We sawed some red oak to 7/8" on our WM bandsaw. We used 1 1/8" lumber for a base, added the 7/8" with .75" X .75" sticks every 6", then finished with 8 rows of 1 1/8". Everything was fine until the kiln schedule went from 120 to 130 at 25%. After 24 hours we noticed the 7/8" had cupped. All had bent up towards the sapwood, even if there was no sapwood on the board. It did not matter which surface was facing up. This is the first time we have had this happen to thin stock. Any thoughts?
I'm not surprised they all warped one direction, towards the sapwood. I wonder about the degree it did, though. What type of kiln are you using? How high are your stacks? What were your previous setpoints?
The type of kiln, the setpoints, and the height of the stack are NOT critical variables, especially in the case you site where you have done this drying before and never had anything like this. I suspect the problem is that you have changed the species of red oak within the red oak grouping of 20 species.
Incidentally, closer sticker spacing (closer than 24") does not reduce cupping with good sticker alignment; it probably does not hurt either, except for the higher cost, and slower drying under each sticker.
Cupping is accentuated by three things:
1) Drying too slowly; however, for oak we always dry fairly slow. Did you dry more slowly than usual? Did you dry based on the MC of the thin stock or the thick stock? If the thick stock, then this would be slow drying and accentuate cupping.
2) Rewetting partially dried wood. There are a variety of ways to do this including mixing MCs in the kiln; using a start-up RH that is too high for the lumber and results in surface MC gain; and using steam spray or water spray at start-up or intermediate levels.
3) Over-drying (which in your case can be eliminated as you are still at a high MC).
Cupping does not show up when green, but becomes worse as drying continues. In fact, it is the only major defect that shows up under 40% MC. I suggest that you read Drying Hardwood Lumber, which covers cupping in a little more detail.
Gene Wengert, forum technical advisor
Gene's thought about the 4/4 holding up the progress of the 7/8" and adding moisture back to the surface is quite possible. With only 100 Sf of 7/8, we did not want to tie up a 4000 ft kiln. Apparently this decision is going to cost me!
Gene Wengert, forum technical advisor
Air-dry shed RH % ranged from 40-60% depending on the day, with natural airflow.
We use a Radio Shack digital thermo-hygro meter in the air-dry shed. Also, we have meters in the kiln, wood shop, retail showroom, and sawmill. Very useful for $25.
Gene Wengert, forum technical advisor
Look's like "start up" is what started the problem. On the 4/4 schedule +50% EMC calls for 110/106 (dry/wet) or 87%RH. I did not make a sample board for the 7/8". I did scan it with a Wagnor 606 pinless and pegged the needle. So it was +30% EMC, but apparently dryer than the 1 1/8". The book also suggests using a pin meter to check the SMC. What if the % is higher then 30%?
Do you think this cupping problem could have been avoided if we had gone directly to the kiln (G ->6%)?
New question:
Is fog and/or multiple rainy days adding too much moisture back to partially air-dried lumber? We saw small amounts of cup or twist in the shed last fall.
So, is a guy better off storing lumber on sticks or building a climate controlled chamber for KD? We like to keep 30,000 ft on hand year round.
Going into the kiln green would have minimized the problem (unless there is another problem). The tendency to cup increases as the pieces are cut from closer to the pith of the tree.
Fog and rain regain is potentially a problem, but not much if the pick-up is gradual. However, such weather may result in very slow drying, which is a problem.
Remember that the biggest risk is only for partially dried wood. So if you D to a low MC and then start the kiln correctly, there will be no problem. In your case, it is likely that after 2 weeks, the 7/8 material had lost a good deal of moisture, and so was "at risk."
KD inventory is much better.
Gene Wengert, forum technical advisor
