Re: gum bichromate

Luis Nadeau (nadeaul@nbnet.nb.ca)
Sat, 10 Jun 1995 10:22:36 +0300

>On Sat, 10 Jun 1995, Dan Shapiro wrote:
>> > I've found exposure times radically longer than seconds, e.g.,
>> minutes (5-10) in sunlight at mid-afternoon. This is with a saturated
>> dichromate solution. Are there hidden variables here, or am I just
>> living further from the sun?>
>
>Among variables for gum exposure are proportion of gum to sensitizer, the
>paper, the thickness of the coat (light has to get through it), the color
>(red through yellow are less sensitive to UV light), denseness of
>negative, drying method (forced heat-drying reduces sensitivity,
>depending on handling up to a factor of five!) and age of sensitiser.
>Also, Gini may be using ammonium dichromate, which is a more saturated
>saturated solution than potassium.

Not necessarily. You can make a 1% sol. of ammonium dichromate, and a 4%
sol. of potassium dichromate...

Gotcha!

Luis
P.S. The gum process is much more forgiving than the carbon transfer (& the
gravure resist). Look up the words "spontaneous hardening" in the index of
any book on carbon printing.