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2.6.1. Discolourations Discolourations can occur in the standing tree, at the time of storing wood, or while drying. Discolourations due to drying can be caused by fungi, physiological reactions in living cells, biochemical (enzymatic) reactions, chemical reactions, or a combination of reactions (Bauch et al., 1991). They are also dependent on tree species and kind of wood: heartwood, sapwood, and wetwood (Ward & Simpson, 1997). 2.6.1.1. Brown and Blue Stain One common oxidative discolouration is brown stain. In this case, it is the oxidation of extractives that causes the colour change (Amburgey, 1994). Kiln drying in a vacuum (i.e., without the presence of oxygen) prevents brown stain from appearing. Schmidt et al. (2002) report that use of chemicals or fungicides could successfully control this discolouration. Furthermore, pre-steaming treatments can be beneficial (Avramidis et al., 1993). Another frequently appearing discolouration is blue stain (see Figure 2). In this case, starches and sugars found in the sapwood are consumed by fungi or bacteria. Although those fungi reduce wood toughness, they do not rot the sapwood and only slightly reduce wood strength (Ward & Simpson, 1997). During drying, blue stain can occur on the boards' surface if temperature is low and relative humidity is high. Controlling temperature and relative humidity can prevent blue stain. Furthermore, chemical substances that kill the fungus can be used. 18PDF Image | KILN DRYING PROBLEMS AND ISSUES IN CANADA
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