The discoloration of wood, simply put, is the color change that occurs on its surface due to the effects of the environment (sunlight, oxygen, moisture, temperature) and microorganisms (fungi). Discoloration may occur on sawn logs, wood products, etc. After trees are felled, discoloration is likely to occur at the ends of the logs and under the imperfect bark. After the original technique is processed, the sawn timber (plate, square timber) is also prone to bluing, browning, mildew, etc. during storage and processing. After wood is made into wood products, discoloration may still occur during use. There are many types of wood color changes, from the natural wood color (white/yellow/light brown, etc.) to pink, red, blue, green, gray, dark gray, brown, taupe, dark brown.
The discoloration of wood can be divided into two categories due to different reasons. One is chemical discoloration, including tannin discoloration and oxidative discoloration; material discoloration). Among them, fungal discoloration is more common and the impact is more severe. Generally speaking, wood discoloration refers to fungal discoloration.
chemical discoloration of wood
The discoloration of wood from many tree species when it has a high moisture content or is exposed to humid air for an extended period of time is not caused by fungal infestation of the wood, but by a chemical reaction of certain components in the wood , called chemical discoloration.
Oxidative condensation reactions 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, water-insoluble condensates are formed, and the color is red, reddish-brown, and brown, so chemical discoloration is also called oxidative discoloration.
Some woods contain tannins, also known as vegetable tannins, which are mixtures of polyphenols that chemically react (complex reaction) with iron to form iron tannins when exposed to iron in wet conditions . Iron tannin is black and is the main raw material used to make ink, thus darkening the color of the wood. Depending on the amount of iron and the length of time the wood has been in contact with the iron, the color of the wood changes from light gray to bluish black. Likewise, this chemical discoloration of wood can also occur when wood is soaked in water that is high in iron.
In addition, the wood is in contact with copper or copper and gold. It also discolors the wood (light red) due to the chemical reaction between the tannins in the wood and the copper to form copper tannins. Chemical discoloration also often occurs during wood drying. This is mainly due to the slow drying rate of the wood, especially in the areas in contact with the dunnage. The characteristics of chemical discoloration are that the discoloration depth is shallow and the discoloration is relatively uniform.
mildew of wood
Wood mold can discolor the surface and sapwood of the wood, but the discoloration of mold is more shallow, and the discoloration is caused by colored spores. Since mold spores only multiply on the surface of the wood, wood mold is limited to the surface of the wood or a very shallow layer near the surface.
Mold often turns the wood green, white, black, and 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 are prone to multiply and grow mold.
The fungi that cause mildew in wood include Trichoderma, Penicillium, Aspergillus, and Mucor. The most important of the Trichoderma fungi is Trichoderma viride, the surface of wood infected with this fungus is green.
There are many species of Penicillium and Aspergillus fungi, the most common being Aspergillus niger. After the wood is infected with this mold, the surface appears black spots, sometimes connected into pieces.
The adaptability and tolerance of mold to the environment and substrates are stronger than that of blue-rot bacteria and decay bacteria. Mildew can still be found on some preservative treated materials.
Since the result of wood mildew is only to discolor the wood surface, and the range of discoloration is 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, but after the mold infects the wood, it can increase the permeability of the liquid to the wood, thereby promoting the formation of blue stain.