Therefore, those species or specimens that exhibit wider bands of earlywood (or diffuse-porous species that exhibit no apparent latewood) will show more weight differential from green to dry than will those with significant bands of latewood. As growth slows, the later cells formed take on a denser form with thicker cell walls and smaller cell lumina. Earlywood cells, formed in the fast-growing early weeks of growth when moisture movement is at its maximum, are necessarily larger with thinner cell walls to allow higher flow volume. For the species in the table, the hardwood heartwood averages 81% moisture content, while the hardwood sapwood averaged 83% the softwood heartwood, however, averages only 60%, while the softwood sapwood averaged 152%!Ī similar situation exists between earlywood and latewood. Table 4-1 of the 2010 Wood Handbook 2 gives the heartwood and sapwood moisture contents of 40 North American hardwoods, and 28 North American softwoods.
Hardwoods vary by species, and on average the moisture content in sapwood is only slightly higher. For softwood, this is practically always true. This leads us also to the understanding that sapwood, with its higher moisture content in the field, often weighs more than heartwood. We see in this example that an increase in moisture content results in the increased weight of green wood. Then its original moisture content would have been (100-40)/40, or 150%.
Now, suppose that original sample had weighed 100 grams. So, to use a simple example, if a sample block of wood weighs 50 grams at original weighing, and 40 grams after being dried to 0% moisture content, then the moisture content of the original sample was (50-40)/40, or 25%. The equation used for calculations is quite simple: This is accomplished by drying the wood to a constant weight in a laboratory oven held at 101 to 105 degrees centigrade. The moisture content calculation is simply a comparison of the mass of a sample of wood at any given moisture to its mass when "oven-dry", or when all the water has been removed from the sample. How, they wonder, can wood have more than 100% moisture? Let's talk about green wood in more detail.įirst, that crazy moisture content calculation that confounds so many beginning wood science students. Now, green wood can have moisture content anywhere from 30% (denser woods) to over 200% (lighter woods). The most commonly referenced "heaviest wood", lignum vitae ( Guaiacum officinale), has a specific gravity of 1.05 when green, which makes its weight about 1365 kg/m 3, or 85 pounds per green cubic foot. And like a sponge, most woods float, and therefore have specific gravities less than 1.0 a few of the denser tropical hardwoods are actually heavier than water and sink. Actual woods, the amazing composite of cell matrix in infinitely different shapes, sizes, and arrangements, much like a sponge made of lignocellulose, have much lower specific gravity than the theoretical maximum due to the amount of space in the matrix filled with air and water. Wood of course, is comprised of cells, of which only the cell walls have the specific gravity stated above. Thus far, we have been talking only of solid wood substance, which is not really wood as we know it.
Together, the amount of ash and extractives in wood can vary from trace amounts to 30% and therefore affect the weight of wood differently according to species. Ash, which makes up 0.5% to 2.0% of most woods, has a specific gravity of 1.6 to 2.8 the specific gravity of extractives varies depending on the substance. These extractives and infiltrates impregnate the lignocellulosic matrix and fill parts of the cavities of the wood. In addition, inorganic compounds such as silicates, carbonates, and phosphates appear in the wood as "infiltrates" and result in ash as the wood substance is decomposed. Wood also contains measurable quantities of organic "extractives" such as such as terpenes, resins, and polyphenols such as tannins, sugars, and oils. With the specific gravity around 1.5, solid wood "substance", or lignocellulose as it is commonly called today, weighs around 1500 kg/m 3 (93.6 lb/ft 3), at theoretical most…no air, water, or other fluids in cell pores, which would decrease the weight of the wood per unit of volume. The nice thing about using water as the reference measure was that early scientists could easily classify materials by whether they float on water (specific gravity less than 1.0) or whether they sink (specific gravity greater than 1.0). Since water conveniently weighs about 1 g/cm 3, variable with temperature, specific gravity was derived as an index metric to state the weight of other substances relative to water.