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