Dear Dieter,
There are some practical considerations with gold metallography. If
you are only looking for features such as porosity and inclusins in
the nuggets, and the distribution of silver, it is probably easiest to
do your microscopy in the SEM/microprobe using both secondary electron
and back-scattered electron images, the latter in Z contrast. You
could also map the sample for, say Ag, Cu.
If you actually want to etch up microstructural features there are two
main etches - aqua regia and potassium cyanide/ammonium persulphate.
Aqua regia is quite reasonable once you get the hang of it but there
are problems if the silver content in the gold gets too high (probably
above 10%). Aqua regia then deposits quite a strong chloride film on
the sample surface; this can be removed with ammonia but the final
etch is less than adequate. Really with Au-Ag you have no real
alternative to cyanide. The safety hazards are obvious but one not
always reported is that a proportion of the population cannot smell
cyanide; I am one so the stuff really scares me. There are other
obscure etches for gold, e.g. one using iodine vapour but I have no
experience of these.
Yours,
Peter Northover
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Peter Northover
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