From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/09/01-09:53:20 AM Z
Hi George and all,
Chromium oxide raises its green head again. Contrary to your surmise, it
most likely *is* related to dichromate, it's the reduced dichromate that
becomes more insoluble as it reduces that is giving you that heavy
green stain. You will probably need to treat this stain with a weak
sulfuric acid to remove it. I don't remember what concentration Pete
Frederick recommends but it's in the archives and not all that long ago.
April or May, I think. At any rate, your pigment didn't actually change
color (somehow this reminds me of that old commercial "You'll wonder
where the yellow went...") and if you remove the stain you'll find that
the yellow is still there. I'm surprised you got this much stain with
potassium dichromate. The only time I've seen a stain like this was
with ammonium dichromate seriously overexposed to direct sunlight. Good
luck,
Katharine Thayer
George Huczek wrote:
>
> I am doing gum prints with Linel helios yellow 1:1 gum and pot. di.
> sensitizer.
>
> After exposure the pigment changes from a bright yellow to an olive green.
> After development, drying and clearing with potassium metabisulfite the
> olive green colour persists, so I don't suspect dichromate stain. The
> highlights cleared to the paper base colour. Can anyone suggest why the
> pigment colour changed after UV exposure?
>
> _
> [o] -GH
This archive was generated by hypermail 2b30 : 08/02/01-11:56:46 AM Z CST