By Dr. Eugene Wengert
I. Moisture and RH
Wood is a hygroscopic material, meaning that it is continually trying to achieve an equilibrium moisture condition with its environment. As the relative humidity (RH) changes, so does the moisture content (MC) of the wood. The following tabulation expresses the relationship between MC and RH.
|5||19 to 25||5|
|6||25 to 32||6|
|7||32 to 39||7|
|8||39 to 46||8|
|9||46 to 52||9|
|10||52 to 58||10|
|11||58 to 64||11|
Note that temperature is not an important factor; it is just the RH. Also, these numbers hold true for all species of wood.
II. Moisture Effects on Wood-Shrinkage
So, what is the big deal about the MC of wood? The truth is that the properties of wood change with changes in MC--most noteworthy is that wood shrinks and swells substantially with MC changes. plus, with the property changes come changes in processing--gluing, machining, fastening, and finishing. Let's briefly look at a few of these changes in order to establish how closely we need to measure and control moisture levels.
Of all the changes, certainly the most troublesome change has to be shrinkage and swelling--wood shrinks when the MC decreases and swells when the MC increases. And to complicate matters, wood shrinks differently in the three directions--lengthwise shrinkage is usually quite close to zero; across the grain, parallel to the rings (width of a flatsawn piece) wood shrinks up to 1% for a 3% to 4% MC change; and across the grain, perpendicular to the rings (thickness of a flatsawn piece) wood shrinks about 1% for a 7% MC change. Of course, there is variation from piece to piece of the same species, as well as variation from species to species. The following tabulation gives the average change in size for a 3 inch wide, flatsawn piece of lumber for a 2% MC change for several important species. Shrinkage is essentially a linear function of MC, so double the values for a 4% MC change, triple them for 6% MC change, and so on.
|SPECIES||SIZE CHANGE||SPECIES||SIZE CHANGE|
|Ash, white||0.016||Maple, hard||0.021|
|Birch, yellow||0.020||Oak, white||0.020|
In summary, we are looking at a 3 inch wide piece changing 1/100 to 3/100 inches in width for a 2% MC change--a very small size change. But such a change is quite large when it comes to edge gluing and to flatness in a high gloss finish, as is discussed later.
Under 12% MC, the commonly used adhesives for wood are not affected by MC directly--that is, the strength of the glue and the chemistry of gluing does not vary with different MCs. However, MC does affect gluing in other ways.
First, when the wood is very dry (under 6% MC), it is very absorptive. When glue is spread on dry wood, the liquid in the glue (usually water) is almost immediately extracted from the adhesive. Unless the glue joint is completed immediately, this rapid absorption of liquid will lead to a very weak joint due to premature thickening and setting of the adhesive. And the reverse is also true--for higher MCs the length of time required for the glue to set is extended.
Second, when the MC of wood changes, the size of the piece changes--shrinks or swells. So, if the wood's MC is not in equilibrium with the air's moisture content, then the pieces of wood being glued will change size. If they change size before the pieces are glued together, then it is likely that the gap between the adjacent pieces will exceed 0.006 inches, meaning that the glue joint between them will be weakened. If they change MC after being glued, then the pieces may shrink or swell differently. Several effects of this delayed shrinking or swelling are a) development of stress on the joint, which can often lead to cracks in the end of glued up panels, and b) uneven shrinkage resulting in different thickness and uneven surfaces. (Note: This delayed shrinkage and swelling is also sometimes called delayed warp.)
The effects of MC on machining quality are well documented, but often are ignored in the manufacturing operation. High MCs, especially with lower density species such as aspen and basswood, lead to an increase in fuzziness when planing, boring, routing, and even sanding. On the other hand, higher MCs, reduce the likelihood of planer or roller splits, torn or chipped grain, and raised grain. A very notable decrease in quality machining occurs at moisture under 6% MC--chipped grain becomes inevitable, shelling (especially in white pine) increases, and dulling of the tools increases.
Machining problems also increase when the lumber is over-dried as warp, especially cup, increases in the lumber when over-dried, leading to movement of the pieces when first machined (such as in a gang rip saw). As a result, edges are not high quality--they are not flat or straight enough for many subsequent manufacturing operations.
Another MC related machining problem is casehardening (or drying stress). Casehardening shows up as immediate warp when machining. If the MC in the kiln is not uniform, then when the casehardening relief treatment (conditioning) is used, the driest lumber will have the stresses relieve quicker than the wetter lumber. If conditioning is too short, then there will be stress in the wetter lumber. If conditioning is extended, then the driest lumber will be subject to excessive regain of MC.
V. How to Control EMC, RH, MC
The first step in controlling moisture is to kiln dry the lumber to the correct MC level. It is important to remember that a check of moisture coming from the kilns includes both the average MC and the spread of MC. Many companies today are specifying an average MC of 6.25 to 7.25% MC with a spread (measured as a standard deviation) of 0.6%. In order to determine the average and spread accurately requires taking no fewer than 20 samples. But, remember that most kiln operators use only 8 or 10 samples and these samples are located on the edge of the pile. It is not a lie to say that often the kiln operator really doesn't know the true MC of the load of lumber--over-drying is common and under-drying is also not unheard of. Of course, the kiln operator gets his instructions from the boss, so it is up to the boss to specify the correct average MC and the desired spread and how it will be measured in the kiln operation. (Or perhaps the customer insist that the boss do this.) If the MC is wrong, it costs the furniture plant many $1000 each year, especially in the winter when the lumber is often too wet.
Once the lumber has been dried correctly and stresses relieved properly, the lumber must be stored under controlled conditions--that is, controlled RH. See the first page for a tabulation of the relationship between RH and MC. Controlling the RH in a shed or storage building can be done by installing expensive humidification and dehumidification equipment. Or, it can be done quite inexpensively by controlling the heat to the storage building.
Two facts help us understand why heating works. First, when air is heated, its RH drops, and when it is cooled, the RH rises. Second, the RH outside in almost the entire U.S. and Canada will be at or close to 100% RH in the early morning hours just before sunrise. So, to control our storage shed, all we need do is keep the shed a few degrees warmer than the outside temperature. The control temperature in the shed can be set every day or two, based on the weather reports, or the heat can be tied into a humidistat, where high RH indications turn on the heat. Specific heating values are:
|Desired EMC||Heat Above The Low Temperature|
(Note: I am often asked how long lumber can be stored at the wrong conditions before there will be a problem. My answer is that the cost of the problem is so high, that the answer is zero days. Further, with as much as 75% of the cost of a furniture piece being wood, can we afford to be casual with the way we treat this basic ingredient? But, as a rule of thumb, the MC of the lumber will change about 2/3 of the way toward the EMC of the air in about 4 weeks of warm storage. In a tight pile, MC changes will be slower--just the exposed ends will change quickly. In the winter, changes will also be slower.)
VI. Checking MC
I could relate many, many stories about the high number of rejects in a plant and how, after getting MC under control the number of rejects was reduced to almost zero. A door company had over 1000 rejects every winter; with an in-line MC meter rejecting wet lumber, their rejects last winter were six. Then there was a furniture company that ran all last winter without any end cracks in their panels (except where the chop saws didn't cut enough off the lumber ends to eliminate the end checks in drying). And so on. Controlling MC works. The correct MC results in better gluing, machining, and finishing and fewer rejects due to warp.
So, what's the best way to check MC? There are two basic approaches--first is to sample the MC of the incoming lumber and eliminate those pieces that are too dry and too wet. A sample of 30 pieces per load will give me a precise estimate of the MC of the entire load, both spread and average MC. [Example: Assume the average was measured to be 7.08% MC. With a standard deviation of 0.4%, then 98% of the pieces are between 6.28% and 7.88% MC.] The second approach is to measure every piece of lumber with an in-line MC meter (usually located in front or behind the rough planer).
There are also two choices on moisture meters--the pin-type meter that measures electrical resistance and relates the resistance to MC, and the dielectric meter that measures a dielectric coefficient and then relates the value to MC. Both meters have certain advantages and disadvantages; I have listed a few. In short, both work well. A comparison of these meters on over 250 pieces of kiln dried lumber did not show a clear preference for one or the other. A plant concerned about MC should have both types.
|Resistance Meter||Dielectric Meter|
|Not highly sensitive to species||Highly sensitive to species variations|
|Not highly sensitive to density||Highly sensitive to density variations|
|Sensitive to temperature||Not highly sensitive to temperature|
|Measures at a very small spot||Measures over a small area|
|Measures at a given depth||Measures an average over 3/4-inch depth or so|
|Best between 7% to 25% MC||Best from 4-1/2% to 25% MC|
|In-line is difficult||In-line is easy, measuring lengthwise |
or cross-wise in 2 or 3 locations
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison