A paper has recently been published that presents a formula to estimate the percentage of health-pertinent ash in a sieved sample. Currently one can only really accurately sieve to 63 um but finding out if volcanic ash is respirable (< 4 um) is critical for health hazard assessment. The formula allows rapid assessment using very basic equipment. The paper can be seen at
(or do a google search for rapid analysis of ash).
Ask Claire J. Orwell for more information: claire.horwelldurham.ac.uk