Blue discoloration 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. In addition to changing to blue, it also includes other color changes, such as black, pink, green, etc.
The fungi that cause blue-staining of wood include Cymbidium spp., Ceratosporium spp. and Chromium spp., etc. The most serious damage to rubber wood is Cocoa spp.
1 Characteristics of blue change
Blue stain occurs in both softwood and hardwood, but usually occurs only in sapwood.
Under suitable conditions, blue staining mostly occurs on the surface of sawn timber and the ends of logs. Under the right conditions, cyanobacteria can penetrate from the surface of the wood into the interior of the wood, resulting in deep discoloration. Light-colored woods are more susceptible to cyanobacteria, such as rubber wood, Korean pine, masson pine, willow tree, 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.
2 Reasons why wood is prone to blue discoloration
The discoloration of wood is caused by the reproduction and growth of discoloration fungi on the wood. The discoloration of wood is affected by:
(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 dried immediately to a moisture content of less than 20%, and the moisture content is always kept below 20% during processing and use, bluestaining can be prevented.
(2) Nutrients (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 addition, trace substances in wood (inorganic salts, nitrogen compounds, etc.) are also necessary for fungal growth, but in very small amounts. Because the starch and monosaccharide content of rubberwood is much higher than other woods, rubberwood is more prone to bluestain than other woods.
(3) Air: Most fungi are aerobic bacteria and must grow in the presence of oxygen. But the amount of oxygen they need is very small. When the oxygen content in the air reaches 1%, the fungi can grow. Therefore, it is unrealistic to control the growth of cyanobacteria by isolating the air (oxygen).
(4) Temperature: Wood microorganisms can only grow within a certain temperature range, and have their optimum growth temperature, maximum growth temperature and minimum growth temperature.
The optimum temperature for fungal growth is 20-30°C, and the growth rate of fungi is slower when the temperature is lower than 10°C and higher than 35°C. Low temperature (cold) cannot kill the fungus, it can only inhibit and make it dormant. High temperatures (eg kiln drying, high temperature sterilization) kill fungi in wood.
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