Re: Gum hardening: top down?

Sandy,
I was simply following your own protocol, quoted below, where you
said you used a "heavily-pigmented" gum emulsion with saturated
potassium dichromate to print through the back of paper, and claimed
that with this emulsion and back-printing you got a "DMax higher than
one could get with a single coat of gum, and ... a full range of
tones." Now, you're saying that it's no wonder I couldn't get a good
DMax because it's very very complicated and you can't just do it with
a heavy pigment mix and saturated dichromate. So, I must admit I'm
confused, and rapidly losing patience.
Katharine

On Apr 5, 2006, at 2:53 PM, Sandy King wrote:
> Although I am not currently involved in gum printing the current
> discussion got me interested in running a test to illustrate the
> concept that in gum printing the hardening of the gum is top down.
> So I mixed up 100ml of a 1:1 solution of saturated potassium
> dichromate and lithorgraphers 14 Baume gum, and added ten grams of
> lampblack to the mix. I applied three thin coating of this to a
> piece of vellum and exposed it, through the back, with one of my
> carbon negatives for 30 minutes with a UV printer. Exposure for
> carbon would have been on the order of 4-5 minutes but the extra
> density of the paper based added about two stops of exposure.
>
> The print is not bad. Dmax is much higher than one could get with a
> single coat of gum, and yet there is a full range of tones. The
> only problem is that I was not able to get a smooth coating so
> there are a few streak in the print. The results were so
> interesting that I plan to do some more experimenting with this
> kind of printing.

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
>
> Sandy
>
Received on Tue Apr 11 02:23:53 2006