Blbdahi - MOISTURE WEIRDNESS

[jimithy]

Better Lumber By Drying at Home Inexpensively - MOISTURE WEIRDNESS

As I have mentioned a few times... as a kid I saw my Dad's greatest work, a Black Magnolia gun cabinet, destroyed because of uneven shrinkage. Since then I have tried to learn all I can about wood and moisture. All wood has moisture but it can create havoc before, during and after making your pride and joy.

First, I want to dispel some myths. Many people believe than a water seal for outside decks prevent moisture penetration. It doesn't. Lots of people believe that penetrating hard sealers will keep out moisture if applied to every surface... they don't. More people believe that a major surface finish such as polyurethane, shellac, oil-based enamel paint, marine varnish, etc. seals out moisture... wrong again. Extremists believe dipping the entire piece in wax or epoxy will absolutely block moisture... nope!

Wood is a sponge and vapor pressure differentials insure that it will reach the Equilibrium Moisture Content (EMC) based on the environment it is in. USFS has studied virtually every type of wood finish and has found nothing that will keep out moisture.

Below is a USFS table from www.fpl.fs.fed.us/documnts/pdf1987/feist87a.pdf

They preface this table with...

The information in our studies relates to coatings that are only a few weeks old and not exposed to prolonged aging or severe conditions, such as outdoor weathering (which will quickly damage most coatings, causing them to lose effectiveness).

The moisture resistance of finishes also depends on the type of exposure. For example, water-repellent treatments are quite ineffective against water vapor but-because they cause water to bead on the surface–they’re fairly effective against liquid water. So, this type of sealer finish would protect your outdoor wood against rain and dew for some time, but not for very long against humidity...

Moisture-excluding effectiveness
This chart shows the moisture-excluding effectiveness (MEE) of a variety of finishes and other materials. Of the 91 finishes tested, these figures are the best for each finish type. The chart is arranged from highest MEE to lowest. Ratings are given for one, two and three coats after 14 days of exposure at 80°F and 90% relative humidity. Negative numbers indicate that the finish itself adsorbed water. (N. A. = not applicable)

USFS research shows that finishes degrade EXTREMELY rapidly. They tested a lot of finishes over a 2-week period. After 2 weeks even robust finishes such as polyurethane (that are thought by many to last many years without major degradation) actually lost 1/3 of their moisture barrier protection in the 2 weeks.

Moisture is more than a "shrinkage/swelling" problem with the size of the board. When a piece of wood doesn't have an equal distribution of moisture inside then it has unequal "shrinkage/swelling" problems internally which creates huge stresses. This causes checks, major splits, warps, etc. The pattern of cuts, the shape of the board, and the particular grain in each part of the board determines how badly it contorts and in what manner it wants to contort.

Very careful woodworkers that respect the power of moisture will use a moisture meter on both the wood they are using and on wood in the environment where the final piece will be going. They use their meter to test a hundred places on their stack when drying wood to make sure that everything has reached the correct EMC. Most people do those tests incorrectly.

Wood in equilibrium with its environment can be tested using surface meters such as a resistive moisture meter. Wood recently introduced to an environment or wood in the drying process is Not In Equilibrium (NIE) and therefore will not test correctly.

Below is a graphic I produced to roughly show how the moisture content of a board (NIE) varies greatly based upon location in the wood. I am showing cross sections of the middle of the board.



If you do not test the internal center of a NIE board (away from the ends of the board) then you do not know the highest MC of the board. Worse is that boards are not perfectly homogeneous as pictured. Variances in grain, splits, knots, heartwood, etc. will create variations in the MC.

This is important because of three major requirements when drying wood.

1. You must know your target EMC.
2. You need to know how much surface drying has already occurred and how much internal moisture there is that must be removed.
3. You need to know when the drying process is done. If the internal center of the board is not at your target EMC or if it is fairly different from EMC readings near the surface of the board then the process is not done.

Items 1 and 2 are necessary to design the correct drying process for your particular needs.

My next post in this series will give you a way to cheaply get internal board measurements without slicing and dicing everything in sight.