Wood Knowledge丨Why does wood change color

Discoloration of wood and its types

The discoloration of wood is the color change on the surface due to the action of the environment (sunlight, oxygen, moisture, temperature) and microorganisms (fungi).

Discoloration can occur on logs, boards, and wood products.

After trees are felled, discoloration is likely to occur at the ends of the logs and under the imperfect bark. After the log is processed, the slabs or square timbers will be also prone to blue, brown, mildew during storage or processing. After wood is made into wood products, discoloration may also occur during use.

There are many types of wood color changes, such as pink, red, blue, green, gray, dark gray, brown, taupe, dark brown, black, etc.

The discoloration of wood can be divided into two categories according to different reasons. The first reason is chemical discoloration, including tannin discoloration and oxidative discoloration. The second reason is fungal discoloration, including mildew and blue stain (also known as blue stain, sapwood discoloration). Among them, fungal discoloration is more common and the impact is more serious.

Generally speaking of wood discoloration is fungal discoloration.

Chemical discoloration of wood

Many wood species will discolor, when these have high MC or prolonged exposure to moist air, That is not caused by fungal discoloration,cause some chemical reaction of certain components in the wood ,which is called chemical discoloration.

Chemical discoloration also often occurs during wood drying. This is mainly due to the slow drying speed, especially in the areas in contact with the skids. The characteristics of chemical discoloration are that the discoloration depth is shallow and the discoloration is relatively uniform.

Mildew of wood

Mildew will discolor the surface and edge of the wood, but the discoloration of mildew is lighter, this kind of discoloration is occurred by colored spores. Due to the mold spores only multiply and grow on the surface of the wood, this type discoloration only is in the wood surface or most very shallow layer near the surface.

Mildew always make wood to occur green, white, black, or occasionally other colors. Discoloration caused by mold is often flocculent or speckled. In warm and humid climates, or in poorly ventilated environments, mold spores deposited on wood surfaces can easily multiply and grow mold.

Wood mildew fungi include Trichoderma (Tirhcodermasp.), Penicillium (Penieilliumspp.), Aspergillus (AspergillussPp.), Mucor and so on.

The most important in trichoderma fungi is trichoderma viride, the wood surface will render green color ater infected by this fuguns. There are many species of Penicillium and Aspergillus, Aspergillus niger is the most common one. After wood is infected with this mold, the surface appears black spots, sometimes connected into pieces areas. The adaptability and tolerance of mold to the environment and substrates are stronger than that of blue-mutant bacteria and decay bacteria. Mildew can still be found on some good preserved wood.

Due to the result of wood mildew is only to discolor the wood surface, and the range of discoloration is shallow, this can be removed with a brush or by planing off the surface layer.

Mildew has little effect on the wood quality, therefore, this is generally not considered as defect.

However, after the mold infects on the wood, it can increase the permeability of the liquid to the wood, which can promote the formation of blue stain.

Blue-stain of wood

Wood blue-stain usually refers to the discoloration of all sapwood in wood, and blue stain is a general term for the discoloration of wood sapwood. Besides blue discoloration, it can also change to other colors, such as black, pink, green, white etc.

Characteristics of blue stain

Blue stain can occur in both soft and hard wood, but usually only occurs in sapwood. Under suitable conditions, blue stain mostly occurs on the surface of sawn timber and the ends of logs. Under the suitable conditions, cyanobacteria can penetrate from the surface of the wood into the interior of the wood, resulting to deep discoloration. Light-colored woods are more susceptible to cyanobacteria, such as oak, red pine, masson pine, willow, maple, etc. Blue stain will not cause loss of wood structure (does not affect wood strength), but finished products made of blue stained wood are difficult for customers to accept.

Reasons why wood is prone to blue strain

The discoloration of wood is caused by the reproduction and growth of discolored fungi on the wood. The discoloration of wood is affected by:

Moisture: Only when the moisture content of wood is higher than 20%, microorganisms such as discoloration fungi can reproduce and grow. Therefore, if the harvested wood can be dried immediately to a moisture content of less than 20%, and the MC keep below 20% during processing and using, bluestaining can be prevented.
Nutrients: The most suitable nutrients required by various wood fungi are different, but all fungi can obtain the nutrients they need from wood. Carbohydrates in wood, namely starch and monosaccharides, are the energy required for the growth of cyanobacteria. In additions, trace substances in wood (inorganic salts, nitrogen compounds, etc.) are also necessary for fungal growth, but the required amount is very small. Because the starch and monosaccharide content of rubberwood is much higher than other woods, rubberwood is more prone to bluestain than other woods.
Air: Most fungi are aerobic bacteria and must grow in the presence of oxygen. But the amount of oxygen they need is very small. Even the oxygen content in the air reaches to 1%, the fungi can still grow. Therefore, it is unrealistic to control the growth of cyanobacteria by isolating the air (oxygen).
Temperature: Wood microorganisms can only grow within a certain temperature range, and have their most suitable growth temperature, maximum growth temperature and minimum growth temperature. The most suitable temperature for fungal growth is 20-30℃, the temperature is lower than 10℃ and higher than 35℃, the growth rate of fungi is slow. Low temperature (cold) cannot kill the fungus, it can only inhibit and make it dormant. High temperatures (eg kiln drying, high temperature sterilization)can kill fungi in wood.

The harm of wood blue-stain

Blue-stained wood is more susceptible to decay. Usually, the wood is blue-stained first, and then decayed. In some cases, only obvious decay defects formed at a later stage of the bluestain may be seen. It can also be said that blue-stain is a harbinger of decay.
The blue-stain increases the permeability of wood. Due to the penetration of blue fungal mycelium, many small pores are formed, which increases the permeability of wood. After drying, the hygroscopicity of blue-stained wood increases, and decay fungi are easy to grow and reproduce after absorbing moisture.
If it is not prevented, the mycelium of cyanobacteria can penetrate deep into the wood and form internal discoloration. The internal discoloration of wood is due to the rapid drying of the wood surface infected with the discoloration fungus, and the dry wood surface does not provide enough moisture to the fungus, allowing the fungus to develop into colored mycelium. In this way, the fungi on the wood surface are colorless (have not developed into colored mycelium), so no discoloration can be seen on the wood surface. However, the dry wood surface did not prevent the mycelium from spreading into the wood. If the interior of the wood is damp, the mycelium will continue to multiply and develop inside the wood, thereby becoming colored mycelium, causing the interior of the wood to discolor.
Reduce the value of wood due to discoloration, the appearance of the wood is not good-looking, and users often refuse to accept such discolored wood or wood products, especially as decorative wood, furniture and other wood used in areas where the appearance of wood is more important, Or ask for a price reduction, so in business, preventing wood discoloration is an important aspect of maintaining the value of wood products.

Prevention and Control of Blue-Stain

After felling, the logs should be processed as soon as possible, the sooner the better. Because discoloration bacteria and mold damage fresh logs, fresh sawn timber, undried and semi-dried boards, logs after harvesting should be sawn as soon as possible to reduce the possibility of biological agents attacking the log ends (fresh surfaces).
The processed wood should be dried as soon as possible. After the log is sawn and processed into boards, the exposed fresh wood surface increases, and its moisture content is suitable for the reproduction and growth of discoloration bacteria. Therefore, the MC of the wood should be reduced to less than 20% as soon as possible.
Treat wood (logs, sawn timber) with anti-discoloration agents in time. If the logs after felling cannot be sawn in time, and these are tree species that are prone to blue staining (such as rubber wood, some pine, maple, etc.), anti-blue staining treatment is required. If the sawn timber after sawn logs cannot be kiln dried in time, or air dried (natural drying, air-drying) before kiln drying, it should be treated with anti-blue-staining agent in time. If the wood is already discolored, anti-bluestaining agents cannot remove the discoloration. However, anti-blue stain agents can prevent the colonization (colony formation) and growth of fungi on unstained wood.