In case it wasn't obvious from the previous discussion:
To distinguish the three leading theories for how the powder sticks
to the surface, observe how much water it takes to unstick it:
a) If you dunk the slide into water and the powder immediately
unsticks, it was held only by surface tension.
b) If you dunk the slide into water, with maybe gentle stirring
to create some washing action, and the powder comes off in a
moderately short time, then it was held by some water-soluble
glue. Perhaps something like sodium silicate that leached out
of the glass.
c) If long washing doesn't budge the powder, then some sort of
direct powder-substrate bond must have formed.
By repeating the above experiment as a function of "drying" time,
you may be able to observe (a) then (b), or (a) then (c).