What makes the lumber blue?

Blue stain is a common cause for the discoloration of lumber. Certain dark-colored microscopic fungi cause a bluish or grayish discoloration in the sapwood of the tree. However, not all blue stains are blue. Common stain shades can be blue to bluish black or gray to brown. Sometimes, the stain coloration in lumber may appear as red, yellow, orange, or purple.

Does it affect the strength properties?

Blue stain has no effect on the performance and strength of lumber. Structural lumber is not downgraded due to the presence of blue stain and should not be labeled inferior lumber.

Where can blue-stained lumber be used?

Because blue stain does not detract from the strength properties of dimension lumber, blue-stained lumber can be used for exactly the same purpose as non-stained lumber. Just be certain that the lumber has a grade mark accredited by the American Lumber Standard Committee (ALSC), which identifies the grade, moisture content, inspection service, species, and mill identification number.

What is it… blue stain or mold?

Blue stain is not mold. However, for a simple test to determine whether or not a piece of lumber is blue-stained or growing mold, lightly rub the affected surface of the wood. Mold grows on the surface and can be brushed off or smeared, where as blue stain penetrates deep into the wood and cannot be removed.

What about health and safety concerns?

Blue stain poses no health risk, and blue-stained lumber is safe to handle. The same safety rules for using non-stained lumber apply to blue-stained lumber:

Wear eye protection and a dust mask when sawing or machining lumber.

Avoid prolonged inhalation of sawdust.

When the work is completed, wash areas of skin contact thoroughly before eating or drinking.

Wash clothing that accumulates sawdust separately from other household clothing and
before reuse.

Does blue stain cause decay?

Blue stain is not a decay fungi. Blue stain fungi live on the nutrients stored in the cells of the wood, not on the cellulose fibers of the tree itself.

Can stained lumber be pressure-treated?

Blue stain fungi have no effect on the treatability of Southern Pine lumber and stained lumber may be treated to guard against decay and termites

Blue stain fungus penetration in Southern Pine log cross-section Clemson University - USDA Cooperative Extension Slide Series, www.forestryimages.org
Blue stain fungus penetration in Southern Pine log cross-section. Clemson University – USDA Cooperative Extension Slide Series, www.forestryimages.org
Blue-stained Southern Pine lumber
Blue-stained Southern Pine lumber