had trouble with Hickory but can't recall any troubles with Beech.
I'm from PA too, its been a dry 30 days here in central Pa.

And to add to your original question, yes it seems that air dried lumber gets none or very little drying stresses because of the high humidity in the early morning hours, but its been a drier than normal summer so maybe you might see a little stress in your air dried lumber.  Do a "prong test" if you can, that will tell you how much stress you're dealing with. 

YellowHammer

Air drying is very forgiving, but I've had it be a little too much for white oak, especially, during the summer, in some cases.
Seems like I was always able to spot the problems when I closely inspected the boards, looking for checks, cracks, warp, etc, and they were minor.
If I think Mother Nature is gong too fast, I simply do pretty much nothing, except maybe rearrange my stacks to bury the pile a little in other stacks so the wood has some company and slows down a little.
When in doubt, pull the boards and cut some samples.
I'd say, odds in they are fine, but I'd probably let them stay in the stacks and stabilize a little lower before I put them in the kiln. 


YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

Den Socling

I agree with cutting samples especially if you are drying 8/4.

GeneWengert-WoodDoc

A prong test for casehardening requires "no moisture gradient."  If there is a gradient, the results are not accurate.

The summer here in GA was the hottest on record, but even so, we had plenty of dew every morning, which means high humidity in the morning and stress relief.

In DRYING HARDWOOD LUMBER, Page 102 shows how to cut samples to determine any damage from drying.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

KenBala

Thank you for the replies gentlemen.

Denny - I use the Lignomat mini with probes in a board in the middle of the stack for a "down and dirty" snapshot of the MC during air drying.  I cut and weigh samples traditionally when in the kiln.  The moisture meter tops out at 54% so I don't know the starting MC, however once it dipped below that (after about a week after it was stacked) I'd say it dropped about 1-2 points a day.

I don't really see checking across the face, however your statement about them closing up is interesting to me.  So during the drying process checks can open up, but then close again as MC drops further?  (Sorry if that's an obvious answer for everybody out there, I'm a novice)

Yellowhammer - Although I don't notice any checking , there does appear to be slightly more warping than usual - not really any twisting, more of bowing across the 8' length to the board.  It's not bad at all really, just more noticeable than past experience for me.  That might be because my stickers should be closer together - they're spaced about 22" apart which might be a little too far for 4/4 beech?  As I said, I'm in no hurry and will likely just let this sit through the winter and finish it off in the kiln in Spring.  If there are any defects that might be brewing in there, I assume the extra time would only be helpful.

Gene, what does it mean to have no moisture gradient for an accurate prong test?  Isn't that what it is assessing?

GeneWengert-WoodDoc

There is no pin meter than is accurate above 30% MC.

The prong test measures drying stress (casehardening).  It does not measure MC or MC gradients.

When lumber first starts to dry, the outside wants to shrink, but the wet core stops that.  If the attempted shrinkage stress is too large (humidity too low and /or velocity and temperature too high) than a check can develop.  This happens only initially.  When the core dries under 30% MC some time later, the core will shrink and pull the surface or end checks closed.  These closed checks are impossible to see.  If they are open at the end of drying, then we know that the wood had some wet-dry-whey-dry cycling.  Further, at the end of drying, the checks are closed but not healed.  Should the surface get some really dry air, the surface shrinkage can reopen checks, but not create new ones.  See DRYING HARDWOOD LUMBER for a discussion of the Four Stages of drying for details.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Denny

Yes when you do a 'prong test' you don't want any moisture gradient.  What I do is microwave my pieces for one minute per sample and then let em sit for a 1\2 hour or so before I cut em up on the band saw. Note I'm not microwaving a big giant kiln sample, only the 1 inch wide samples I'm using for the prong test.

I have a mini-Ligno and I don't use it much, I believe that the prongs are too short to reach the core and perhaps you're only getting a reading from the shell.  I might be wrong tho.

Den Socling

Some manufactures of moisture meters are claiming they can read above 30% but I don't believe it either. Neither do I think that those high readings are any value. When you are doing conventional drying, you want to know when the free water is gone and that's around 28% and below. When I mentioned cutting samples, I wasn't talking about looking for stress. I meant that you need to bake samples from the core to know where you are MC wise. If you don't overdry the shell covering a wet core, you won't develop excessive stress. However, just how conventional drying works, some stress is always developed.

GeneWengert-WoodDoc

Regarding the prong test, you can cut the prong and then put it in a microwave for 15 to 20 seconds and then let it cool for several minutes before reading.  More discussion of this technique in DRYING HARDWOOD LUMBER.

"When the free water is gone..." Has various meanings.  For example, in conventional drying when the AVERAGE MC is 30% MC we drop the RH and raise the temperature.  At this point, there is still plenty of free water in the core.  All the free water is gone in conventional drying when we get to around 20% MC.

We use pins in the kiln sometimes that are fairly shallow, so they reach under 30% MC when the average is maybe 40% MC.

Over 30% MC means that there is liquid water in some or more cells, so, there is little resistance to electricity at 35% MC and About the same resistance at 65%. This is why no meter can measure high MCs.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Den Socling

The fact that there is no definite point of no free water is why we normally ramp temperature linearly in our vacuum kilns. The proportion of free vs bound water does not change abruptly.

Denny

If we'd air dry RO down to say... 15-20% range and there is no case hardening in the lumber, then what happens when we put that air dried lumber into the kiln and take it down to the 6-8% range. Do stresses form at these lower MC numbers ?

I ask because I don't air dry my lumber, my oaks go immediately into a pre-dryer... and the pre-dryer definitely puts stress in the lumber.

Den Socling

I can't see you air drying down to 15-20% with "no" case hardening. If results were very good, maybe it wouldn't be the definition of "case hardened" but there will be stress.

GeneWengert-WoodDoc

Air drying relieves most semi-permanent drying stress, which is also called casehardening.  This is because of the high humidity condition every morning. You will see no casehardening stress at about 20 to 25% MC.  You do not put new semi-permanent drying stress in lumber below this level as the wood is so much stronger that shrinkage stress will not exceed the proportional limit.\

Thepredryer puts in huge casehardening stress as there is no high humidity daily.  Plus most pre dryers do not reverse fans so the wood has no rest period.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Denny

Den I should have said "and there is no or very little case hardening in the lumber"


"You do not put new semi-permanent drying stress in lumber below this level as the wood is so much stronger that shrinkage stress will not exceed the proportional limit."
That answers my question and its what I assumed.
And my pre dryer fans do not reverse and we don't rotate the packs in the pre dryer. If I 'prong test' my PD samples, there's always plenty of stress in the samples.  My kilns do a great job of conditioning though. Big ol' steam pipe injecting steam works great. I prong test my kiln samples at the end of every kiln charge.

Den Socling

I would expect that your predryers always do a better job than air drying. You should have conditions inside that barely skim water. We have put control systems on predryers where airflow was so slight we couldn't use wet bulb wicks. We use humidity sensors.

Denny

Yeah the air movement is slow in the pre dryer, it barely moves my air velocity meter, I haven't checked it in awhile but if I recall it was pushing at best around 125 ft\minute. I have baffle systems under all the fans. I keep the conditions mild in there.

The pre-dryer is a Irvington Moore, 500,000 bd-ft and it originally came with wet bulbs. We added small fans to blow over the wicks to ensure we're getting good evaporation off the wicks. But we canned that wick\fan system many years ago and now use humidity sensors.  I can choose which sensor I want to use to monitor humidity via the control rooms controller.

I broke out the old air velocity meters for this reply. I have two of em, one is an Alnor Velometer Jr. and the other one is an old Floret brand. Neat little toys.

Den Socling

I see you're in central PA and your control system sounds like one of ours. Did we build your predryer controller?

Denny

I'm pretty sure our electrician modified the Irvington Moore control panel for the RH sensors, I don't see your company tags anywhere but.... PC Specialties built all eight of my kiln controllers. Got seven MRC5000 controllers and one MRC7000 using Partlow loop controllers.

Den Socling


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