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Glossary of Terms


    A unit of concentration for a solution prepared by dissolving one liquid in another liquid, e.g. 20% v/v is equivalent to 20 mL in a total of 100 mL or 20 L in a total of 100 L.


    A unit of concentration for a solution prepared by dissolving a solid in a liquid, e.g. 20% w/v is equivalent to 20 g in a total of 100 mL or 20 kg in a total of 100L.

Actual Wood Density (Rg)

    The mass of degraded wood per unit volume such as g/cm3; this value can be calculated by PEGcon.

Anti-Shrink Efficiency (ASE)

    An indicator for comparing the effectiveness of different treatments for waterlogged wood independent of wood species or amount of deterioration. To calculate the ASE: first air-dry a sample of the piece of wood that is to be treated and measure the shrinkage; then treat the rest of the piece of wood and measure its shrinkage; then compare the difference between the untreated and treated wood using the formula:

    ASE (%) = (Sa - S) / Sa * 100 where:
    Sa = air-dried untreated wood shrinkage (%) and
    S = treated wood shrinkage (%).

    An ASE of 100% indicates that the treatment caused no shrinkage. The ASE can be calculated in either the radial or tangential direction; longitudinal shrinkage is usually very small and can be ignored.

Botanical Name

    A unique Latin name for a tree; it usually includes the genus and the species, and may also indicate the name of the researcher who classified the tree.

Bound Water

    Water that is found within the cell walls and is associated with the cell wall structure; it is the general term for all water in the cell wall and includes water that is chemisorbed and that which is condensed within capillaries. It is difficult to remove completely.

Capillary Tension

    The large compression forces exerted on the cell walls as water evaporates from the capillary-like wood cells; these forces result from the surface tension of water.

Cell Lumen

    The cavity inside a wood cell; when wood is waterlogged this space is filled with water.

Cell Wall

    The multilayered membrane, composed primarily of cellulose, hemicellulose, and lignin, that encloses the cell lumen.

Cell Wall Bulking

    Used here in the same manner as "bulk" is employed in the wood-technology literature, to refer only to the filling of the cell wall and not the filling of lumens.

Common Name

    The term by which a wood species is generally called. Nearly all wood species have a common name; many have several.

Compression Wood

    Wood that is formed on the underside of branches and curved stems as a result of the tree's resistance to gravity. It is different from normal wood, e.g. it has high longitudinal shrinkage.

Eutectic Mixture

    The composition of a mixture at which the melting point is lowest; with PEG and water this composition is approximately 55% (range is 52-70% depending on the molecular weight of the PEG). The PEG literature states that at temperatures below -25°C, PEG/water mixtures supercool and have no definite freezing point.

Fibre Saturation Point (FSP)

    The moisture content at which all cell lumens are dry and the cell walls are fully saturated. The cell walls will be fully expanded at FSP, and removal of water will result in cell wall shrinkage.


    The material from wood that remains after the lignin and solvent-extractable substances are removed; this carbohydrate fraction includes the alpha cellulose and the hemicellulose.

Juvenile Wood

    Wood that is formed near the pith (centre) of deciduous trees and results in a core of varying size; there may be differences in cell size and arrangement and in the density.

Libriform Fibre

    A particular cell found in deciduous trees; it is elongated with thick walls and simple pits.


    A major organic component of woody cell walls; it is a non-carbohydrate with a complex macromolecular structure that varies from species to species, and is concentrated in the regions between adjacent cells. There is mounting evidence that it has antioxidant properties.

Maximum Water Content (µmax)

    The maximum amount of water that can be contained in the combined spaces in the cell wall, the lumen, and the intercellular spaces; it is generally based on the oven-dry weight of the wood and is calculated by the formula:

    µmax (%) = (weight waterlogged - weight oven-dried) / weight oven-dried * 100

Moisture Content (MC)

    The weight of the moisture in wood expressed as a percentage of its oven-dry weight.

Molecular Weight

    The sum of the atomic weights of all of the atoms in a molecule; molecular weights for commercially available PEGs generally range from 200 to 20 000.

Normal Wood Density

    The mass of undeteriorated wood per unit volume such as g/cm3; it is specific for each species of wood. Use of the word "normal" indicates that this is the density of undeteriorated wood. The numerical value is the same as the specific gravity of the wood, and is calculated by oven-drying the wood.

Osmotic Collapse
    An undesirable reaction of dense or undeteriorated wood to immersion in concentrated PEG solutions. When there is a large difference between the amount of PEG in the cell and the amount in the solution, the cells in the wood collapse and the surface of the wood is pushed inward. For example, when a piece of sound oak is removed from water and placed in a 40% PEG 3350 solution it will collapse. The PEG cannot easily get into the cells and exerts a pressure on the cell walls that can be high enough to collapse them. If enough cells collapse the exterior of the wood will appear concave.

PEG (polyethylene glycol)

    Polyethylene glycol, polyoxyethylene, or poly(ethylene oxide) is a water-soluble wax that replaces the water in the wood and supports the cell structure during drying, which helps the object to retain its shape and size. It has several trade names depending on which company sells it.

PEG Increments

    When PEG is used to treat waterlogged wood the concentration of the solution is usually increased in a stepwise fashion. This is a generally recognized precaution to reduce the risk of osmotic collapse of the wood. The question then arises as to what the concentration of each increment should be. Generally PEG 400 is added in steps of 10-20% and PEG 3350 is added in steps of 5-10%. The concentration of each increment is affected by several factors:
    • The size of the objects - the larger the artifact the smaller the increment. Ship's timbers are considered to be large objects and barrel staves are small ones.
    • The extent of the wood deterioration - more deterioration allows the use of higher concentrations in each increment. Actual densities that approach 1/4 of the normal density indicate deterioration while actual densities that are close to the normal density suggest little deterioration.
    • The speed at which the treatment must be carried out - shorter treatment times require a higher concentration in each increment.
    • The normal density of the wood species - less dense (<0.4 g/cm3) species can have higher concentrations of PEG in each increment while more dense (>0.6 g/cm3) species require lower concentrations.

Second-order Space:

    The volume of the micro capillaries in the cell wall; this term is preferred over "cell wall volume" which could include the voids caused by deterioration.

Secondary Cell Wall

    The location of the internal thick layers of the cell wall; it is formed after the cell has enlarged to its normal size after cell division, and consists mainly of cellulose.


    The change in dimension (as a % of the swollen state of the wood) that occurs during drying. It may be in a radial, tangential, or longitudinal direction or be volumetric.