2 quarter sawn pcs about 5/8"t each that cupped. Still cupped after about
and....
I don't know if this is a general occurrence or just a few funny boards in Steve's stash. Why wasn't the original 5/4 board already cupped after 2 years in his garage? :icon_scra
OK, let me elaborate on my original post. Sometimes this concept can be difficult to follow if you're not in the lumber drying business.
For starters, Danmart the board in question is quartersawn, but both QS and FS lumber will cup after resawing if differences in MC% exist between the shell and the core of the board and the board is resawn into two equal thickness pieces. The direction of the cup will be based upon which part of the board had the higher MC% before resawing.
Let's start off with a 5/4 thick quartersawn white oak board that has been kiln dried down to 6% MC throughout, and stored in a humidity controlled warehouse. Wally Woodworker purchases said board and takes it home and stores it in his non-humidity controlled garage shop. Being online, Wally has "learned" from the Internet that he should store his lumber stickered in his shop (which is not a good idea in this instance. I'll elaborate more below).
Wally's shop is a garage, and it's not humidity controlled. For discussion purposes let's say that the relative humidity in Wally's shop averages around 53% relative humidity and the temperature averages around 75 degrees F. In this environment, the equilibrium moisture content for lumber will be 10%, so Wally's lumber starts to increase in moisture content from the shell inward.
Jeff, in answer to your question Wally's boards didn't cup while being stored because each face is regaining the same amount of moisture in his shop. Moisture related cupping is caused by unequal MC% on each face of the board that subsequently equalize. In Wally's situation both faces have regained the same amount of MC% in his non climate controlled garage because they were stored stickered. This creates internal stresses on the board from core to shell that show up after resawing (or if not resawn but face jointed and planed they wood will move if you don't remove the same amount of thickness from each face of the board.)
Because Wally's lumber is white oak, it is very resistant to changes in MC%. White oak is a close pored lumber, with the cellular openings filled with a product called Tyloses. Think of tyloses as similar to plaque in a human artery. They fill up the artery and block the flow of blood. In the case of wood, tyloses block the conveyance of moisture, which is one of the reasons that white oak is so difficult (and time consuming) to dry. Kiln operators typically have to really crank up the temperature in their kilns in order to help cook the moisture out of the wood. Tyloses are also why white oak is very rot and water resistant and used to build ships and whiskey barrels.
So, after a few months in Wally's garage, stickered and exposed to ambient RH% on both faces of the board, a cross section of the MC% gradient in the board will look like this:
Note that the outer portions of the board have equalized with their environment in Wally's garage and regained some moisture - up to 10%MC or so around the shell of the board. However, because of the tyloses the core of the board has not regained moisture and is still at 6% MC.
Wally wants to use thinner boards in a project, so he resaws his 5/4 thick QSWO board through the center. When he does so, the two resawn pieces have unequal MC% on each face, with one face of the resawn pieces being 10% and the other (resawn) face being 6% MC. See this diagram:
After resawing, the new, thinner boards spend some time stickered in Wally's garage. Because the shell of the original board had reached equilibrium with it's environment in Wally's garage, it's MC% is 10%. However, because the former core of the board is at 6%, it will gain moisture on that face in order to reach equlibrium with it's environment. In short, the resawn face gains 4 percentage points of moisture, while the opposite face remains the same. This causes the face to swell and cup the board. See this diagram:
QSWO will move approximately .0018 per inch per 1% moisture change, so in our example above one face of the resawn boards will expand by about 1/16" per inch of width, while the other face remains the same. This 1/16" per inch expansion causes the resawn pieces to cup.
Now in reality, based upon the average RH% in our part of the world lumber will typically equalize in the summer to around 12% - 14% MC. So that means that the amount of expansion on the former core of the board will be about 1/8" per inch on the resawn faces, causing noticeable cup.
Certain species that are porous (red oak, pine, mahogany, poplar, etc) will tend to equalize across the entire board over time, thus they are not as prone to cupping after resawing unless they have not fully equalized with their environment.
Now, if the board was not properly dried and the core had a MC% that was higher than the shell, the cupping would be in the opposite direction as the former core of the board lost MC% after resawing. This would cause the resawn boards to cup towards the center, instead of away from the center as the former core of the 5/4 board lost MC and shrunk while doing so..
For this reason, it is best NOT to sticker kiln dried lumber in a non-climate controlled shop after purchasing. Instead wrap the stack in plastic to retard the regain of MC%, and this will help to keep the boards not on the perimeter of the stack from regaining moisture. Even better yet, store the lumber in a humidity controlled location or in a high temp/low RH% location (such as an uninsulated attic).
Clear as mud?!
Scott
