Moisture in wood

Wood is divided, according to its botanical origin, into two kinds: Softwoods from coniferous trees and hardwoods from broadleaved trees. Structurally softwoods are generally simple in structure and lighter whereas hardwoods are generally complex in structure and harder. Softwood (like pine wood) is much lighter and easier to process than the heavy hardwood (like fruit tree wood). The density of softwoods generally ranges between 350-700 kg/m³, while hardwoods are 450-1250 kg/m³. Both consist of approximately 12 % moisture. Due to the more dense and complex structure of hardwood, the permeability is very low in comparison to softwood, thus making it more difficult to dry. The timber of living trees and freshly felled logs contains a large amount of water, which often constitutes more weight than the actual wood. Water has a significant influence on wood. Wood continually exchanges liquid and gas phase moisture (water) with its surroundings, although the rate of exchange is strongly affected by the degree wood is sealed. Dried timbers that are coated with deeply penetrating hydrophobic (water resisting) CD50 demonstrate a dramatically slowed rate of moisture exchange.

Why Do We Dry Timber?

Drying, if carried out promptly after the felling of trees, protects timber against primary decay, fungal stain and attack by certain kinds of insects. Organisms, which cause decay and stain, generally cannot thrive in timber with a moisture content below 20%. Several, though not all, insect pests can live only in green timber. Dried wood is less susceptible to decay than green wood (above 20% moisture content). Apart from the above important advantages of drying timber, the following points are also significant:

  • Dried timber is lighter, and hence the transportation and handling costs are reduced.

  • Dried timber is stronger than green timber in most strength properties.

  • Timbers for impregnation with preservatives have to be properly dried if proper penetration is to be accomplished, particularly in the case of oil-type preservatives.

  • In the field of chemical modification of wood and wood products, the material should be dried to a certain moisture content for the appropriate reactions to occur.

  • Dry wood works, machines, finishes and glues better than green timber. Paints and finishes last longer on dry timber.

  • The electrical and thermal insulation properties of wood are improved by drying.





Fibre Saturation Point

Fibre saturation point is a term used in wood mechanics and especially wood drying, to denote the point in the drying process at which only water bound in the cell walls remains – all other water, called free water, having been removed from the cell cavities. Further drying of the wood results in strengthening of the wood fibres, and is usually accompanied by shrinkage. Wood is normally dried to a point where it is in equilibrium with the atmospheric moisture content or relative humidity, and since this varies so does the equilibrium moisture content.

Equilibrium Moisture Content

Wood is a hygroscopic substance. It has the ability to take in or give off moisture in the form of vapour. The water contained in wood exerts a vapour pressure of its own, which is determined by the maximum size of the capillaries filled with water at any time. If the water vapour pressure in the ambient space is lower than the vapour pressure within wood, desorption takes place. The largest sized capillaries, which are full of water at the time, empty first. The vapour pressure within the wood falls as water is successively contained in smaller and smaller sized capillaries. A stage is eventually reached when the vapour pressure within the wood equals the vapour pressure in the ambient space above the wood, and further desorption ceases. The amount of moisture that remains in the wood at this stage is in equilibrium with the water vapour pressure in the ambient space, and is termed the equilibrium moisture content or EMC.. Because of its hygroscopicity, wood tends to reach a moisture content that is in equilibrium with the relative humidity and temperature of the surrounding air. The EMC of wood varies with the ambient relative humidity to a lesser degree with the temperature. EMC also varies very slightly with species, mechanical stress, drying history of wood, density, extractives content and the direction of absorption in which the moisture change takes place (i.e. adsorption or desorption).

Moisture Content Of Wood In Service

Wood retains its hygroscopic characteristics after it is put into use. It is then subjected to fluctuating humidity, the dominant factor in determining its EMC. These fluctuations may be more or less cyclical, such as diurnal changes or annual seasonal changes. In order to minimise the changes in wood moisture content or the movement of wooden objects in service, wood is usually dried to a moisture content that is close to the average EMC conditions to which it will be exposed. These conditions vary for interior uses compared with exterior uses in a given geographic location. The EMC is recommended to be 10-12% for the majority of Australian states, although extreme cases may be up to 15 to 18% for some places in Queensland, Northern Territory, Western Australia and Tasmania. However, the EMC may be as low as 6 to 7% in dry centrally heated houses and offices or in permanently air-conditioned buildings. The primary reason for drying wood to a moisture content equivalent to its mean EMC under use conditions is to minimise the dimensional changes (or movement) in the final product. Dried timbers that are coated with CD50 are less susceptible to dimensional changes because the deeply penetrating hydrophobic (water resisting) nature of CD50 minimises the free absorption and desorbtion of liquid and gas phase moisture.

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Shrinkage and Swelling

Shrinkage and swelling may occur in wood when the moisture content of timber is changed. Shrinkage occurs as moisture content decreases, while swelling takes place when it increases. Volume change is not equal in all directions. The greatest dimensional change occurs in a direction tangential to the growth rings. Shrinkage from the pith outwards, or radially, is usually considerably less than tangential shrinkage, while longitudinal (along the grain) shrinkage is so slight as to be usually neglected. The longitudinal shrinkage is 0.1 to 0.3%, in contrast to transverse shrinkages, which is 2-10%. Tangential shrinkage is often about twice as great as in the radial direction, although in some species it may be as much as five times as great. The shrinkage is species dependent and can be typically 5 to 10% in the tangential direction and 2 to 6% in the radial direction. Dried timbers that are coated with CD50 are less susceptible to dimensional changes because the deeply penetrating hydrophobic (water resisting) nature of CD50 minimises the free absorption and desorbtion of liquid and gas phase moisture therefore assisting with maintaining the dimensional integrity of moisture stabilised timbers.

Factors Affecting the Dried Appearance, and Dimensional Integrity of Wood

Factors that significantly affect the drying, appearance and dimensional integrity of dried timbers are:

  • The species; because of the variations in physical, mechanical and moisture transport properties between species.

  • The thickness of the timber; because the drying time is approximately proportional to thickness and, to some extent, is also influenced by the width of the timber.

  • Whether the timber boards are quarter-sawn, back-sawn or mixed-sawn; because sawing pattern influences the distortion due to tangential and radial shrinkage. This leads to warping, cupping, bowing, twisting, spring and diamonding. (see image)

  • Defects that arise due to uneven drying such as rupture of the wood tissue, checks(surface, end and internal), end splits, honey-combing, case hardening and collapse.

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Iron stain

Iron stain, is an unsightly blue–black or grey discoloration that is often incorrectly described as “mould” because of its frequently “spotty” appearance. Iron stain can occur on nearly all woods, however some timbers are particularly prone to iron stain because they contain large amounts of tannin-like extractives. The discolouration is usually caused by a chemical reaction between extractives and iron content in steel products, such as nails, screws, and other fasteners. Steel used in contact with wood must be protected from corrosion by using stainless steel or processes such as hot dip galvanising. Problems with iron contamination can come from traces of iron left on wood from cutting, grinding or slicing; cleaning the surface with steel wool, wire brushes, or iron tools; using finishes stored in rusty containers; and using previous iron-containing or iron-contaminated finishes. Iron dust from metalworking and even plant fertilizers can be sources of iron along with removal of old rusted guttering, handrail construction and contact by steel capped boots. Merely striking wood with a hammer can cause iron stain on some timbers. Urine on wood floors will also hasten the reaction with iron and wood extractives, producing the typical iron stain discolouration. Unprotected timbers that get wet on or off site prior to fixing are particularly vulnerable as the water soluble extractives are more readily mobilised to react with any iron contamination.

iron stain.jpg

Testing For Iron Stain

A simple test can be used to determine whether wood discolouration is caused by iron. Apply undiluted CD50 DEEPClean, scrubbing into the stained wood surface. If the solution removes the stain after approximately one hour, then iron is present on the wood. If the solution does not remove the stain, try applying bleach to the stained area. If the iron stain is spotty, try to view the stained wood under a 10x magnifying glass. ”Chunky” discoloration is usually a result of molten metal and looks like clinkers from a grinding operation. Stain that resembles slivers or flakes could be from steel wool. An even discolouration throughout the stain indicates that the iron was in solution when it contaminated the wood, probably in a contaminated finish or iron contaminated water.

iron stain 1.jpg

Removing Iron Stain

It is easy for iron to contaminate wood, as there are so many possible sources of iron contamination on a building site that are often not initially recognised. To remove iron discolouration scrub stained timbers with undiluted CD50 DEEPClean and leave to soak for one hour. After one hour thoroughly wash the surface with fresh water (preferably with a power washer) to remove excess CD50 DEEPClean. It is very important to rinse the CD50 DEEPClean off thoroughly because if all sources of iron are not removed or protected from corrosion, staining will occur again. In other words, treatment with CD50 DEEPClean is only a temporary solution if iron remains on or in the wood. CD50 DEEPClean  reacts with iron tannates to form a colorless complex. In time, the residual unrinsed CD50 DEEPClean /iron complex will break down, permitting the iron to react with the extractives to form a dark-coloured stain again.

Broken down film former on garage door.JPG

What about oil - is it good for timber? 

Oils have been used for years to protect timber. Nearly all work by cross-linking close to the surface to provide a barrier which is similar in its effect to the film forming products. Some oils provide nutrients that support the growth of fungi, mould and bacteria. Fungi, mould and bacteria thriving within the oil can lead to unsightly stains, rot and decay.

What sets CD50 apart from other products?

CD50 is totally unique and different from other products. Most importantly, it is not a film forming product. CD50  is an oil based product that:

  • Contains Copper Quinolinolate (Copper 8) - an extremely safe preservative which is extremely, effective on bacteria, mould and fungi.

  • CD50's unique formulation allows it to penetrate deep into the timber where it works from within

How does CD50 protect my timber?

  • CD50 penetrates deep within the timber structure, coats the fibres and prevents the moisture movement into and out of the cells.

  • This controls warping, cupping, and splitting. In other words, the structure and shape of the timber is protected. CD50 gives stability to the timber.

  • In some cases CD50 can return cupped and warped timber back to near its original shape. With sufficient applications of CD50 cupped and warped weatherboards can be renailed straight again.

  • CD50 gives protection against termites and borer

  • CD50 controls mould, fungus and bacterial growth within the timber, giving protection against rot and decay.

  • CD50 does not break down in the UV rays of sunlight. This is particularly important in countries that have extremely high UV conditions, like New Zealand.

  • There is no "film" to break down, by sunlight, weather or mechanical effects.

  • Copper 8 is recognised worldwide for its effectiveness, yet when applied to timber, has little or no toxicity to humans and mammals. It is even suitable for timber that has incidental contact with food or potable rain water. As a liquid, CD50 is rated as a non-hazardous product. It is safe for humans, mammals, and for the environment.

Cracked, split and warped

The top board has been treated with CD50. The bottom board has had nothing. Both boards were left exposed to the elements together for the same amount of time.

CD50 will not crack, peel or flake and will assist the control of warping, cupping, and splitting, thus enhancing the service life of the timber.

Weather conditions affect timber

Weather conditions affect timber. The following illustrations show the difference that CD50 can make.


This is what happens to timber without protection.


This is how the timber is protected when CD50 is applied.