While I was researching the term "spalting" in wood, I found that there is a very big interest in spalted wood.
When is it good for the wood and when isn't it good? I'm referring to another post on WW SE that asked how it can be avoided.
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When is [spalting] good for the wood and when isn't it good?
Spalting can provide a lovely patterning to wood that would otherwise be more plain. Please see the image below for an example of spalted maple.
Maple is usually otherwise fairly plain-looking (assuming it's not figured maple), and the spalting provides a bit of interest in an otherwise plain-grained wood.
The problem with spalting is that it's a fungal attack on the wood fibers. It's fine when the fungus only has superficially attacked the wood and has really only added coloration. However, if left too long, the fungus can break down the wood fibers, making the wood spongy, punky, and rotten. If you're expecting the wood to last, rot is of course a bad thing. Also, punky wood is much harder to work since it has a tendency to crumble.
If you were working on a project where you expected consistent, even grain, having spalting in a piece of wood would be detrimental. For example, if you wanted clear maple and it had a streak of spalting, that would not be good.
Another facet of spalted wood that people sometimes overlook is that working with it can be hazardous to your health. Breathing in fungal spores is generally a bad idea, so a dust mask is suggested when working spalted wood.
Turners use spalted wood all the time because of the pretty colors and patterns that arise. In lumber it is more difficult. because logs don't tend to rot evenly, so while you might get some nice pretty boards, you will also get clean boards and boards that are rotted to uselessness.
Ultimately, spalting is a discoloration of the wood by fungus. It can be a hard job getting enough colors and still keep the wood solid. Very light spalting is (IMO) ugly and I would prefer heavier or none.