The single most revealing property of wood as an indicator of wood quality is specific gravity (Timell 1986), as both pulp yield and lumber strength are determined by it. Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water; density is the ratio of a mass of a quantity of a substance to the volume of that quantity and is expressed in mass per unit substance, e.g., grams per milliliter . The terms are essentially equivalent as long as the metric system is used. Upon drying, wood shrinks and its density increases.
It is common to classify wood as either softwood or hardwood. The wood from conifers is called softwood, and the wood from dicotyledons is called hardwood. These names are a bit misleading, as hardwoods are not necessarily hard, and softwoods are not necessarily soft. The well-known balsa is actually softer than any commercial softwood. Conversely, some softwoods are harder than many hardwoods.There is a strong relationship between the properties of wood and the properties of the particular tree that yielded it. The density of wood varies with species. The density of a wood correlates with its strength . For example, mahogany is a medium-dense hardwood that is excellent for fine furniture crafting, whereas balsa is light, making it useful for model building. One of the densest woods is black ironwood.
Aside from the structural polymers, i.e. cellulose, hemicellulose and lignin (lignocellulose), wood contains a large variety of non-structural constituents, composed of low molecular weight organic compounds, called extractives. These compounds are present in the extracellular space and can be extracted from the wood using different neutral solvents, such as acetone. Analogous content is present in the so-called exudate produced by trees in response to mechanical damage or after being attacked by insects or fungi. Unlike the structural constituents, the composition of extractives varies over wide ranges and depends on many factors. The amount and composition of extractives differs between tree species, various parts of the same tree, and depends on genetic factors and growth conditions, such as climate and geography.
For example, slower growing trees and higher parts of trees have higher content of extractives. Generally, the softwood is richer in extractives than the hardwood. Their concentration increases from the cambium to the pith. Barks and branches also contain extractives. Although extractives represent a small fraction of the wood content, usually less than 10%, they are extraordinarily diverse and thus characterize the chemistry of the wood species. Most extractives are secondary metabolites and some of them serve as precursors to other chemicals. Wood extractives display different activities, some of them are produced in response to wounds, and some of them participate in natural defense against insects and fungi.
These compounds contribute to various physical and chemical properties of the wood, such as wood color, fragnance, durability, acoustic properties, hygroscopicity, adhesion, and drying. Considering these impacts, wood extractives also affect the properties of pulp and paper, and importantly cause many problems in paper industry. Some extractives are surface-active substances and unavoidably affect the surface properties of paper, such as water adsorption, friction and strength. Lipophilic extractives often give rise to sticky deposits during kraft pulping and may leave spots on paper. Extractives also account for paper smell, which is important when making food contact materials.