Moisture Gradient

[Kierman Dimitri]

Does anyone know what kind of a moisture gradient can be present from surface to 3" needle depth on 8x8 DF before degrade starts to become an issue? or likewise for 1" needle depth.

[MichaelM]

First, I don't have an answer.  But I like the question.

People have tried to make moisture meters that do what you want to do - measure at two depths and determine a moisture gradient (% MC change per distance).  Commercially, I don't think these have been popular.

The thick Douglas-fir is difficult partly because it takes a long time and most people do not want to dry that slowly.  It would not surprise me if you will spend 4 to 6 weeks drying 8" DF and even then the core will be at 20-25 with the surface at 10%.

The handling of the wood prior to drying is important.  You must prevent rapid air drying or surface checks will start.  Most boards with the pith in will develop spits no matter how you dry it. 

End seal might be advisable to reduce end checks if the load is of high value.  End seal needs to be applied within hours after the end cut is made if it is to be effective.  AnchorSeal from UC Coatings is one brand.  Butt the loads together well in the kiln to help prevent the ends from drying.  End checks can grow internally and be in the interior of the board 6-8' from the end if they are not controlled.

The USDA schedule is #288 in "Dry Kiln Schedules for Commercial Woods"  That schedule might be a little too aggressive for the thick stock.  I would reduce the dry- and wet-bulb byt 10-15F in the first 1/3 to 1/2 of the schedule to reduce the potential for internal checking later in the schedule.

If you are not in a hurry, consider air drying with the pieces out of the rain.  Nursury cloth can be used to reduce airflow during the summer months.

When your customer orders it at XX% moisture content, make sure they specify how that is measured.  Is it 2" in, the piece average, or at the core?  If it the piece average or at the core, allow enough time to get the core moisture content low enough.  The first half of the moisture comes out a lot faster than the second half.

Maybe others have some good suggestions?