From: Judy Seigel (jseigel@panix.com)
Date: 11/06/00-01:03:12 PM Z
On Mon, 6 Nov 2000, J. Wayde Allen wrote:
> 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>.
Wayde, a good teacher doesn't say you are a stupid doodyhead. She softens
the correction or deflects it, as with the "editorial" we.
> > 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.
But another similarity is that both use gelatin -- the carbon printing
incorporates it with the pigment, gum printing (usually) has it as the
paper size. (The particular gelatin used can be an *enormous* variable in
gum printing, perhaps not in carbon.)
> 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.
My sense of the situation, incidentally, is that the *mechanical*
differences cause large chemical differences, and I mean within a given
medium, too -- as a change in method often changes results beyond what
seems "logical."
> > 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.
Exactly. But the sum total of *ingredients* much the same. (Unless Sandy &
co were just making that up.)
> > 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>.
Firstly, explain to the backward what you mean by "highly correlated
variables." In gum printing, changing more than one variable at a time may
give you a good printing method, but little information about more than
that exact combination. If we ("we") keep all conditions the same and
change one variable, say the pigment, we ("we") at least see the changes
wrought by that pigment in the given combo....
Pigment character changes with the environment, so it isn't clear why you
scorn one-variable-at-a-time. It's not crucial to a beginner, who needs to
find a general modus operandi-- but for fine tuning & control, also
trouble shooting, we (I) don't see how, or why, you dismiss it.
Judy
>
> 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.
>
> - Wayde
> (wallen@lug.boulder.co.us)
>
>
This archive was generated by hypermail 2b30 : 12/01/00-11:46:56 AM Z CST