Blue-stained wood at risk of being discarded unnecessarily

ARTICLE Blue-stained wood, with its characteristic blue grain, has become increasingly common as bark beetle infestations increasingly hit forests around the world. While often rejected by Swedish consumers, it is exported to other countries for use in their construction projects.

A dark blue discolouration is not always a reason to reject a board. If blue stain fungus is the cause, the material can still have a wide range of uses.

»There’s a lack of knowledge to some extent. People think the wood is rotting and throw it away, even though blue stain doesn’t affect strength, because the fungus doesn’t attack the cell walls of the wood. But it’s important for the construction industry to think more about resource efficiency and put all viable wood to the best use possible,« says Gabriel Eriksson, technical project manager at Swedish Wood.

He believes that the Swedish construction industry needs to be better at not wasting good raw materials unnecessarily and that there are uses even for the blue-striped timber.

»We’re currently working on projects where we’re looking at the possibilities of incorporating blue-stained timber into the innermost layers of CLT elements.«

Blue-stained timber shouldn’t be used in places where the wood may be exposed to moisture, such as in exterior cladding, windows and bargeboards, because the blue stain increases the permeability of the wood when exposed to water, which means that it absorbs water more easily. From an aesthetic point of view, it should also be borne in mind that the blue stain will show through if the wood is stained or painted.

Works just like any other timber

But for internal, dry structures, such as stud walls, beams and roof trusses, bluestained timber works just like any other timber, explains Anders Svensson, Quality Manager at the Vida sawmill group:

»Used correctly, it’s not a problem, and if you have blue-stained timber in a stud wall, it has no effect on the final finish because wood panelling or plaster and wallpaper will be going on the outside, so no one will see it anyway.«

He feels the general perception of bluestained wood means that it is not used in Sweden to the extent that it could be. However, many other markets don’t see blue stain as a major problem, and Vida exports to the US, Australia and the UK, among others.

»We don’t discard anything. We sort and sell to the accepting markets, and the global market price is unaffected.«

Two types of blue stain

A distinction is commonly drawn between two types of blue stain, both caused by the blue stain fungus – sap staining and timberyard blue stain – depending on whether the staining occurs in the log or in the sawn timber. In a living and undamaged tree, the occurrence of blue stain is prevented by the limited availability of oxygen.

The blue stain fungus requires water and oxygen to grow, and the tree protects itself by keeping the bark sealed and the wood saturated with water. If, however, the tree suffers damage such that the sapwood begins to dry out and the structure opens up, the blue stain fungus can begin to get a foothold. To prevent the development of sap staining after harvesting, it is important to minimise the time between felling and the log being processed at the sawmill.

Spruce bark beetle attacks also cause blue stain. The damage they cause to the tree helps to dry out and open up the structure, allowing oxygen and moisture into the wood. The bark beetles can also spread spores and fungi to the tree, and the Swedish sawmill industry has seen an increased amount of blue-stained wood in recent years, as the insect’s distribution has widened.

Timberyard blue stain, on the other hand, can occur after the log has been sawn, if the wood is left in a wet environment before it is dried. The staining often occurs during the period of general decay, from July into October.

»As with many other things, it grows more slowly when it’s cold. During hot, humid August nights, blue stain can appear within a week, while later in the autumn when it’s a little cooler, it may take a month,« says Anders Svensson.

Increase knwoledge about the materials

Erica B loom, microbiologist and senior researcher at RISE Research Institutes of Sweden, agrees that blue-stained wood is a material that could be used more widely in Sweden:

»There are questions that need to be answered, and as the climate changes, we’ll face new challenges. I think it’s important for the wood industry and researchers to work together to increase knowledge about the materials we surround ourselves with, so we make sure that we use the right material in the right place. Blue-stained timber has the same strength as any other. It’s important to conserve natural resources, and the aim should always be to avoid discarding wood that can be used for other purposes.«

Blue stain and mould

Blue stain and mould can often – but not always – be the same species of fungus.

The availability of water is what determines whether the fungus grows as one or the other. The mould variant only grows on the surface and can take up water wherever it comes from, for example from the air or condensed on the surface.

But what determines whether it can grow into the material and take the form of blue stain is whether the wood has a sufficiently high moisture content. In other words, it is not enough for there to be moisture on the surface – for blue stain to occur, there also has to be water deeper in wood.

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