MM Flynn:
And when you read the 100 reports you will see in virtually every incident that the aircraft flew through or 'just around' an ash cloud with clearly visible features (St Elmos fire, sand blasting of windows, cabin filling with dust, black cloud of bits of gravel from volcano directly underneath, etc). The crews may have not noticed the cloud was ash vs. water or they may not have been able to avoid due to being night ... but they were clear clouds rather than very low ackground concentrations in clear blue 100km viz sky that are the basis of your concern.
(Note - for the record, there are a few incidents where the details from the crew/airline/NAA are sufficiently sketchy you can draw no conclusion as to the flight conditions - or sometimes the day or if the crew noticed at the time)
Go ahead, buy an aircraft fitted with jet engines worth millions of dollars and become your own test pilot.
The Gurus have spoken, and their word is law.