From: J. Wayde Allen (email@example.com)
Date: 11/06/00-10:33:49 AM Z
On Mon, 6 Nov 2000, Judy Seigel wrote:
> I think we tend to think that just because gum printing and carbon use
> many of the same materials, they work the same,
What do you mean "we"? Maybe "you" tend to think they work the same, but
those of us who've tried both don't <grin>.
> but I get the impression they're completely different !!!
Let's see ... the processes both use paper, dichromate, and
pigments. They also are both water developed. Besides that though the
colloid used is different: gum arabic for gum printing and gelatin for
carbon printing. That right there changes the chemistry a
bit. Mechanically these are also different too. For gum printing the
pigmented gum is applied to the final paper support, dried, exposed, and
then the un-hardened gum/pigment washed away to make the image.
In carbon printing the gelatin/pigment is coated on a temporary backing
where it is exposed. The gelatin/pigment film is then mechanically
transfered either to the final support paper, or alternatively to an
intermediate support, where it is developed by removing the temporary
backing and washing the un-hardened gelatin away. One critical difference
is that the gelatin UNDERNEATH the exposed layer is washed away with this
process, rather than trying to wash the un-hardened material away from the
surface of the print as is done with gum. That changes the mechanical
requirements quite a bit too.
> I remember when we did some exposure
> tests years ago, Sandy King & company found a whole other profile for bulb
> range for carbon than I found for gum, a BETTER scale, with, as I recall a
> daylight fluorescent. For gum the exposure time with daylight fluorescent
> tripled and the scale about vanished.
We'll the chemistry IS different.
> In carbon transfer, I take it that you mean the exposure was done on a
> different surface, then transfered to the Buxton for development.... which
> is, I gather, purely mechanical.
> I would assume elements don't get into
> each other the same way...
> Although of course it's possible that a
> different worker, using different ingredients and a different gelatin
> might well have results very different from mine. (In fact as we know,
> just opening the door can cause something completely different.)
In a process with highly correlated variables, this is true, and is
exactly why one-variable-at-a-time testing is NOT a good approach. You
can never see variable correlation with this kind of testing, and as such
changing anything throws everything off. You don't have to take my word
for it though see <http://www.pdlab.com/experimentx.htm>.
For those of you interested in the carbon process I'd highly recommend
subscribing to the carbon mailing list at
<http://rmp.opusis.com/mailman/listinfo/carbon>. I've also been
collecting carbon printing info at
<http://rmp.opusis.com/carbon/carbon.html>. One of the more recent
additions has been a primer written by Sandy King.
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