Spalting

Spalting is any form of wood coloration caused by fungi. Although primarily found in dead trees, spalting can also occur in living trees under stress or even in thriving trees. Although spalting can cause weight loss and strength loss in the wood, the unique coloration and patterns of spalted wood are sought after by woodworkers.

Spalting in hardwoods is divided into three main types: pigmentation, white rot and zone lines. Spalted wood may exhibit one or all of these types in varying degrees. Softwoods are susceptible to brown rot which degrades the wood too quickly to be used for woodworking.

Also known as sapstain, or in its most common form, bluestain, this type of spalting occurs when the darkly-pigmented fungal hyphae grow in the sapwood parenchyma of a tree. A visible color change can be seen if enough hyphae are concentrated in an area. These pigmentation fungi often colonize wood via the rays, but are not considered decay fungi due to their non-destructive use of easily available wood carbohydrates. The most common groups of pigmentation fungi are the imperfect fungi and the Ascomycetes. Mold fungi, such as Trichoderma spp., are not considered to be spalting fungi, as their hyphae do not colonize the wood internally.

While pigmentation fungi do not degrade the wood cell wall, this type of decay can lead to a reduction in toughness (amount of energy absorbed before breaking), and increased permeability. Pigmentation can occur on both hardwood and softwood, unlike other types of spalting which are more host specific.

The mottled white pockets and bleaching effect seen in spalted wood is due to white rot fungi. Primarily found on hardwoods, these fungi 'bleach' by consuming lignin, which is the slightly pigmented area of a wood cell wall. Some white rotting can also be caused by an effect similar to pigmentation, in which the white hyphae of a fungus, such as Trametes versicolor (Fr.) Pil., is so concentrated in an area that a visual effect is created.

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