Drying Schedule for 8/4 Walnut
Be careful when drying thick Walnut — drying too fast can damage the wood. January 14, 2013
Question
Can someone tell me what schedule is used for 8/4 walnut and what the safe drying rate is? Also, how long does 8/4 walnut take to dry in kilns on average?
Forum Responses
From Gene Wengert, forum technical advisor:
Green or air dried? What type of kiln? Do you have samples and are weighing them, or do you use a meter? Do you have a copy of “Drying Hardwood Lumber”, as both schedule and rate would be there.
From the original questioner:
They are approximately 60% moisture content. I have six samples in the dry kiln. The first day I lost 6% on all samples (I thought that was ok as we had lots of snow that day that saturated the wood) but since then we have lost approximately 3% max on all samples.
From contributor K:
Be careful. I just wrecked a bunch of 8/4 walnut when I allowed 6% moisture drop even when it was already reading 33% MC. I just switched to Lignomat’s core probes so I wasn't used to the system and wasn't too careful. Max daily for walnut in 8/4 is 2.5% MC or such. I have lots of horrible end checks and possibly honeycomb now. I try to air dry most of my stock so as to avoid some of the worst defects, and as Gene has mentioned the defects occur over 40% MC.
From Gene Wengert, forum technical advisor:
Honeycomb (interior checks) in thick walnut is quite likely if the wood is dried too quickly, including end checks when drying too fast. Note that too fast drying can occur when air drying too - mainly above 30% MC. Under 30% MC, the risk of new damage drops to almost zero. The only safe way to dry thick walnut above 30% MC is using properly prepared weighing samples, as electronic techniques are not close enough.
From the original questioner:
We also use Lignomat control but when running the charge at the beginning we keep it on hold so we can control ourselves. Like I said we lost 6% the first day and then under 3% for the rest. We dropped the EMC and raised the temp only while watching the moisture lost per day. I believe that it may have happened in the steamer or air drying.
From Gene Wengert, forum technical advisor:
You can have a few small checks from improper (under 100% RH) steaming. They cause no problem unless subsequent drying is too fast or too hot. The proper kiln schedule is T3-D3 for 8/4, I believe. So, you cannot change the EMC much from the schedule values. You cannot under any circumstances raise the temperature over 110 F until under 30% MC. The safe drying rate is a maximum for any sample. It is not a target that you seek to maintain. You would therefore not drop the EMC to maintain a 3% daily rate. In practice, the average drying rate would be around 2% MC per day at higher MC’s. The rate is used to fine tune and not used as the primary control.
