Water and Wood

Water and Wood Probably the most important but most overlooked issue facing the woodworker Wood is Hygroscopic –The ability to absorb or desorb water Growing trees contain a lot of water most of which is in the sap.

Water and Wood Two kinds of water in a growing tree: Free water – water found in the cell cavities Bound water – water found in the cell walls Together they equal the Moisture Content

Water and Wood Calculating moisture content (MC) WW = wet weigh DW = dry weight WW-DW x 100 = MC DW Red pine has 200% moisture content

Water and Wood Fiber Saturation Point (FSP)- All water is removed from the cell cavities but the cell walls are still saturated. Wood is at its weakest state at this point It’s only when the bound water is removed that the wood begins to shrink and increase in strength. The loss of bound water depends on Relative Humidity

Water and Wood 100% humidity – No bound water is lost To remove all bound water requires an oven or vacuum at 0% humidity In reality wood doesn’t exist in either condition Remember – Wood is hygroscopic As relative humidity increases the amount of bound water increases As relative humidity decreases the amount of bound water decreases

Water and Wood For a given relative humidity wood reaches an equilibrium where the amount of bound water stays constant. This condition is called Equilibrium Moisture Content (EMC)

Water and Wood At 25% humidity the EMC = 5% * EMC = 18% of relative humidity

Water and Wood Relative Humidity in Portland (interior) January Relative Humidity=~ 40% EMC = ~8% July Relative Humidity=~ 50% EMC = ~9% Note: Any wood above fiber saturation point is considered green

Water and Wood Longitudinal shrinkage is negligible Tangential shrinkage is the greatest Radial shrinkage is lesser

Water and Wood

Water and Wood

Water and Wood The difference between tangential and radial shrinkage will effect warpage. The greater the difference the greater tendency to warp

Water and Wood Dim. Change from Green to Oven Dry Stable Woods (Number represents percent of green dimension) Tangential Radial Tan/Rad Teak 4.0 2.2 1.8 Mahogany 5.1 3.7 1.4 Redwood 4.4 2.6 1.7 Potentially Troublesome Beech 11.9 5.5 S. Red Oak 11.3 4.7 2.4 E. Pine 6.1 2.1 2.9

Water and Wood Methods for getting Moisture Content below FSP FSP (average) 28% Air Dry 14% Kiln Dry 7% Oven Dry 0%

Water and Wood To the cabinetmaker Kiln Dried suggests moisture content below 10% Kiln drying is needed in most locals to achieve levels appropriate for interior use Exterior grade materials may be kiln dried to only 19% or less Reduce weight for shipping Kill fungus Speed up drying process