From: Philippe Monnoyer (monnoyer@imec.be)
Date: 04/08/02-08:16:59 AM Z
Witho,
>I have two questions concerning the coating of gelatin on papers.
>First : Last night I tried to coat Rives BFk with a 5% gelatin/ 1%
>aluminiumpotassiumsulfate solution. I coated the papers with a hake brush.
>The paper should be used for carbon double transfer or, after some extra
>coats, as an oilpaper. Very tiny bubbles now show as little shiny specks.
>Will these specks harm the final print ? or should I use those papers just
>for tests ? Is there a remedy against these specks or bubbles ?
I had similar results when I tried to size some papers w/ different
gelatins or colloids. These little shiny specks are not bublles, but I
am pretty sure (99 %) that they are actual gelatin microcrystals. Their
crystalline faces reflect the light and give shiny specks. I don't
remember who told me this and how to avoid that neither. If I don't
forget, I will check in my gelatin science handbook.
>I have the impression that the alum creates more viscosity in the gelatin
>and is therefore harder to coat. Is it not simpler to give a paper coated
>with a plain solution of gelatin a hardening bath afterwards (I am thinking
>about carbon now) in formaline or glyoxal?
It is not an impression. Alum changes drastically the viscosity of a
gelatin solution. After one day, you can even not melt it back.
Hardening afterwards is a good idea I would say, at high pH.
>Second question : Nze Christian wrote that Rifes BFk can be made suitable as
>a platinum paper by coating it with a 1% gelatin/1% alum/1% oxalic acid
>solution. Why is there need for the alum ? Why should the gelatin get
>hardened ?
To become a part of the paper support I guess. If gelatin is not
hardened, papers will simply loose a part it on processing, along with a
part of your platinum image.
Moreover, I would expect a non hardened sizing to give warmer tones.
With chrysotypes, I had red images when adding some gelatin, and brown
to black w/o gelatin.
In emulsion making, the hardening action is slow and is complete in
weeks for chromium alun, and in days for formaldehyde. Most probably
even quicker with glyoxal. The doses are very low.
For sizing, the dose is massive and can be even accelerated with high pH
as far as I remember.
Philippe
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