Re: "speckling" v "staining "

From: "Christina Z. Anderson" <zphoto@montana.net>
Subject: Re: "speckling" v "staining "
Date: Sun, 04 Sep 2005 15:24:42 -0600

> Can you explain in layman's terms why the sodium disulphite might
> interfere with dichromate or sizing? Are you thinking it might
> soften the gelatin? Or what? If it is in a hardener made for
> hardening gelatin why might it interfere with a sizing process used
> to harden gelatin? Inquiring minds want to know.

The reason I would avoid sulfites of any kind in paper, sizing and
emulsion (or dichromated colloids) is because ot their reducing
property. Hexavalent chromium such as dichromate is readily reduced by
very mild reducing agent such as primary alcohols (ethanol, etc.) to
become trivalent chromium. So, sulfite added to the sizing layer could
react with gum layer to reduce available dichromate and increase
Cr(III) before light exposure. Whether this effect can be taken
advantage of in a creative way, I am not sure. But I do suspect that
sulfite interfares with dichromate process in some way. Two
possibilities are base fogging (nonimagewise stain) and lowered
contrast. But experimentalists may want to verify or exclude these.

I am aware that some people add ethanol to dichromated gum as a
coating aid. Depending on the temperature, quantity, etc., it may have
slight influence for the same reason. But ethanol-dichromate reaction
is probably not very active at room temp, and ethanol probably
evaporates too fast, hopefully. I'd make that addition as absolutely
last step immediately before coating. One way to avoid this
possibility altogether is to use tertiary alcohols, like tert-butyl
alcohol (a common, old time defoamer) in place of ethanol.

I suppose Maco's emulsion is a crude bromide or iodobromide. In that
case, small amount of sulfite in sizing layer might not do much. But
if sulfite comes in contact with optimally digested emulsions,
especially with chlorobromide, chloroiodide, chlorobromoiodide and
chloride emulsions, it can cause excessive incidental reduction
sensitization on the grain surface, which is very easy to grow to fog
centers (particularly if the emulsion is gold sensitized).
Received on Sun Sep 4 16:14:44 2005