The following pages provide some background information and possible further experimentation that you may choose to do with your class.

Shrinkage and Swelling of Wood Shrinkage of the fibre wall, and therefore of the whole wood, occurs as bound-water molecules escape from spaces between cellulose (and lignin) molecules. These cellulose (and lignin) molecules can then move closer together. Swelling is simply the reverse of this process. The amount of dimensional change that occurs is generally proportional to the amount of water that is removed (or added) from the wood, and the orientation of microfibrils in the cell wall. Troublesome uneven shrinkage results when shrinkage properties vary within a piece of wood, a situation associated with juvenile and reaction wood. That might not be a problem if the degree of abnormality were uniform, but it never is. Typically, the extent of juvenile wood or the severity of reaction wood varies within a piece. Bow and crook are commonly traceable to the abnormal longitudinal shrinkage of these abnormal woods. The longitudinal shrinkage of normal wood is negligible for most practical purposes. Here microfibril orientation is about seven degrees or less from the cell axis. Usually, some longitudinal shrinkage does occur in drying from green to oven-dry condition, but this is only 0.1-0.2% for most species. METHODS: Radial and Tangential Shrinkage and Swelling demonstration shall be made on air-dry 100 x 100 x 5 mm specimens, cut from a 4 x 4 in. post with truly edge grain and flat grain ring orientation. The air-dry condition is assumed to be between 8-12% moisture content. Take two identical cross-sections and measure the width of all four sides for both pieces, using a caliper (or a ruler). Prepare photocopy images for both, being careful to orient the ring patterns in an identical fashion. Soak one of the specimens in water for 5-10 minutes. Wipe excess water with a wet sponge or paper towel. Prepare another photocopy image, placing the wet specimen beside the air-dry one. Using a ruler, measure all four sides of the wet specimen, and record these swollen dimensions on the photocopy. Calculate the difference By subtracting the dry dimension from the wet dimension. Express dimensional change in percentages. On average, flat grain (rings running parallel with the edge of specimen) swelling is twice as large as the edge-grain (rings running at right angle to the edge). % swelling = wet width - dry width / dry width x 100% Longitudinal shrinkage/swelling measurements shall be made on material taken from the strength and stiffness experiment, after the 1 x 2 x 100 cm sample had broken. Cut about 20-30 cm segment, being careful to have nice smooth ends. Measure air-dry length precisely and record the dimension. Soak specimen in water for about 24 hours, because the side-grain is not as permeable as the cross section in the above experiment. Measure and record the length of the wet stick. Longitudinal swelling is expressed as a percent length gain, based on the original air-dry length: Longitudinal swelling = green length-dry length/dry length x 100% Longitudinal shrinkage = green length-dry length/wet length x 100% TB 1 and 2 WOOD TECHNOLOGY LAB #1 RADIAL, TANGENTIAL, AND LONGITUDINAL CHANGE IN WOOD DIMENSIONS WITH MOISTURE CONTENT CHANGE Purpose: To demonstrate and measure the anisotropic (exhibiting properties with different values when measured along axes in different directions) nature of wood. Shrinkage (and swelling) of wood occurs when cell-bound-water molecules escape (or enter) from the spaces between cellulose and lignin molecules which make up the cell wall. When water escapes the molecular structure, the elements move closer together and shrinkage occurs. Swelling is simply the reverse of this process. Once the cell walls are completely saturated, the wood reaches its maximum swollen size. Additional ("free") water in the cell cavities does not create more swelling. Conversely, shrinkage does not occur until the free water in cell cavities has exited, and cell-bound water starts to leave. Apparatus: A bowl of water, paper towels (or a sponge), caliper (or a ruler with a millimeter-scale), and access to a photocopying machine. Materials: 1) A pair of identical-size cross-sections, measuring approximately 100 x 100 x 5 mm length (along the grain), cut from a 4x4 in. post. The specimens should have the annual rings are at right angle to one edge (commonly known as edge-grain), and parallel to the other edge (commonly known as flat-grain). The cross-sections should be air-dried to about 8% moisture content. 1) About a 20-cm-long strip of wood from the 1 x 2 cm wood sample that was used in the strength and stiffness measurements, again at about 8% moisture content. Please ensure that the two ends are smoothly cut and sanded. Hook: In all woods shrinkage and swelling is at a maximum in the tangential direction. In British Columbia softwoods, the average tangential shrinkage is twice as much as the radial direction (8% compared with 4%, going from wet to oven-dry condition). Along the grain longitudinal shrinkage in normal wood is only 0.1-0.2%. Procedure: Take two identical cross-sections and prepare a photocopy image, being careful to orient the ring patterns in an identical fashion. Measure and record the width of all four sides for both pieces on the photocopy, using a caliper or a ruler. Now soak one of the specimens in a bowl of water for about 5 minutes, making certain that all sides get thoroughly wetted. Wipe off excess water with a wet sponge or paper towel and prepare another photocopy image. Lining up the wet specimen beside the air dry one gives an excellent visual display of dimensional change. Measure all four sides of the wet specimen and record these swollen dimensions on the photocopy. Calculate the tangential dimensional change (i.e. swelling) by subtracting the dry dimensions from the wet ones. Do the same for the radial direction. Express dimensional change in percentages. % swelling = wet dimension - dry dimension / dry dimension x 100 (Conversely, % shrinkage = wet dimension - dry dimension / wet dimension x 100) For longitudinal swelling demonstration the specimen has to soak for about 24 hours, because the side-grain is not as permeable as the cross sections in the above experiment. An interesting addition to the above experiments could include the oven-dry (bone-dry) condition as well, because maximum shrinkage occurs at 0% moisture content. To arrive at the oven-dry condition the wood cross-sections should be placed in an oven for about one-half hour at 105°C, before swelling in water (but after the air-dry measurements). Please note that an ordinary kitchen stove could be used for this experiment, set at 105° C (or about 220° F). Significance: Because of differential shrinkage in the three planes in wood, cupping and bowing occurs in lumber as wood is being dried. A VERY INTERESTING EXPERIMENT CAN BE PERFORMED BY THE FOLLOWING STEPS: Wet sample completely. Remove from water, towel off excess water, weigh sample and measure tangential face. Using a hair-dryer, force hot air onto both sides for about 30 seconds. Weigh and measure again. Repeat step 3 a number of times. When no more weight loss (or shrinkage) is evident, consider moisture content to be zero (0%), and stop measuring. Plot weight (on "y" axis) and shrinkage (millimeters on "x" axis). Calculate moisture content at which shrinkage began: % moisture = wet weight - dry weight /dry weight x 100 TB 3 and 4 DETERMINATION OF WOOD DENSITY Purpose: To measure the density of different species of woods because this property is important in describing wood quality. For example, wood density affects strength and stiffness, shrinkage and swelling, nailing resistance, wood machining, thermal, acoustical, electrical, and other basic wood properties. Apparatus: 500 or 1000 ml (0.5 or 1.0 liter) graduated cylinder, caliper (or a ruler with a millimeter-scale), oven (or a kitchen stove), and a balance. Materials: About a 20-30-cm-long strips of wood from the 1x2 cm wood sample that was used in the strength and stiffness (or shrinkage) measurements. Hook: Density is the mass contained in a unit volume of a material, and relative density is the ratio of the density of the material to the density of water. In the Metric system, density is measured in grams per cubic centimeter (g/cm3), or kilograms per cubic meter (kg/m3). Water has a density of 1 g/cm3 or 1000 kg/m3 (1 ton). Wood weighing 0.4 g/cm3 is thus four-tenths as heavy as water, and has a relative density of 0.4. In experiments dealing with tree and wood samples, relative density is usually expressed on an oven-dry weight and green-volume basis. The practical reason for this is that both the oven-dry weight and the fully-swollen green volume condition are easily reproduced. If, for some reason, the green condition can not be assumed (moisture content > 30%), soak the sample in water before volume measurement. Please note that the maximum volume is achieved at fiber saturation point, at about 30% moisture, and more water will not make the sample bigger. Dense woods generally shrink and swell more, and usually present greater problems in lumber drying. The densest woods also make the best fuel wood. In British Columbia the average density of softwoods ranges from 0.33 (western redcedar and subalpine fir) to 0.55 (western larch). Procedure: We will determine wood density by three methods. First, by flotation, which will be an estimate of actual density. This experiment will be carried out in the air-dry (as is) condition. Second, by determining the volume of wood by the difference in the water level before and after immersion. Third, by measuring the dimensions with a caliper or a ruler to calculate wood volume. Please note: the last two methods of volume determination shall be carried out in the wet, fully swollen condition. Therefore, samples should be soaked in water over-night, by holding them under water with a heavy weight. 1) Flotation Method. Take a prismatic wood sample (1x2 cm across, and 20-30 cm long) and mark its length in 10 equal parts. Place sample length-wise very gently into a graduated cylinder of water, filled approximately 3/4 full (Figure 1). Read and mark water-immersion line. The relationship of the length immersed to the total length is an approximate measure of density. 2) Determining volume by water immersion. Fill graduated cylinder with water to about half to 3/4 level. Read and record water level. Place wet wood sample into the graduated cylinder, and force the sample below the water-line with the tip of a pencil Figure 2). Record the water level. The difference in water levels in milliliters is the volume of the wood sample in cubic centimeters. 3) Determining volume by measuring dimensions. For a sample that is regular in shape, such as our prismatic strips of wood, the simplest method is to measure the dimensions as accurately as possible, and calculate the volume: Volume (cm3) = thickness (cm) x width (cm) x length (cm) Calculating basic relative density: After the above measurements are complete, place wood samples in the oven and dry until no more weight loss is evident (usually overnight). Drying shall be done in an oven maintained at 103 ± 2° C. Basic relative density = oven-dry weight in grams / green volume in cm3 TB 5 and 6