Re: Carbon Printing

nadeaul@nbnet.nb.ca
Fri, 10 Nov 1995 12:24:54 +0300

>On Fri, 10 Nov 1995, J. Wayde Allen 303-497-5871 wrote:
>
>> I was under the impression that it took around 12-13 hours to dry freshly
>> coated carbon tissue. However, I was wondering about this? It seems
>> that once the gelatin has set and is reasonably dry it could be
>> sensitized, allowed to dry for say another 3 hours or so, and then used.
>> Is this true? It seems that the longer I let the tissue dry the worse it
>> wants to curl, and the more brittle it gets. Is this the reason that you
>> recommend drying for the shorter amount of time?
>>
>> I have also been experimenting with the addition of varying amounts of
>> sugar in the mix. It looks to me like the sugar decreases the amount of
>> contraction of the gelatin layer, or at least allows it to dry slower
>> and more evenly. It does seem to make the gelatin layer a bit more brittle
>> though. Any thoughts on this?
>
>Sugar is highly hygroscopic so it keeps a certain amount of humidity in
>the emulsion (glycerin has the same property).
>The curling of paper is caused by different drying speed of paperbase and
>Gelatin emulsion and by different properties of paper fibers and gelatin, so
>slow and even drying could avoid curling.
>Sensitized pigmentpaper must be perfectly dry for printing

That is a myth. Check Wet Carbon Process in my Encyclopedia or better, a
full chapter, pp. 157-164 in _History and Practice of Carbon Processes_
(1982)

The funny thing about this process is that an "inventor" was granted a US
patent in 1945 for the "discovery" of the sensitivity of wet dichromated
gelatin. In fact it was mentioned in Poitevin's first patent of 1855...
Autotype made specially formulated tricolor tissues for the wet process
until ca. 1950. Darryl Jones experimented with it when we were teaching in
Maine ca. 1980. I never liked it myself as it was a quick and dirty
(indeed, messy) method. You soak the pigment tissue in a tray of dichromate
say, three minutes and then pull it out holding it by one corner for a
minute or so then you lay it down (pigment down) on a sheet of cellophane
(polyester would do) and squeegee the back of the tissue to remove excess
dichromate. (Wear gloves for all of this) Then you lift the cellophane and
lay it in the exposure frame pigment side up and put your neg on top of the
dry side of the cellophane. Exposure is about three times as long as the
normal dry method for conventional pigment tissues. I seem to recall that
in my research for the History book mentioned above that I had found
patents and scientific papers on additives to the pigment formulas that
increased the sensitivity.

This method tends to increase the contrast so negs and coating formulas
have to be adjusted. The Autotype process was used by amateurs and the
results were always inferior to the conventional process.

(gelatin
>contains every time a certain amount of humidity).

about 10% of its weight

Luis Nadeau
NADEAUL@NBNET.NB.CA
Fredericton, New Brunswick, Canada

>The problem occures only with pre-sensitized material because drying time
>should be as short as possible when gelatin contains dichromate but
>freshly hot coated gelatin contains 3-4 times more water than a tissue
>soaked for a few minutes in a cold dicromate solution.
>So this is the problem!
>I make pre-sensitized material because I use baryta paper as a final and
>on its smooth and white surface every kind of uneven sensitizing is visible.
>I tried to replace 40% of water by alcohol as a drying agent in the hot
>emulsion but the result was an uneven coating with made tissue useless.
>
>Stefan Stecher
>stefan.stecher@htw-kempten.de