Discoloration of wood and its types
The discoloration of wood, in simple terms, is the color change on its surface due to the effects of the environment (sunlight, oxygen, water, temperature) and microorganisms (fungi).
Logs, sawn timber, and wood products may all be discolored.
After the trees are felled, the ends of the logs and under the intact bark are prone to discoloration. After logs are processed, sawn timber (board, square timber) is also prone to blue stain, brown stain, and mildew during storage and processing. After the wood is made into wood products, it may still be discolored during use.
There are many types of wood color changes, from the natural color of the wood (white/light yellow/beige, etc.) to pink, red, blue, green, gray, dark gray, brown, taupe, dark brown, black, etc.
The discoloration of wood can be divided into two categories due to different causes. One is chemical discoloration, including tannin discoloration and oxidative discoloration; the other is fungal discoloration, including mildew and blue discoloration (also known as blue discoloration and edge discoloration). Material discoloration). Among them, fungal discoloration is more common and the impact is more serious.
Generally speaking, wood discoloration refers to fungal discoloration.
02 Chemical discoloration of wood
The wood of many tree species will change color when its moisture content is high or exposed to humid air for a long time. It is not caused by fungus infecting the wood, but caused by the chemical reaction of certain components in the wood. Is called chemical discoloration.
Oxidative condensation reaction of tannins, pigments, alkaloids, sugars, phenols and other organic substances in wood, the most important of which is the oxidation of phenolic substances in wood.
Phenolic compounds have a benzene ring structure and are easily oxidized, which is the cause of chemical discoloration. Phenolic compounds are colorless before oxidation, and some are soluble in water; after oxidation, they form water-insoluble condensates, which are red, reddish-brown, and brown in color, so chemical discoloration is also called oxidative discoloration.
Some wood contains tannins, also known as plant matter, which is a mixture of polyphenols. When it comes in contact with iron under humid conditions, the tannins in it will react chemically with iron (complexation reaction) to form iron tannins. . Iron tannin is black and is the main raw material used to make ink, which makes the color of wood black. Depending on the amount of iron and the length of time the wood is in contact with iron, the color of the wood changes from light gray to blue-black. Similarly, immersing wood in water with a high iron content can cause this chemical discoloration of wood.
In addition, when wood is in contact with copper or copper alloys, the tannins in the wood react with copper to produce copper tannins, which will also discolor the wood (light red).
During the drying process of wood, chemical discoloration often occurs. This is mainly due to the slow drying speed of the wood, especially the parts in contact with the skid. The characteristic of chemical discoloration is that the depth of discoloration is shallow, and the discoloration is relatively uniform.
03 mildew of wood
Moldy wood will discolor the surface and sapwood, but the range of moldy discoloration is lighter, and the discoloration is caused by colored spores. Because the mold spores only multiply and grow on the surface of the wood, the moldy of the wood is limited to the surface of the wood or a very shallow layer close to the surface.
Mold often makes the wood green, white, black, and occasionally other colors. The discoloration caused by mold is often flocculent or spotted. In a warm and humid climate, or in a poorly ventilated environment, mold spores deposited on the wood surface are prone to multiply and mold.
Wood mildew fungi include Trichoderma (Tirhcodermasp.), Penicillium (Penieilliumspp.), Aspergillus (AspergillussPp.), Mucor (Mucor) and the like.
The most important fungus of the genus Trichoderma is Trichoderma viride, and the surface of wood infected by this fungus is green. There are many kinds of fungi of Penicillium and Aspergillus, the most common one is Aspergillus niger. After wood is infected with this fungus, the surface shows black spots, sometimes connected into pieces. The adaptability and endurance of molds to the environment and substrates are stronger than that of blue mutants and decaying bacteria. The molds are also resistant to chemical drugs, and they can grow even when they come into contact with certain toxic chemical drugs. Mildew can still be found on some wood after preservative treatment.
As the result of wood mildew is only the discoloration of the wood surface, and the range of discoloration is relatively shallow, it can be removed with a brush or by planing off the surface layer.
Mildew has little effect on the quality of the wood itself, so it is usually not considered a defect.
However, after mold infects the wood, it can increase the permeability of the liquid to the wood, which can promote the formation of blue stain.
04 Blue change of wood
Wood blue stain usually refers to all sapwood discoloration that occurs in wood, and blue stain is a general term for wood sapwood discoloration. In addition to changing to blue, it also includes changes in other colors, such as black, pink, and green.
The fungi that cause wood blue change include Botryo-diplodia Sp., Ceratoeystissp., iDploda Sp.), etc., and the most serious damage to rubber wood is Cocoa diplodia Sp. tryodiplodiatheobromaepat,).
4.1 The characteristics of blue change
Blue change can occur in both softwood and hardwood, but usually only occurs in sapwood.
Under suitable conditions, blue change occurs more often on the surface of sawn timber and the ends of logs. If the conditions are right, blue stain bacteria can penetrate from the surface of the wood into the interior of the wood, causing deep discoloration. Light-colored wood is more susceptible to blue stains, such as rubber wood, red pine, masson pine, willow, maple and so on.
The blue stain will not cause the loss of the wood structure (it does not affect the strength of the wood), but the finished product made of blue stain wood is difficult for customers to accept.
4.2 Reasons why wood is easy to blue
The discoloration of wood is caused by the reproduction and growth of discoloring fungi on the wood.
The discoloration of wood is affected by the following factors:
1) 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 immediately dried to a moisture content below 20%, and the moisture content is always kept below 20% during processing and use, blue staining can be prevented.
2) Nutrients (nutrients): The most suitable nutrients required by various wood fungi are different, but all fungi can get the nutrients they need from wood. The carbohydrates in wood, namely starch and monosaccharides, are the energy needed for the growth of blue stain bacteria.
In addition, trace substances (inorganic salts, nitrogen compounds, etc.) in wood are also necessary for fungal growth, but the amount is very small. As the starch and monosaccharide content in rubber wood is much higher than other woods, rubber wood is more prone to blue stain than other woods.
3) Air: Most fungi are aerobic bacteria, which can only grow in the presence of oxygen. However, they require very little oxygen. When the oxygen content in the air reaches 1%, fungi can grow. Therefore, it is unrealistic to control the growth of blue stain bacteria by isolating air (oxygen).
4) Temperature: Wood microorganisms can only grow within a certain temperature range, and have their most suitable growth temperature, maximum growth temperature and maximum growth temperature.
The most suitable temperature for fungus growth is 20-30°C, and the temperature is lower than 10°C and higher than 35°C, and the growth rate of fungi is slow. Low temperature (cold) cannot kill the fungus, but can only inhibit it and make it dormant. High temperature (such as kiln drying, high temperature sterilization) can kill fungi in wood.
4.3 Harm of blue change
1) Blue-stained wood is more perishable
Usually, the wood undergoes blue stain first, and then decays. Sometimes you may only see the obvious decay defects formed in the late blue transformation period. It can also be said that discoloration is a sign of decay.
2) Discoloration increases the permeability of wood
Due to the penetration of blue stain fungus hyphae, many small holes are formed, which increases the permeability of wood. After drying, the hygroscopicity of the blue-stained wood increases, and the decaying bacteria are easy to grow and multiply after absorbing moisture.
3) If it is not prevented, the hyphae of blue change bacteria can penetrate deeply
To the depths of the wood, internal discoloration is formed. The internal discoloration of wood is due to the rapid drying of the surface of the wood infected with the discoloration fungus, and the dry wood surface does not have enough water to provide the fungus, so that the fungus develops into colored hyphae.
In this way, the fungus on the wood surface is colorless (has not yet developed into colored hyphae), so no discoloration can be seen on the wood surface. However, the dry wood surface cannot prevent the mycelium from spreading into the wood. If the interior of the wood is damp, then the mycelium will continue to multiply and develop inside the wood, which will become colored hyphae, causing discoloration inside the wood.
4) Reduce the value of wood
Because of the discoloration, the appearance of the wood is not good, and users often refuse to accept this discolored wood or wood products, especially wood used as decorative wood, furniture, and other areas where the appearance of wood is more important, or require a price reduction, so Commercially, preventing wood discoloration is an important aspect of maintaining the value of wood products.
4.4 Prevention and control of blue change
1) After felling, the logs should be processed as soon as possible, the sooner the better.
Because discoloration bacteria and molds harm fresh logs, fresh sawn timber, undried and semi-dried boards, logs after harvesting should be sawn and processed as soon as possible to reduce the possibility of biological factors invading the ends of the logs (fresh surfaces).
2) The processed wood should be dried as soon as possible.
After the logs are sawn and processed into sawn timber, the exposed fresh wood surface increases, and its moisture content is suitable for the reproduction and growth of discoloration bacteria. Therefore, the moisture content of the wood should be reduced to less than 20 rings as soon as possible, that is, the wood should be dried as soon as possible .
3) Treat timber (logs, sawn timber) with anti-tarnishing agents in time.
If the felled logs cannot be sawn and processed in time, and they are tree species that are prone to blue change (such as rubber wood, certain pine wood, maple, etc.), they need to be treated with blue stain prevention.
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 used in time to prevent blue
Change agent treatment.
If the wood has been discolored, the anti-blue discoloration agent cannot remove the discoloration. However, anti-blue staining agents can prevent the colonization (colonization) and growth of fungi on undiscolored wood.
05Anti-blue change agent/anti-draining agent
Sodium pentachlorophenol (NaPCP) has been used in my country to control (prevent) blue stain and mildew. Polyphenols such as tetrachlorophenol, pentachlorophenol and sodium pentachlorophenate have been used in wood protection for more than 50 years, and they are very effective in inhibiting wood blue stain, mildew and decay. However, since the discovery of carcinogenic idoixn compounds in phenolic compounds, chlorinated phenolic compounds have gradually been banned in more and more countries.
