What are Black-Blue Discolorations on Stretcher Bars?
If you’ve ever unpacked your stretcher bars and noticed dark, bluish, or gray discolorations, you might have wondered:
Is this mold? Is my wood damaged? Or just a cosmetic flaw?
Don’t worry – in most cases, it’s not mold, but something completely different: blue stain fungi (mycelium growth).
What Exactly Is This Bluish Discoloration?
The dark or bluish-gray streaks or spots on stretcher bars are caused by mycelial structures of specific wood fungi, known as blue stain fungi.
They grow within the surface layers of the wood, typically following the wood grain, and create this characteristic discoloration.
The fungus doesn’t actually “eat” the wood – it simply changes the color by depositing its metabolic byproducts in the fibers.
Is Blue Stain Bad or Dangerous?
No – blue stain is not mold in the usual sense.
While mold fungi attack moist surfaces and can produce harmful spores, blue stain fungi only penetrate the wood without affecting its stability or safety.
So, the wood remains mechanically stable and perfectly suitable for use in stretcher bars.
However, the discoloration indicates that the wood was exposed to a bit too much moisture at some point during processing – the ideal condition for mycelium to form.
Why Does Blue Stain Appear on Stretcher Bars?
The cause usually lies in natural wood storage and drying processes:
🌧️ Excess wood moisture after sawing or drying
🌲 Humidity and temperature fluctuations in storage areas
🪵 Contact with slightly affected wood
📦 Insufficient air circulation during transport
Especially with untreated softwoods such as spruce or pine, blue stain can occur before the bars are primed or assembled.
It doesn’t affect durability or tensioning strength, only the appearance of the wood.
How Can Blue Stain Be Prevented?
In industrial woodworking, blue stain fungi are prevented through consistent drying and storage below 20% wood moisture.
At CRAZY CANVAS, our wood is dried to below 10% moisture content and sourced from sustainable forestry.
Still, with natural wood, minor visual variations can occasionally occur – they’re not a defect, but rather a natural property of the material.
Conclusion: No Reason to Worry – Wood Is a Natural Product, and Every Piece Is Unique
Bluish or gray discolorations on stretcher bars are not quality defects, but natural traces of genuine wood.
Wood is alive, it changes – and sometimes, it shows its history.
The functionality and stability of our stretcher bars remain fully intact – so you can focus on what really matters: your art.