I would guess they are considered practical ways to get an opaque, microscopically even, pigment 
deposit on the object without having to paint it on (which might obscure details if a paint was 
used); presumably one that can be brushed off, as opposed to sticking.  But I have no experience 
with it.

>
> But I still wish someone would be willing to answer a few questions -- Like
> why ammonium chloride or burning magnesium strips?
>
> best, Julia Morgan Scott
>

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