Air-Drying and Kiln-Drying Thick Maple

      Drying 12/4 hard Maple, a kiln operator walks a fine line between checking and staining. Here's more. July 29, 2011

Question
We are going to be drying a kiln load of 12/4 white hard maple soon. I have never dried maple thicker than 8/4. The schedule I have found for the 12/4 is T3-A1. Does anyone have suggestions on what to expect? The material will be loading at dead green MC. It will be trimmed and waxed. I need to maintain the good white color. My fear is this will end split very easy.

Forum Responses
(Sawing and Drying Forum)
From contributor D:
You are correct that the 12/4 hard maple will tend to end split. If the boards contain boxed heart, they will split toward the boxed heart. To keep the lumber white, you should begin with low temperature (around 90 degrees F) with at least a 10 degree depression. I have had over 30 years of hands-on industrial drying experience (20 years with a large furniture manufacturer drying about 1.5 million board-feet per week), and drying thick lumber is still a challenge.



From Professor Gene Wengert, forum technical advisor:
The kiln schedule will likely result in quite a bit of stain. You should consider low temperature, high air speed drying if you want white color. For those who do not know, T3 starts at 110 F and A1 starts around 90% RH. This high RH is needed at 110 F to prevent checks, but it will result in stain.

The typical schedule for 4/4 hard maple is 100 F dry-bulb and a 10 F depression. I cannot imagine how a 10 F depression for 12/4 will work even at 90 F without severe checking on the faces and probably the ends.

One common problem with thick hard maple is that the surface may be dried fast enough to give a white color, but unless the interior is also dried at the same rate, which means the RH must be lowered soon, the interior color will darken, giving a two-toned color to the wood. We can also see this darkened core color with other species, especially when the wood is partly air dried and then kiln dried. The outside is dried at one rate and temperature, while the wet core is dried at another rate and temperature.

Back to thick hard maple, shed drying with high air flow (when RH is not high or too low) to a fairly low MC and then kiln drying will give the best results on the average. Of course, proper stacking, uniform air flow, protection from rain, good outside weather, and other proper drying techniques are required, along with freshly harvested and sawn wood.



From the original questioner:
Thanks for the responses. I feel like I'm going to be walking a fine line between stain and checking. My experience tells me that starting at 90 DB with a 10 depression gives me 11%RH, 2.9EMC. Having no experience with these thicker items still tells me that I'm going to check, crack and split. I think the shell will dry too fast compared to the core. This is just my gut feeling.

I think the schedule I was looking at starting with 110DB 97WB, 90 RH, 19.0EMC is on the other end of the spectrum. I do agree after more research that I'm going to stain with those settings.

I would like to start with a lower EMC and RH, but shouldn't I start with a higher temp so the material is drying more uniformly throughout the boards? I'm afraid this material will be going into the kiln at close to 100% MC, cut in winter. How does 110DB 105WB sound? 84%RH, 16.2EMC.



From contributor B:
90 F dry bulb with a 10 degree depression gives 65% RH and 11.5% EMC. I agree with Gene that the high % RH from the 3 and 5 degree WB depression schedules you note will likely cause unacceptable darkening, especially when starting at 110 F. I have seen lots of green 4/4 maple dried without surface checking at greater than 10 WBD, so maybe you will be okay starting off your 12/4 stock at 90/80 or, if checks worry you more than color, perhaps 90/82 or 84. Monitor core % MC in your samples and don't raise DB over 110 until MC is under 15%. This will take a while. Be sure you have end sealed well, and that stacking, loading and baffling is correct.


From the original questioner:
Wow - I'm not sure what I was looking at when I made my last post. The EMC and RH aren't even close to what I was thinking. Thanks, contributor B, for the suggestions.

I like the idea of starting at a lower temp to maintain the color. I am still concerned about what Gene said. On this thick stock, the possibility of having a white shell and a caramel looking core is great! I really need this to come out of the kiln as white as possible through the whole board. The lumber will be getting machined, ripped, resawn - some will be a natural or light finish.

Keeping that temp lower than 110 until the core drops below 15% I'm hoping will help the color all through the piece.



From the original questioner:
Can you tell me how much air velocity I should be using? My thought was between 350 and 400 fpm. Also after the free water has dropped off, what kind of moisture losses per should I shoot for?


From contributor N:
Wouldn't it be better to resaw now?


From the original questioner:
I wish we could, but unfortunately most will not be resawn.

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Have you reviewed the related Knowledge Base areas below?
  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Air Drying Lumber

  • KnowledgeBase: Primary Processing: Kiln Operation


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