Re: Gum hardening: top down?

On Apr 5, 2006, at 10:21 AM, Sandy King wrote:
> If the jury is still out on whether hardening of gum on exposure to
> UV light takes place from the top to the bottom of the emulsion
> layer, or from the bottom to the top, I suggest someone go to the
> jury room and check on the condition of the jury. They may have
> left the building.
>
> Were there any gum system that would result in hardening of the gum
> emulsion from the surface of the paper up to the top of the
> emulsion it would be possible to make gum prints with one coating
> and exposure that have both high Dmax (log 1.80 or above as is
> possible with carbon), and a complete range of tones from the
> shadows to the highlights. No one has ever been able to do that in
> gum printing, and unless the laws of photochemical reactions are
> reversed, they never will.
>
> I have suggested a very simple test that will clearly prove that
> hardening of a gum emulsion on exposure to UV light is a top to
> bottom phenomenon, same as it is in carbon printing. Just expose
> the gum print from the back through the surface of the paper. If
> you do this you will find that it is possible to get very high Dmax
> prints with a complete range of tones, though printing times will
> be very long and some detail will be lost from the texture of the
> paper.
>
> Sandy

On Apr 10, 2006, at 4:56 PM, Sandy King wrote:
>
> Thank you for taking the time to do the test.
>
> Let me remark that in carbon photography, where image formation
> should be very similar to your test in exposing from the bottom,
> just coating with a heavily pigmented emulsion would not
> necessarily result in high Dmax. There must be the correct balance
> between negative contrast, thickness of the coating, how heavily
> the coating is pigmented, and the strength of the dichromate
> sensitizer.
>
> Many people find it very difficult to match all of this up when
> they first start printing with carbon, primarily because they use a
> dichromate sensitizer that is much too strong for this type of
> exposure. If they do, the resulting image will be very weak (low
> Dmax) because the strength of the sensitizer is so great that all
> of the hardening takes place right on the surface of the print (on
> the top at the end with carbon because we transfer to another
> surface, but at the bottom with your experiments). The result is
> you have a very thin layer with relatively little pigment, and you
> can recognize this because a very high percentage of the pigmented
> carbon layer just washes away during development.
> Assuming that there is some kinship between the mechanism of carbon
> transfer, and exposing a gum colloid from the bottom (which I
> strongly believe there is), you should find that the strength of
> dichromate that is used in regular gum printing is much to strong
> and will result in a fairly low Dmax when exposing through the
> bottom. In carbon, for example a sheet of 8X10 carbon tissue might
> contain 80ml of an 8% gelatin solution, but we would need no more
> than about 10ml of a 2% ammonium dichromate solution to sensitize.
> In gum, where typically you expose from the top, and do not
> transfer the image, you use a much higher ratio of
> dichroamte:colloid. So I would think that if anyone wants to
> optimize a gum coating for exposing from the rear they need to
> think more in terms of the carbon paradigm that gives maximum
> relief and Dmax, moderately pigmented, thick emulsion sensitized
> with very dilute dichromate solutions
>
Received on Mon Apr 10 18:38:30 2006