1 Attachment(s)
Blue Stain in Southern Yellow Pine: Facts and Observations
As some of you may have seen I got started on a workbench about a month ago using some SYP that I found at a local sawmill. Since the initiation of this project I've been kind of fascinated with SYP--its properties, its economic importance, etc. Here in Kansas I never saw SYP lumber apart from the pressure-treated stuff. Framing lumber is almost all Hem-Fir or Douglas Fir so SYP is virtually an exotic to me. How a sawmill here in town got ahold of a yellow pine tree I don't know. But Mr. Sawmill claims that this tree was sawed a decade ago and has been sitting ever since.
Attachment 385626
A lot of the sapwood has blue stain in it. On some boards the stain goes all the way through. I read a little bit about it and saw a lot of conflicting information ranging from "don't use anything that is stained because it is weakened and unsuitable" to "it makes zero difference in mechanical properties and is actually desirable because it looks pretty." Personally I think it looks kinda cool but structural defects are not worth pretty colors so I did a little bit more digging. I found more than a few research papers on the subject going back over 100 years. Apparently the economic cost of blue stain can be significant for a lumber company so the subject gets a some attention. As it turns out there are a number of fungus and mold species that result in staining, and you cannot always determine the type of fungus/mold by color. That said, the blue stain common in pine is generally a fungus. Some studies concluded that the fungus can decrease the strength or toughness (shock resistance) of the wood and some studies concluded no change in mechanical properties.
The most informative study I found gave a little clarity to some of the inconsistent results I'd seen before. But, like so many helpful studies, it ultimately muddied the water by raising new issues. It was done by Dale Chapman and Theodore Scheffer, if anyone is actually interested in reading a scientific article about fungus and wood. Anyhow, they actually tested multiple species of fungus and multiple species of pine. Awesome. What they found is that different species of fungus were detrimental in different ways and to different extents. Sigh. It gets messy but these are the points that may be useful to the woodworker:
- compression, bending strength, hardness, and specific gravity were not really affected that much
- the only property that was seriously affected was toughness AKA shock resistance. In their tests toughness was reduced by between 9 and 75%. However, in the conclusion they state that in naturally infected wood that we would encounter they estimate the potential loss in toughness to be 15-30% at most. The tests involved heating that they think caused significant degradation on its own.
- determining the species of fungus that caused the stain...no easy way to do it so don't even worry about it.
- the intensity of the color did not correlate to the extent of degradation. In fact, the fungi that caused the darkest stains affected the wood the least apparently.
- the extent of the staining (i.e. the amount of discolored wood) is a decent indicator of the magnitude of the infection and thus may have value as a means of estimating strength losses
So, at the end of the day, blue stain is probably not worth getting too worked up about. If the stain is extensive, or strength and toughness are critically important to the thing you're building then it is probably worth trying to avoid but I wouldn't go rejecting a stained piece of pine that is otherwise ideal for most projects. I suppose I'll find out a little more as I spend more time working with the pine I have but based on the research and my experience thus far I am betting that the blue-stained stuff is just fine structurally-speaking. SYP seems to be tough enough that even with a reduction of 15-30% it'll probably be fine for most applications.
On related note, I have noticed that some of the blue-stained sections on the pine that I have appear to be more porous. Almost like they have little micro-checks sprinkled around. Another brief study said that blue-stained pine caused by beetles (the beetles carry the fungus--they do not cause the stain themselves) becomes more permeable to liquid. Whether or not this is caused by the fungus or the beetles I did not figure out. I suppose I'll find out when I get further along on my workbench. My prediction is that the stained wood will be a little bit more permeable and will suck up more finish but I base that prediction only on handling and looking at the pine that I have.
I apologize if this got a little too nerdy and sciencey for some folks but hopefully someone will find it useful or at least interesting as I did. After seeing how detailed some people here get over steel composition and sharpening media I figure I have a decent shot of hooking at least a few folks!
