Just a small point to add to the number of comments about the dichromates -
Judy as usual is correct (just pass over her blind spot on gloy - maybe it
doesn't travel).
<< >I've read Ammonium is more dangerous to use. >>
All the dichromates need to be treated carefully. As well as the health
risks mentioned by others they are powerful oxidising agents and can form
dangerous mixtures with readily oxidisable substances. However once in
solution you don't need to worry on this score - just don't get anything in
the powders/crystals.
Don't try crushing ammonium dichromate in a pestle and mortar either. Or
heating the solid - it used to be used to give demonstrations of a volcano
like effect - now frowned upon for health reasons.
Peter Marshall
On Fixing Shadows, Dragonfire and elsewhere:
http://faraday.clas.virginia.edu/cgi-uva/cgiwrap/~ds8s/Niepce/peter-m.cgi
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