TUESDAY, SEPTEMBER 8, 2009
How Dry is Dry
Water occurs in wood in two places, First, there’s the free water that fills the inside of the wood cells. That’s like water in a bucket. Second, water also infiltrates the cell walls. That’s called bound water. Imagine squeezing a piece of cotton Cheeses cloth until all the free water is drained away. The cloth though, remains damp because the material continues to contain moisture-the bound water.
When wood contains bound water it is said to be in its fiber saturation point. And the bound water can be eliminated completely only by drying it. We would select a place where we have low relative humidity, as in an air-tight oven.
Wood likes water, wood rates as a hygroscopic substance; it has an affinity for water and readily absorbs it as a liquid and vapor. This ability directly depends on the humidity of the surrounding atmosphere. The amount of moisture in wood changes as the humidity in the atmosphere changes. For example in CA when we get a “Santa Ana Wind” the air is coming from the desert and is very dry causing most wood to shrink due to the decrease in Humidity.
The term Kiln-Dried means that wood moisture was removed in a chamber where air circulation, humidity and temperature were controlled.
Bow, Cup, Twist Crook
What Shrinkage does to wood?
Woodworkers call the change in shape of a piece of wood warp.
It takes several common forms, all of which distort the wood.
Bow, as its name implies, describes the lengthwise curvature for board-end to end along its face. Twist, means that all of a boards corners won’t lie equally flat. Crook, all the curvature runs end to end alone its edge.
Cup, is when a board no longer flat from edge to edge.
Cup always occurs in the opposite direction of a flatsawn board’s annual growth rings. Checking, Although not a distortion like any form of warp, checking refers to the Small splits along the grain. You’ll most often see checks in the ends of boards but they can occur on surfaces too.
This is due to the fact that wood dries 10 times faster along it length compared to drying across the width. One of the things we will learn in our class project is to project wood movement and design and build with this in mind.
Defects in Wood
Knots are limbs separated from the tree trunk during tree growth. Wane is bark on the edge of a board. Pitch Pocket is an opening in a board containing pitch. Splits are the tearing apart of wood cells due to improper storage or handling. Checks are the lengthwise separation of wood due to shrinkage.
The standard method of measuring hardwood is given in board feet. We will be calculating the board feet we will need for class project. Including any projected waste as part of our calculations.
One board foot is described as 12 inches by 12 inches by 1 inch thick or 144 cubic inches. This is calculated by multiplying length x width x thickness.. examples of a board foot are 3×4x12 and 2×4x18 and also 1×6x24. All these measurements give 144 cubic inches of lumber. We will be using a bill of materials for the workshop to determine how much wood we will need for each project.
Grading Wood FAS (first and Seconds) This is the best grade, boards are 6” and wider and 8’ and longer. Almost clear and yields 8 32/3 percent clear face cuttings 4” or wider by 5’ or longer and 3” or wider and 7’ or longer.
Selects, The boards 4” and wider, 6’ and longer. one side is FAS the is no 1 common. Yields 83 2/3 percent clear face cuttings.
No 1 common, Boards are 3” and wider, 4’ and longer and yields 66 2/3 percent clear face cuttings 4” or wider by 2’ or longer.
Urban Forestry in California
In 1989 the legislature passed a bill to reduce solid waste by 50% by the year 2000. One unique outcome put forward by Eric Oldar of the California Dept of Forestry was to promote the value of the lumber cut from trees in Urban locations and to promote organizations to harvest urban lumber as an economic venture and to reduce solid waste.
Palomar College Participates in Urban Forestry with a Wood=Miser portable ban saw and Kiln for drying the lumber. Student participation in the Urban Forest Program is part of the class requirements and more will be covered on this subject in future blogs.
The Portable Saw Mill at Palomar College
-- Woodworker in Progress, Oceanside CA