Free Water / Bound Water - Shrinkage (warping)

[John Tisdale]

The following is from Gene Wengert's "Principles and Practices for Drying Lumber" as shown on Lignomats website. As many woodturners work with green wood, a good understanding of the dynamics is important to quality work. After all, we want our works to outlive us.

The change in dimension of wood that occurs when it dries is called shrinkage. The individual
cells begin shrinking when they are at the fiber saturation point (28% MC) and the shrinkage
continues linearly with a decrease in MC to 0% MC. Lumber, because some cells are at fsp very
early in drying, begins shrinking very early in drying at very high MCs (within 5% of its "tree
green" MC) and continues shrinking until equilibrium is reached. Whenever the relative
humidity changes, wood will lose MC and therefore shrink if the humidity is lowered, or gain
MC and therefore swell if the humidity is raised. Temperature change has no important effect on
shrinking or swelling of wood.

Sometimes during the early stage of drying, water is removed too quickly from a cell and the cell
is sucked inward (like sucking on a straw that has one end closed). The cell collapses.
Shrinkage, where many cells collapse, appears much greater than normal or expected. After
drying, when collapsed cells are exposed to liquid water or wet steam, they will usually return to
their expected size as if they hadn't collapsed (i.e., the collapse is recovered).

Fiber Saturation Point

Water is held in wood in two locations. One is in the hollow center region (the lumen) of the
cell. This water is held due to the physical size of the cell. Theoretically, this water could be
blown out of the cell just like blowing the soda out of a soda straw. This water is called "free
water." Water is also held in the cell wall, but this water is chemically held in the wall. It is
called bound water.

When a cell is dried, the free water is removed first. When all the free water is removed, but the
cell wall is still saturated, the cell is at the fiber saturation point (fsp). The approximate MC of
the fsp is 28%, but this varies with species. When drying up to this point (i.e., from "tree green"
to the fsp), the wood cell has not shrunk (unless it has collapsed, which is rare). As the cell
begins to dry below the fsp, bound water is removed from the cell wall and the cell begins to
shrink. Shrinkage continues until 0% MC.

Note that the discussion above has referenced the wood cell. In a piece of drying lumber, cells
on the outside will dry below the fsp and begin shrinking quite quickly. Cells in the center of the
piece may require weeks, in some cases, before they reach the fsp and begin shrinking.
Therefore, it is incorrect to say that a piece of wood is "at the fsp" when it is at 28% average
MC. In fact, at 28% MC, some of the cells are well below the fsp and have shrunk, or tried to
shrink quite a bit, while other cells are still well above the fsp and haven't begun to think about
shrinking.

 

[Bill Boehme]

The gist of the description appears to be that during the process of going from sopping wet green to bone dry, except at the beginning and end, no two points will have the same moisture content. Some parts are below the FSP while other parts still has free water. The result is internal stresses leading to cracking and warping. There's not a homogenous point where we can say that an entire piece of wood is at FSP.

Drying is only one thing that causes cracking and splitting. Simply cutting a piece of green "reaction" wood is enough to cause that.

 

[hockenbery]

The gist of the description appears to be that during the process of going from sopping wet green to bone dry, except at the beginning and end, no two points will have the same moisture content. Some parts are below the FSP while other parts still has free water. The result is internal stresses leading to cracking and warping. There's not a homogenous point where we can say that an entire piece of wood is at FSP. Drying is only one thing that causes cracking and splitting. Simply cutting a piece of green "reaction" wood is enough to cause that.

Turning natural edge bowls from green wood to 1/8 - 3/16 wall thickness will illustrate all the points you make.

While the wood is full of water light woods will pass light through the fiber optic effect of the free water.
The turner hers sprayed as the free water is released.

The end grain fibers are 1/8-3/16 thick and dry quickest the bottoms and sides are long fibers and dry slowest.
When hollowing there is often some reaction wood that lets the bowl open more as it is hollowed.

We can mitigate some of these.
Misting the end grain area slows the moisture loss.
Doing some rough hollowing before finish turning the outside mitigates the reaction wood

When the turning is finished, washing with water west the endgrain areas.
Putting it in a cardboard box or paper bag creates a humidity chamber to let the moisture coming off the long grain areas hydrate the endgrain some learning he moisture differential which lessens the movement differtial.
When dry they are too thick to be translucent.


This also illustrates why the vertical sidewall flat bottom bowls are so hard to dry crack free. There is the shortest end grain adjacent to the longest grain.
The hemispherical bowls rarely crack because their is a gradual transition from short to long.

John, excellent post on the process of warping.

 

[Mark Mandell]

Something of a stub article, but a good discussion starter.

Shrinkage occurs in two directions, radially and tangentially in relation to the grain, and the amount for each is different for every wood species. Radial shrink occurs at a high percentage as the annular rings' cell structure collapse with the moisture loss, and makes green-turned face grain bowls and hollowforms go oval because wood does not shrink longitudinally. Tangential shrink is slower, is more effected by differences in wood density and growth direction, and may cause warping. When turning spindles, it is often recommended that the wood be "riven" (split rather than sawn into blanks) as this will reduce tangential warping and maintain more of the woods structural strength.

As wood looses water, its cell structure becomes stiffer. Spinning a "green" bowl blank on the lathe accelerates the loss of "free" water exactly like a centrifuge. A roughed-green bowl then begins to shrink in two directions at once from the two exposed surfaces. The inside surface shrinks and pushes outwards, making the inside diameter larger in one direction, while the outside surface shrinks in the opposite direction, making the outside diameter less. This puts the "squeeze" on the still wet wood fibers in the middle of the bowl walls. With the inside surface becoming stiffer faster (less wood fibers to shrink) the outside surface fibers are then pushing against hardened wood, setting up the possibility for cracking in the bowl due to the drying stress. This drying stress will be channeled to the weakest section of the wood, the end grain, where the wood fibers are shortest and most easily pulled apart. When we coat the outside surface of the bowl with something like Anchorseal we substantially slow down the rate of outside shrinkage and allow the bowl to dry from the "inside out" while keeping the outside surface more pliable and less likely to crack. We can further reduce the drying stress when we place the bowl in a paper bag which creates a micro-atmosphere that further slows the drying process allowing the wood to more gently adjust to its new dimensions as the MC lowers.

Worth noting too, that wood will never get to 0% MC unless it is placed in an oven and specifically baked until all water is driven out of its structure. Wood will otherwise dry to its EMC "Equalized Moisture Content" depending upon where it is located. The oven-dry (0% MC) wood will pull moisture from the air until is reaches EMC. Kiln-dried wood is usually taken down to 7%MC depending on the species. It too will then pull moisture from the air to reach EMC. If the wood is located in Arizona, its EMC will likely be 5-7% whereas here in New Jersey EMC is 9-11%. All wood that has reached EMC in a given location will have the exact same MC, regardless of species.

Thanks for posting this, John. It's important for our readers to get a better understanding of the material we are working with here.

 

[hockenbery]

In the bowl there are two spots that don't move and don't shrink
( they may get pulled slightly)

The top of the rim in the endgrain does not shrink.
Turn a 12" diameter bowl 1" thick. When it is dry the top of the rim where the end grain is will be1" thick and 12" OD 10" ID.
These two point will be at a height less the radial shrinkage on the wood with a balance grain bowl.

As Mark stated the wood does not shrink in the vertical direction.

These are the two peaks you get on the rim and turn away to make a round bowl in the second turning the simple way to view warping /shrinkage is the growth rings want to flatten out. Tangential shrinkage being more than radial.

 

[Mark Mandell]

Tangential shrinkage being more than radial.

Just went back and looked! I always seem to get those two conflated, even after 30+ years at this wood stuff and knowing better. Thanks for the correction, Al.

Values for shrinkage most often show radial at about half of that for tangential. This is why quarter-sawn wood is more dimensionally stable than flat or rift-sawn stock.

My duh 🙄

 

[Rob Wallace]

Slight Correction.....

Worth noting too, that wood will never get to 0% MC unless it is placed in an oven and specifically baked until all water is driven out of its structure. Wood will otherwise dry to its EMC "Equalized Moisture Content" depending upon where it is located. The oven-dry (0% MC) wood will pull moisture from the air until is reaches EMC. Kiln-dried wood is usually taken down to 7%MC depending on the species. It too will then pull moisture from the air to reach EMC. If the wood is located in Arizona, its EMC will likely be 5-7% whereas here in New Jersey EMC is 9-11%. All wood that has reached EMC in a given location will have the exact same MC, regardless of species.

In case readers wish to pursue this topic further, EMC is actually Equilibrium Moisture Content* - that is, the amount of moisture (reported in percent moisture content) found in the wood which is at equilibrium with the water that is gained or lost with the surrounding air (and thus is at equilibrium with it), and is dependent upon the relative (or absolute) humidity in said air surrounding the wood. Mark is correct that unless baked in an oven, the MC will never reach zero ("Oven" is also a relative term - I have been in southern Arizona, Mexico, and the Atacama desert of Chile at various times, which I immediately likened to an oven due to high temperature and lack of humidity). The exact same stack of wood at 6% MC in Phoenix will change to 9% or 11% when taken to a more humid environment like Missouri or Alabama; this is, of course, also dependent upon temperature, given that warm air has the capability of holding more water (relatively speaking!) than cold air does. (Meteorology 101)

Concepts of wood shrinkage (which is inherently different in nearly all species, due to the magnitude of tangential versus radial dimension changes) and moisture content relationships are among the most misunderstood concepts in woodworking. When I give my Wood Identification and Wood Properties demonstration, there are invariably questions asked nearly every time about these attributes of wood as it transitions from full water saturation when alive to "dry" and at EMC, as well as what the compensatory responses of the wood are during that process.

Rob

*quod vide: http://en.wikipedia.org/wiki/Equilibrium_moisture_content

 

[Mark Mandell]

In case readers wish to pursue this topic further, EMC is actually Equilibrium Moisture Content* - that is, the amount of moisture (reported in percent moisture content) found in the wood which is at equilibrium with the water that is gained or lost with the surrounding air (and thus is at equilibrium with it), and is dependent upon the relative (or absolute) humidity in said air surrounding the wood. Mark is correct that unless baked in an oven, the MC will never reach zero ("Oven" is also a relative term - I have been in southern Arizona, Mexico, and the Atacama desert of Chile at various times, which I immediately likened to an oven due to high temperature and lack of humidity). The exact same stack of wood at 6% MC in Phoenix will change to 9% or 11% when taken to a more humid environment like Missouri or Alabama; this is, of course, also dependent upon temperature, given that warm air has the capability of holding more water (relatively speaking!) than cold air does. (Meteorology 101)

Concepts of wood shrinkage (which is inherently different in nearly all species, due to the magnitude of tangential versus radial dimension changes) and moisture content relationships are among the most misunderstood concepts in woodworking. When I give my Wood Identification and Wood Properties demonstration, there are invariably questions asked nearly every time about these attributes of wood as it transitions from full water saturation when alive to "dry" and at EMC, as well as what the compensatory responses of the wood are during that process.

Rob

*quod vide: http://en.wikipedia.org/wiki/Equilibrium_moisture_content

Well . . .

You can get all scientific on us if you must, but I like "equalized" better. "Equilibrium" sounds something you're supposed to ask your doctor "if it's right for you". 😀 😀

 

[Bill Boehme]

Well . . .

You can get all scientific on us if you must, but I like "equalized" better. "Equilibrium" sounds something you're supposed to ask your doctor "if it's right for you". 😀 😀

Mark, I don't think that your heart is healthy enough for equilibrium. 🙄

 

[John Tisdale]

As many of the forum contributors have helped me over various learning curves, I'll toss out a few tidbits on my drying experiences. As I do large hollow-forms, consistency/uniformity is critical to product quality. My goal is to "second turn" when the rough is 6%MC - compromise is not an option. In Cliff notes:
- a 20"-dia hollow-form will be turned to about 1.25" thickness if walnut, maple or anything but mesquite - a bit less for mesquite
- boil for about 2-hours (see Steve Russell website)
- Piece is placed in a 24x24 cardboard box
- 12V computer fan is epoxied onto a piece of length of 2" PVC and, a dowel inserted through the PVC, and placed into the hole of the vessel
- The fan must pull air out of the vessel and into the closed box - keeps the inside of the vessel at the same humidity and the air circulation evens the process.
- Do NOT have the fan blow into the vessel - that's dumb - I only did it twice (ok, maybe three times) - results in major cracks across the bottom
- Seal the box with only the fan wire coming out
- tape and put in a room or closet with a dehumidifier and leave for 3 or 4 months - run fan 24/7
- in 3-4 months the piece will be a bit less than 20%MC - at that time you can remove it from the box and dry more aggressively in the dehumidified room
- a moisture meter is a great tool and Lignomat has good ones. Get one that will cable-connect - the pin-type are worthless. I drill/drive little 18-gauge nails into the bottom - my Lignomat has a cable to which I soldered alligator-clips that I attach to the nail heads

According to Gene Wengert, wood is the most vulnerable to defects in drying from saturation to around 20% - the above process is slow but not as slow and more consistent than air drying which is not consistent at all. Believe it or not, the boiling removes a lot of pent-up stress and dramatically reduces cracking. And perhaps counter-intuitively, the drying is maybe 3X faster.

 

[Bill Boehme]

I don't turn any hollow forms anywhere close to 20" diameter so I can't really comment on your method. My hollow forms are mostly tall and smaller diameter, somewhat like what Steve Sinner does. I don't do anything special when drying them. I can imagine that something that is 20" diameter would force me to do more than I currently need to do -- which is basically nothing.

 

[KellyDunn]

Since its so wet where I live my cheap fridge kiln is a must for me to get out bound water in rough turned blanks. Since most of my work does not stay in Hawaii I also work on the finished piece while just fresh from the kiln and keep it in the kiln each evening until the finishing process is done. Once in the house to finish degassing of finish the bowls can move drastically. But will round out in a dry climate. If I even pull a bowl off the shelf that was once kiln dried it has so much moisture from the wet air it can go way wonky on me during finishing. Send it Colorado and it can go oval and S shaped. Bummer. So we each have to come up with what works for us where we live or where we expect our work to end up and how we want it to look in its new home. I also use a moisture meter. Can be 12 to 18% on the shelf. I take it to under 10 in the kiln. 10 is Ok, 6 is better.

 

[Odie]

Since its so wet where I live my cheap fridge kiln is a must for me to get out bound water in rough turned blanks. Since most of my work does not stay in Hawaii I also work on the finished piece while just fresh from the kiln and keep it in the kiln each evening until the finishing process is done. Once in the house to finish degassing of finish the bowls can move drastically. But will round out in a dry climate. If I even pull a bowl off the shelf that was once kiln dried it has so much moisture from the wet air it can go way wonky on me during finishing. Send it Colorado and it can go oval and S shaped. Bummer. So we each have to come up with what works for us where we live or where we expect our work to end up and how we want it to look in its new home. I also use a moisture meter. Can be 12 to 18% on the shelf. I take it to under 10 in the kiln. 10 is Ok, 6 is better.

Just wondering here, Kelly......

What happens to lathe turnings that come to Hawaii from dryer areas of the states? Is it a matter of how well the wood is sealed, or will it necessarily take on a higher MC?

I can see the difficulties you face with the humidity in Hawaii. Here in MT, I seldom see my seasoned roughed bowls go below 12%, and KD lumber generally is around 8-10%. I've never been aware of problems with my bowls because a change in climate.

ooc

 

[KellyDunn]

Odie, they grow. They swell up. A bowl say 12 in. in diameter in the kiln can grow to 12 1/2 in. when I take it in the house. Any trade I do with someone of a lidded nature I ask them to make the lid very sloppy. Turners who want to impress another turner make those really snug lids. If I ever want to take those lids off to sand them I will have to put them in the kiln. Then I risk damaging the whole piece as almost no turner kiln dries roughed work. When Jimmie clewes was here he did one of his snap lid boxes. I knew I was going to bid for it later so asked him after he showed how it snapped to make it more sloppy. He refused. I got it home and sanded so it would come off easy. In two days even that had swoll so tight the lid has not been off since. One side of each island is rainforest and one side is bone dry desert. I live on the cusp but average humidity is still over 80%. A gallery a number of us deal with in the desert area is a real testing place. It may be fine over near the volcano where they get more than 200 inches of rain a year. Take it to Kawaihae and it can come apart. Lumber can get to 8% or less air dried there. The only thing that grows along that coast away from the ocean is Kiawe(mesquite). So my work does just fine because I have created that condition in the finishing. So thats why I say each of us has to come up with what works for them.

 

[Mark Mandell]

Odie, they grow. They swell up. A bowl say 12 in. in diameter in the kiln can grow to 12 1/2 in. when I take it in the house. Any trade I do with someone of a lidded nature I ask them to make the lid very sloppy. Turners who want to impress another turner make those really snug lids. If I ever want to take those lids off to sand them I will have to put them in the kiln. Then I risk damaging the whole piece as almost no turner kiln dries roughed work. When Jimmie clewes was here he did one of his snap lid boxes. I knew I was going to bid for it later so asked him after he showed how it snapped to make it more sloppy. He refused. I got it home and sanded so it would come off easy. In two days even that had swoll so tight the lid has not been off since. One side of each island is rainforest and one side is bone dry desert. I live on the cusp but average humidity is still over 80%. A gallery a number of us deal with in the desert area is a real testing place. It may be fine over near the volcano where they get more than 200 inches of rain a year. Take it to Kawaihae and it can come apart. Lumber can get to 8% or less air dried there. The only thing that grows along that coast away from the ocean is Kiawe(mesquite). So my work does just fine because I have created that condition in the finishing. So thats why I say each of us has to come up with what works for them.

Knew a guy in Phoenix (AZ) who had a nice business repairing "Snow Bird Furniture" which consisted of wood items brought by retirees from Minnesota and New York when they moved to Arizona and found their furniture falling apart when the glue joints all failed.

Anybody who thinks that wood becomes inert when the tree "dies" is destined to be sitting on the floor surrounded by chair parts.