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February 2004


Working with blue-stained wood

Research organization Forintek is working with nature’s “blue tone,” evaluating blue-stained lumber manufactured from mountain pine beetle-affected timber for strength, treatability, gluing and finishing.

By Sean Goldie

As nature’s true building material, wood is both strong and aesthetically pleasing. However, by virtue of being a natural product, wood is also subject to variability—unlike the deliberate consistency of a manufactured product such as plastic. Variability in wood can be seen as the natural colour differences within a single tree, or through varying susceptibilities to biological attack by insects and disease. Insects typically utilize trees for one of two purposes: for consumption (a food item for termites) or for habitat.

The subject of habitat is of immense interest in both British Columbia and Alberta where an infestation of the mountain pine beetle has persisted for some years and, more recently, has undergone an explosion of epidemic proportions. This attack represents a significant volume of standing timber and shows no sign of slowing. As the beetle’s name would suggest, it prefers pine—primarily mature lodgepole pine, but other species, including western white pine and limber pine, are also attacked by the beetle. The beetle carries a specific blue-stain fungus which it introduces into the tree. The insect carries these spores on its body and deposits them under the protective bark, where they quickly germinate. As the fungus grows into the wood it clogs the water passages in the tree, interrupting the sap flow. Death of the tree follows and the needles turn red.

These red-coloured trees are the most visible evidence of mountain pine beetle attack. The fungus leaves a blue-coloured stain in the sapwood of the tree. This blue stain not only detracts from the aesthetic value of the wood when it is sawn, but may also diminish the commercial value of the timber. The current approach to stemming the extent of blue-stain damage to wood is to harvest the affected trees as quickly as possible and to process them into dried lumber with a moisture content (MC) of 19 per cent or less. This practice and the continued maintenance of the lumber’s low moisture level will virtually eliminate any further staining. However, the bluish hue will persist in the products.

Consumer Confusion
The large volume of affected timber has resulted in more stained lumber and processed wood products reaching the marketplace which in turn has caused consumer confusion. Although blue-stain is not a mould, consumers are concerned over the effect mould has on human health, especially in enclosed environments. Some large retailers, particularly in the US, are requesting stain-free lumber—avoiding the issue altogether. Although inaccurate, the consumer perceives blue-stain to be a mould, particularly as it appears bluish-black, and distinguishing between blue-stain and mould can be challenging for the lay person. While some may view the appearance of blue-stain negatively, others have successfully turned this phenomenon into a marketing opportunity. “Denim Pine” (www.denimpine.ca ) has taken this unique feature and is setting up a process to market a variety of blue-stained products such as flooring, paneling and furniture.

Is it Blue-Stain or Mould?
Fundamentally, the blue-stain fungus is contained within the wood cells, whereas a mould occurs mainly on the surface where favourable conditions such as acceptable moisture and temperature exist in order for mould to grow. While in fact blue-stain is commonplace, users of blue-stained lumber need the reassurance that this wood is substantially unchanged from non-stained wood. Until Forintek Canada Corp (www.forintek.ca) set out to investigate the performance characteristics of blue-stained lumber, little work had been done to answer the concerns of consumers and provide usable information to lumber manufacturers. Since 1979, Forintek has been providing the Canadian wood products industry with technical advice on manufacturing processes, research on new manufacturing practices, products and equipment, and market research and information. As Canada’s national wood products research institute, Forintek works closely with industry to provide objective and scientific advice.

Tests for Strength, Treatability, Gluing and Finishing
Anticipating the need for clarification, Forintek undertook the first comprehensive study on the properties of wood infested by beetle-transmitted blue-stain. The project examined two fundamental areas: the impact on selected lumber strength characteristics and issues arising from an anticipated increase in permeability. Past research into specific varieties of blue-stain had indicated some influence on selected strength properties, compared to non-stained wood. Some of this research, however, was dated and perhaps not valid for the beetle-transmitted fungi found in Western Canada.

Strength properties of wood are important because if the physical properties of wood are compromised, structural systems such as roof trusses and I-beams could be impacted. These systems can have highly stressed members and, whether they were assembled using glue and/or held together with mechanical connections, they must maintain their structural integrity as safety is of primary importance. Increased permeability was expected due to the manner in which the stain spreads through the sapwood. An increase in permeability could influence such activities as gluing, surface finishing and liquid absorption during preservative treatment. The study collected ordinary “mill run” lumber samples from sawmills located in British Columbia’s Interior. Samples of lodgepole pine that displayed typical blue-stain characteristics were prepared using standardized test procedures under such variables as moisture content, physical sample geometry and size, and grain orientation.

Strength Unaffected by Blue-Stain
A variety of tests were chosen to detect changes in the mechanical performance of lumber resulting from the blue-stain. Small clear samples were used to remove variables that would otherwise be introduced by the use of full-sized pieces. One test compared the relative performance of blue-stained to non-stained lumber, as it related to the holding capacity of truss connector plates loaded in tension. The results indicated a comparable truss plate grip capacity between the two groups. Further tests on Modulus of Rupture (MOR) and Modulus of Elasticity (MOE), found comparable results between both blue-stained and non-stained samples.

No Impact on Treatability
Blue-stained wood is more permeable than non-stained wood. In general, successful preservative treatment consists of two conditions being met. The first condition is that an appropriate amount of chemical is absorbed, as outlined by industry standards. The second condition is that the chemical be deposited in the appropriate location within the lumber. Researchers investigated the liquid uptake of such products as chromated copper arsenate (CCA) to understand the consequences of excess absorption. Although this preservative is being phased out for certain uses, the results seen would apply to alternative preservatives. Over-absorption not only incurs additional cost but also may not result in any additional benefits. Some tree species do not absorb liquids as well as others and are referred to as “refractory.” Lodgepole pine is a refractory species in that its heartwood does not absorb chemicals very well. Since blue-stain cultivates and spreads in the sapwood, any enhanced absorption would be restricted to this area and not within the adjoining heartwood.

Gluing Bond Strength Unaffected
With increased permeability, the question of possible side-effects on adhesive bond quality and surface finishes was investigated. Laminated wood products represent a significant value-added opportunity—one that is dependent on the ability of the component surfaces to successfully interact with adhesives and finishes. The use of adhesives depends on the product’s end use either in non-structural or structural applications. Non-structural bonds were tested with a polyvinyl acetate (PVA) adhesive formulated for radio frequency (RF) drying, while structural bonds were tested using phenol resorcinol formaldehyde (PRF). During the study of adhesive properties, the dried lodgepole pine samples were assembled into test panels having blue-stain-to-blue-stain and non-stained-to-non-stained joints. Both adhesives types were subjected to a standardized vacuum pressure delamination test to investigate any resulting open gluelines. An assessment of bond quality was then carried out using the block shear test (BST). Both tests showed the integrity of the glue joint was unaffected when blue-stained-to-blue-stained wood was glued together.

Finishing Blue-Stained Wood Adds to its Unique Appeal
The finishing test was designed to determine what single finish or combination of finishes could be applied to either complement or de-emphasize the unique colouring. Emphasis was placed on non-paint finishes suitable for residential furniture. As this is a subjective area, there were no test protocols available that could quantify the aesthetic value. A number of different stains/toners were applied, either separately or in combination, followed with a clear sealer. These finishes were then tested for surface adhesion using standardized protocols. The results showed that, blue-stained or not, both samples had comparable finish adhesion qualities. In total, thirty different surface treatments were investigated, resulting in suggested combinations of colours and finishes for either enhancing or minimizing the appearance of the blue-stain. In general, tints that successfully masked the phenomenon were combinations of reddish, bluish or charcoal tones. The appearance of blue-stained wood could be enhanced or highlighted using simple standard clear furniture finishes.

For Further Information:
For more details on mountain pine beetle or beetle-transmitted blue-stain, or for additional technical information, please contact a Forintek Industry Advisor (e-mail: lumber@van.forintek.ca ) or visit Forintek online: http://www.forintek.ca , http://www.durable-wood.com/publications/index.php  (select the publication entitled “Properties of Lumber with Beetle-Transmitted Blue-Stain”).

Sean Goldie is a Vancouver-based consultant in the forest sector. He may be contacted by tel/fax: (604) 266-3254 or e-mail: sgoldie@telus.net.

Conclusion: Blue-stained lumber exhibits same characteristics as non-stained lumber

Forintek conducted the study on blue-stained lumber to provide timely objective research on the effects of blue-stain on common wood performance characteristics. With higher volumes of blue-stained products entering the marketplace, it was important to provide sound scientific advice to manufacturers and consumers. Samples from both blue-stained and non-stained wood yielded similar findings. Blue-stained lumber continued to exhibit the same strength, fastening, gluing and finishing characteristics as non-blue-stained lumber. For manufacturers intent on finishing blue-stained lumber with a stain, a variety of techniques can be used. Depending on the desired end effect, colour contrasts between blue-stain and non-blue-stained lumber can be de-emphasized or enhanced with standard furniture finishes. While working with and marketing blue-stained lumber poses some challenges, the underlying message is that the performance characteristics of blue-stained lumber are not significantly different than those of non-stained lumber.


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