In my experience, sodium dichromate does not behave very well for gum printing (but I did not test it extensively). I primarily use ammonium dichromate because it seems to have a longer scale (lower contrast) than potassium dichromate. Gum printing is inherently a fairly short scale process, so I usually prefer the longer scale of ammonium. I use potassium on later dark coats when I just want to kick the shadows a little darker.
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