Pattern, I agree with your comment about the pilots on the spot being able to differentiate between fire ash and volcanic ash - but they better not get it wrong.
Prior to the KLM event, the belief was that volcanic ash would react similar to rain and be centrifuged out sufficiently to avoid it being a danger to the engine. However in the aftermath we discovered that much of the volcanic ash is so fine that centrifugal designs don't work reliably.
After Mt. Saint Helens went boom 40 years ago, I saw a lot (too much, actually) volcanic ash up close and personal. The stuff is amazing yet frightening.