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Wood and Moisture (Read 161 times)
 
Don Stephan
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Wood and Moisture
Sep 28th, 2017 at 2:57pm
 
In Understanding Wood Bruce Hoadley devotes and entire chapter to "Water and Wood."  Seems like every few months I am referring back to this material, so I thought I would summarize for anyone else who might be interested.

(I think the following applies to dry" wood as lumber, bowls, finished furniture, et cetera.)  . . .  "As woodworkers, we have two goals . . .  First, we must dry wood (and thereby preshrink it) to a moisture content consistent with its eventual environment, and second, we must control any subsequent gain or loss of moisture in order to minimize dimensional change."  . . .  "In a nutshell, the atmospheric humidity determines the moisture content of wood, and the moisture content, in turn, determines the dimension of wood."

In a living tree, the wood could be compared to a saturated, swollen kitchen sponge.  In the tree, the cell walls are fully saturated and there is also free water in the cell cavities.  All of this free water can be removed without the wood shrinking.  Free water is not attached to the cell walls in any way, and is the liquid that can be flung out when turning green wood on the lathe.

The bound water in cell walls is held by  "physical forces of attraction" and only leaves the wood when the relative humidity of the surrounding atmosphere decreases.  The moisture content of wood attempts to reach an equilibrium with the relative humidity of the surrounding air. 

He suggests as generalities a 50% relative humidity (RH) will eventually yield about a 9% equilibrium moisture content (EMC) in wood, a 25% RH about 5% EMC, and a 75% RH about 14% EMC.

Although the prior paragraph may not make clear, dry wood is ALWAYS going to have increasing or decreasing EMC according to the surrounding RH.  Some finishes are more effective than others at slowing the rate of moisture movement into and out of wood, but none totally block moisture movement.  And Mr. Hoadley states that this is also true of kiln-dried wood, which can realize an increased EMC when in a higher RH environment.

"Wood shrinks or swells due to loss or gain of bound water from the cell walls.  The amount of movement varies according to the orientation of the wood cells and is typically measured separately in the three principal directions:  tangential [parallel to the growth rings], radial [perpendicular to the growth rings], and longitudinally [between the roots and the leaves].  . . .  The orientation of the long-chain celllulosic structure in the cell wall is nearly parallel the long axis of the cells.  As water molecules enter and leave the cell wall, the resulting swelling or shrinkage is mainly perpendicular to the cell walls and does not influence their length.  Similarly, pushing marbles into a straw broom would make the broom head wider but would have little effect on the overall length of the broom head."


"Total shrinkage of wood along the grain is normally only about 0.1%."  Tangential and radial shrinkage rates vary from species to species, and a representative table for hardwoods, softwoods, and imported woods is included in the chapter.  [In the table] "Tangential shrinkage ranges from 4% in teak to 12.7% in overcup oak, with an overall average of 7.95%.  [Again in the table "Radially, shrinkage values range from 2.2% for teak or redwood to 8.5% for eastern hophornbeam, averaging 4.39%.  It is reasonable to think of wood as having roughly 8% tangential shrinkage and 4% radial shrinkage."

"The difference between tangential and radial shrinkage is caused by anatomical structure, principally the restraining effect of the wood rays, whose long axes are radially oriented.  The magnitude of the differential shrinkage is critical to development of certain forms of warp and defects.  The woodworker soon learns the importance of the ratio of tangential to radial shrinkage.  Table 6.2 reveals that the tangential shrinkage/radial shrinkage ratio averages 1.8, but individual species range from 1.1 (greenheart) to 2.9 (eastern white  pine)."

Differences in the ratio of tangential to radial shrinkage show up in once turned flat cut bowls (the growth rings are parallel to the bowl rim).  Black cherry (1.9 ratio) and red oak (2.2 ratio) show more warping after drying than black walnut (1.4 ratio) and white ash (1.6 ratio).  Warping occurs because the growth rings form arcs of a circle within the bowl.  But quarter cut bowls (the growth rings are perpendicular to the bowl rim) tend to stay fairly round and the rim flat because the growth rings remain straight within the bowl.  Using wood with a higher tangential to radial shrinkage ratio, cut a tallish rift cut blank and turn.  The growth rings are straight within the bowl, but diagonal to the rim.  After drying, the bowl might be leaning.

This is a relatively expensive book, but might be found in pubic library systems.  One can argue it is most applicable to flat work, furniture and building construction, but it is fascinating and I would argue that turners cannot know and understand too much about wood.




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« Last Edit: Sep 28th, 2017 at 3:32pm by Don Stephan »  
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Ed Weber
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Re: Wood and Moisture
Reply #1 - Sep 28th, 2017 at 3:21pm
 
If you could build up a finish to the point where you actually  'encase'  the wood within the finish, (no moisture transfer in or out) you can all but eliminate wood movement.
The only problem is that in all the various processes of keeping wood from moving (making it more stable) you seem to simultaneously take away the characteristics that drew you to use wood in the first place, namely the look and the feel. If you don't want wood to move, you end up using plastic, wood composite or some other 'man altered" wood-like product.
just my two cents
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Tom Coghill
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Re: Wood and Moisture
Reply #2 - Sep 28th, 2017 at 5:29pm
 
Ed, I agree with you.  Also, if the moisture of the wood is high enough when encased, it could quickly decompose (rot) from the inside-out.  One may see this as a mold that appears beneath the finish.

I prefer enough wood exposed (the finish to be substantially thin and porous) to let the turning "breathe" with the changing seasons and humidity.
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« Last Edit: Sep 28th, 2017 at 5:29pm by Tom Coghill »  
 
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Ed Weber
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Re: Wood and Moisture
Reply #3 - Sep 28th, 2017 at 7:29pm
 
It's a balancing act, just another one of those things to deal with when working with a natural product.
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Ron Sardo
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Re: Wood and Moisture
Reply #4 - Sep 29th, 2017 at 7:47am
 
Well written Don, thanks.

Good point Ed.

I would add that in frame and panel doors the panels are slightly undersized and float within a rabbet on the four sides of a frame and is not glued in. This way when the inevitable happens (the wood expands or contracts due to moisture and/or temperature) the panels won't crack. The same goes for a wooden bread board with end caps, the caps are only glued a couple of inches in the center so the wood can expand and contract.

Understanding this helps when you are cutting and gluing pieces together to build a segmented form, understanding the relationship between the bottom of a box and its lid or why a bowl warps when it dries.
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