From: Katharine Thayer (kthayer@pacifier.com)
Date: 02/14/02-02:09:32 AM Z
I've never tried it myself but I assume it's true that a high dichromate
dilution may result in staining, since others have reported this. I've
never wanted or needed to go beyond 1:2 myself, and very rarely that,
and have seen no staining at that dilution. And of course it's true
that a gross excess of pigment in the mix will result in flaking (which
I prefer to call sloughing; when I think of flaking I think of dry paint
coming off bit by bit, whereas what happens with a too-thick emulsion is
that it all comes off at once, phloff) without staining.
While beginning gum printers should be aware of thse phenomena and avoid
gross excesses in either direction, both are outside the ranges of
concentration I'm talking about in discussing the adjustment of pigment
concentration to avoid staining.
Katharine Thayer
Keith Gerling wrote:
>
> Lately, I've learned quite a bit about these phenomena in gum printing.
> Most of my prior gum printing has been with RGB or CMYK-produced negatives
> with the goal of producing a color-accurate print. Achieving this requires
> an element of measured consistency with regard to the mixture or pigment,
> gum, and dichromate. Recently I have been working on a series of duotones
> produced from black and white negatives. My goals here are somewhat
> different. Color accuracy is no longer important, and what I hope to
> achieve is a variety of interesting color effects from the use of a
> combination of different pigments, applied in a manner that will afford me a
> variety of contrast. Accepting serendipitous results, I have been willing
> to experiment with some "wild" mixture combinations, at least when compared
> to my, otherwise, more controlled process. I have gone back to pigments and
> papers that have been problematic in the past.
>
> What I have learned from this is that, contrary to my previous notion,
> staining and flaking are not so much a factor of the pigment itself, but of
> the concentration of pigment or dichromate in the emulsion mixture. In
> efforts to achieve density, I have occasionally mixed gross amounts of
> pigment into the mixture (quadruple what I would have used in the past).
> This usually results in flaking. (My understanding of "flaking" is when the
> emulsion comes of the print in areas where the printer does not desire it to
> wash off). This usually manifests itself in great densities sticking to the
> paper in the shadows, nothing sticking in the highlight, and high level of
> graininess in the midtones. There seems to be an "all or nothing" effect".
> I attribute this to a masking effect by the great level of pigment: the
> pigment blocks the light, and the emulsion cannot absorb enough to become
> hardened. Likewise, when levels of normally-flaking pigments (which for me
> include lamp black and certain indigos) are reduced to small amounts, no
> flaking occurs. In no case have I ever observed staining of the paper when
> using high levels of pigment. There is always a "paper white" where the
> flaking occurs.
>
> In cases where I have hoped to achieve either an extreme low contrast, or
> wanting to apply a "tint" over the print, I have occasionally increased the
> amount of dichromate to levels I haven't used before, or diluted the
> emulsion with water. In these cases, the risk of staining is very high. I
> have determined that regardless of the pigment used, any time I reach a
> point where the combination of pigment and gum arabic is less that a third
> of the total mix, I will likely get some degree of staining. I attribute
> this to the fact that the viscosity of the emulsion has been lowered to the
> point where it can easily penetrate the gelatin. For the record, I use a
> concentrated Potassium Dichromate, Daniel Smith gum, a variety of pigments,
> and various gelatin-sized papers.
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