Quality of drying, drying defects and preventing them

Requirements set for timber drying quality

According to Nordic grading rules, the following requirements apply to timber moisture content:

1. The maximum moisture content for all timber dimensions is 24%. A total of 93.5% of the timber batch must be within the moisture content range determined by EN 14298.
2. The methods described, for example, in the Nordic standard INSTA 141 or standard EN 14298 can be used to determine the moisture content of an individual piece or batch of timber. Any requirements that deviate from these standards must be identified in the relevant trade agreement.
3. When the moisture content is above 20%, the following phenomena must be taken into consideration:
   1. In terms of the actual dimensions, the diameter and width of the piece can be lower than what they are with 20% moisture content. The rule is roughly this: a change of 4 percentage points in moisture content is equivalent to a 1% change in the dimensions of the piece. For example, the nominal dimensions 50 x 100mm with 20% moisture content is equivalent to actual dimensions 49 x98mm with 12% moisture content.
   2. The risk of checks and deformations may increase and exceed the allowed limits if the timber is dried to a moisture content below 20%.

Requirements for timber moisture content arising from intended use

The different timber drying qualities are commonly called “shipping dry” and “carpenter dry”.

1. The moisture content of shipping-dry timber is 18–24%.
2. The moisture content of carpenter-dry timber is 8–14%.
   

Precise definitions for these moisture content requirements exist in national and EN standards and norms.

The lowest moisture contents are required by the furniture industry and cabinet, door and window manufacturers. Many applications in the carpentry industry also have special requirements for dried timber.

When wood is glued and treated in an industrial process, the glues, paints and varnish all set their own requirements on the timberdrying process. There are also regulations governing the moisture content of floor boards and inner lining boards at the installation stage.

The moisture content requirements for structural timber, external cladding boards and logs are in line with the requirements for shipping-dry timber.

Preventing drying defects

Timber drying can cause the defects described below. Ways to prevent these drying defects are also discussed.

Checks and splits

Checking is the most common defect that affects the value of timber. Checking is usually a visibly observable breaking of the wood but it can also be microscopic splitting between cells and even cell walls.

Most commonly, checks are caused by drying. Drying tension is caused by moisture gradient (distribution of moisture inside the piece of timber) when the piece dries unevenly or when wood restricts drying in the surface layers. Hardwood can also have internal checks. This can also happen with softwood during high temperature drying.

The basic reason for checking is drying that happens too fast. In addition to this, there are many other reasons such as shrinkage in different directions in a particular species, drying conditions, storage and spontaneous drying before kiln drying, sawing patterns, the proportion of heartwood and deviations in the wood structure (e.g. reaction wood).

Different types of checks and splits are end splits, heart shakes, surface checks, internal checks, knot checks and microchecks.

1. End splitting can be seen in both boards and logs when they are dried fast. End splitting is caused by the wood drying faster with the grain than against the grain. In logs especially, this is combined with anisotropic shrinkage. In anisotropic materials, the radial, tangential and longitudinal properties are different. If the air flow in the kiln over the ends of the dried pieces is fast due to poor controls, end splitting can occur, especially in thick pieces. End splits occur within the 30cm from the ends of the piece. These splits can be minimised by using appropriate air flow controls and positioning the outermost laths as close to the ends of the dried pieces as possible.
2. Heart shakes are internal splits that occur when the tree is still growing in the forest. Heart shakes are basically caused by the same reason as end splits i.e. tension due to anisotropic shrinkage where the core of the timber dries to below the fibre saturation point. The fibre saturation point is the moisture content at which the cell walls are saturated and the cell cavities have no free water.
3. Surface checks can occur in sawn timber when the surface wood dries to below the fibre saturation point. The higher the moisture content in the middle of the wood, the slower the shrinkage. The checks are caused by tensile stress between the fibres. If the stress exceeds the wood’s tensile strength, a check appears. This problem occurs at the beginning of the drying process when the moisture content in the middle of the piece of timber is still over the fibre saturation point. The appearance of surface checks depends on the moisture gradient between the surface and core of the piece.
4. Internal checks are not very common in softwood if it is dried in low temperatures. In some hardwood, such as eucalyptus, internal checks are a common drying defect. Internal checks can be prevented by ensuring a low moisture gradient between the surface and core.
5. Knot checks. A lower grade can be caused by knot checks or knots that fall off and leave a hole in the wood. There can be resin in and, especially in pines, around the hole. Knots dry fast longitudinally and shrink more than the surrounding wood material. Differences in radial and tangential shrinkage cause tensile stress in the knot, which often results in radial checks. These checks can be difficult to prevent but it is recommended to use higher temperatures to soften wood and avoid an excessively dry surface if knot checks are a problem. This will cause the resin to boil and loosen the dead knots. An alternative is to use slower drying in conditions that are as humid as possible.
6. Microchecks are tiny checks that are difficult to detect. They are often on the surface. They may become visible during planing or when the wood is painted. Even though these checks are very narrow, they can be deep. Microchecks occur in the same way as the other surface checks i.e. drying of the surface before kiln drying, too rapid drying at the beginning of the drying process or fast cooling after drying, especially in cold weather.

Case hardening

Case hardening refers to the hardening of the surface of sawn timber during drying. This phenomenon is due to fast drying, and it can be prevented by using a slower drying process and stabilising loads.

Twisting

Twisting is a deformation that can occur in a piece of timber during sawing, drying or storing.

The most common cause of twisting is aniso tropic shrinkage which occurs during or after drying. The underlying reason for this is crossgrain or variation of longitudinal shrinkage in different parts of the cross-section.

Bad stacking etc. leaves room for twisting if the material is already inclined to do so. Bad stacking can also expose timber to bending due to a load on top of the piece. Different parts of wood, such as reaction wood, shrink differently and cause bending.

Cupping

The anisotropic shrinkage of wood also causes cupping. Tangential shrinkage is roughly double radial shrinkage, which leads to curved sides. The greater the difference between radial and tangential shrinkage, the stronger the cupping after drying. Cupping is a natural reaction to drying and is not considered a drying defect.

Discolouration

Artificially dried timber stains slightly compared to fresh timber. The higher the temperature and the longer the drying time, the stronger the discolouration of light-coloured wood.

Discolouration is caused by chemical changes in the wood’s structure, especially in the heartwood of pine as resin travels towards the sap. There is great variation in the discolouration of different species due to structural changes. Discolouration can be affected, for example, by fungi such as mould and blue stain.

If timber is dried in high temperature conditions, discolouration is stronger and there can also be other changes.

In addition to these factors, another significant cause for discolouration is the exposure to sunlight of freshly cut timber.

Resin flow

Resin and other extractives can flow out of the wood during drying as organic solvents. Resin in softwood softens during drying and starts to flow inside the wood. Eventually, easily evaporating terpenes vaporise.

As a result, resin flows onto the surface of the wood through knots, resin canals and end grain, especially in resinrich pine heartwood. Once the terpenes have vaporised, resin crystallises and stiffens on the surface of the wood.