Cupping of newly cut lumber

Why would all the lumber in one kiln load cup toward the sapwood? January 16, 2002

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?

Warping is caused by the natural tendency in wood for the bark (or sapwood) side of lumber to shrink more than the heartwood (or pith) side. This tendency is made worse when the radial shrinkage is much smaller than the tangential shrinkage value. This can happen with red oak, as there are 20 species of red oak and some have very high tangential shrinkage, while all have about the same radial shrinkage. Hence, you have some red oak that is going to cup more than other red oak.

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

From the original questioner:
The cup was 3/16 to 1/4 on boards 8" and wider. Our kiln is a hot water type at 180 water temp. Chamber temp to 145. Controlled by a custom built NYLE wet/dry bulb computer.
We did follow the 4/4 T4-D2 red oak schedule, using OD weight of the 4/4. There was only 100 surface feet of the 7/8" mixed with 2000 BDFT of 1 1/8. We moved right along at 3 to 4 % per day. All others times we dried 4000 SF by itself.

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!

Was the incoming MC uniform between the various pieces of lumber or was the thin material partially dried already?

Gene Wengert, forum technical advisor

From the original questioner:
All the lumber was sawn together, placed on sticks right off the saw. The lumber was moved directly to an air-dry shed for 2 weeks until the kiln was ready.

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.

This may be the clue we are looking for. Did you read the section in Drying Hardwood Lumber on how to start a kiln when the lumber has had some drying beforehand? Check the top of page 86.

Gene Wengert, forum technical advisor

From the original questioner:
Just read page 86 and 87.

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.

To measure the surface MC SMC, if there has been substantial drying, then the meter will read under 30% MC.

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

I have found that letting the lumber air dry for at least 60 days prior to the kiln helps a great deal with cupping and honey-combing. I also start the heat at only 70 degrees for the first 4or 5 days, increasing to a temp of 120 over the next 15 days (slowly). From this point I move on to 135 degrees for the next 7 days. For the next and last 3 days I raise the temp to 155 degrees and then cool slowly for 3 days. I have some very well kiln-dried lumber.