Tax Time to Turkey Time: An Article on Blue Stain
The dog days of summer bring a seasonal problem to sawmills, stain. I was told the hard and fast rule for stain season is “tax time to turkey time.” I assume April 15 until Thanksgiving. April 15 is aggressively early for the stain to set into hardwood logs, as most mills are still cutting white hardwoods that were cut in the wintertime. Winter-cut logs often stay cold enough into May to negate the presence of fungi that cause a stain in the sapwood of logs.
What is the stain that occurs in the sapwood of hardwood logs? Three types of fungal growth cause blue stain: Ophiostoma, Ceratocystis, and Sphaeropsis. The fungi thrive when the temperature reaches 50 degrees and moisture is in the air. Combine these two conditions with oxygen, and fungal stain will appear in the wood. As temperatures rise, the intensity of the stain will increase. Logs that have airflow on the ends will dry the ends enough to stall the stain somewhat. However, once the stain begins in the log and the temperature is above 50 degrees, the fungi imbed themselves deeper into the sapwood and will not stop until the wood is dried enough to kill the fungi. I've seen logs cut on Friday, left end to end over the weekend when temperatures are above 80 degrees, and on Monday, have blue stain that is already six inches deep at the end of the log.
The appearance of the stain is a blue/gray color that will intensify as temperatures rise and humidity increases. The fungal stain will not damage the wood, but it’s not accepted in the general lumber marketplace. Additionally, the standard hardwood grading rules only allow stain if it can be surfaced out at standard thicknesses in most species of hardwood lumber.
I don't mean to disrespect softwood producers, as hardwood production is all I know, but it is my understanding that the same fungi that grow in hardwood grow in softwood. As in hardwood, when the lumber is dried, the staining process is stopped.
Interestingly, spalted maple is accepted in the marketplace for its unique appearance, color variation, and wild patterns that appear in the wood. For a long time, I thought spalting had the same fungal composition as a regular fungal stain. Like a lot of things in this world, I was wrong. There are three separate fungi in spalted maple: Trametes versicolor (known as turkey tail), Xylaria polymorpha (dead man's fingers), and Peniophora. As opposed to blue stain, each of these fungi has wood-decaying properties. However, once the lumber is dried, the decay stops. There is evidence that spalting can be performed in a controlled environment. However, it is costly, time-consuming, and not guaranteed. According to some research I have done on this topic, the oxygen level is difficult to control and does not promote a guaranteed spalting process.
Anyway, back to the topic of blue stain. In the summer months, sawmills do everything they can to bring logs into their mills as expediently as possible. Typically, a mill will try to keep lower levels of “whitewood” inventory during the summer to avoid exposing the logs to the stain environment. This is not an exact science, as the woods giveth what it may, but most mills operate with steady production of whitewood this time of year. Sawing and drying whitewood to prevent blue stain is the flavor in these parts.
In conclusion, if you've read this far without being bored to tears, I hope markets treat everyone well in the upcoming fall and winter months.
Troy Brown,
GLTPA President