Re: Dots of gum?

Thanks for your thoughtful post, Etiene, some things there to chew on
for a while.
kt

Etienne Garbaux wrote:
>
> Katharine wrote:
>
> > Another thing I've been thinking about lately is this idea that gum
> > sticks better * * * if one uses a half-tone or stochastic dot negative.
> >
> > But I realized that something isn't making sense to me about this. The
> > argument has always run this way: somehow these discrete dots of gum
> > that are laid down by halftone or stochastic dot negatives stick to the
> > substrate better than gum that's not laid down this way. But here's the
> > problem I have with this argument: since it's the negative that is made
> > of discrete dots, what prints on the gum isn't the discrete dots. What
> > prints is the space around the dots. So I don't get this argument; could
> > someone make this work for me?
>
> Both Mark and Judy have identified the mechanism, at least as my
> understanding has it to a first approximation.
>
> Printing gum is in some ways very much like making dye transfer matrices or
> carbon prints. In all three, one exposes a layer of soluble gel to harden
> it and make it insoluble. In all three, there is a depth effect to the
> exposure -- the surface closest to the exposing light hardens first, and
> the hardening proceeds through the thickness of the gel layer according to
> exposure. The remaining soluble gel is then washed away, leaving a
> representation of the tonal values in the amount of gel remaining at any
> point on the image.
>
> In DT matrix preparation and carbon printing, the process is arranged so
> that the maximally-hardened surface of the gel is bonded to a support
> before developing. In carbon printing, this is usually done by
> transferring the exposed pigmented gel to another support before
> development. DT matrix film is exposed through the base, so the
> maximally-hardened layer is already next to the support.
>
> Gum, on the other hand, is developed with its hardened layer away from the
> support and soluble gel next to the support. If one tries developing
> carbon tissue this way, or exposing DT matrix film from the emulsion side,
> the whole image washes away, except for a few spots where it was exposed
> sufficiently to harden the gel all the way through to the support. Gum,
> therefore, depends on the tooth of the support -- having substrate fibers
> more or less throughout the depth of the coating -- so that the hardened
> gel has something to grab onto to prevent being washed away. Thus, it
> tends to work best with rather woolly papers, not as well with hot press,
> and even less well on toothless substrates like glass. This feature of gum
> is widely credited with making it more difficult to get a wide and smooth
> tonal range with gum than it is with other processes, particularly on
> smooth paper -- which is believable. [Obviously, some practitioners are
> able to get a very nice tonal range with gum -- it is not my intention to
> start a "gum has limited tonal range" war, and shame on anyone who takes
> this discussion in that direction.]
>
> The advantage, if there is one, of a halftone image is that wherever the
> gum is exposed, it is more or less exposed all the way through to the
> support. Even in the lighter midtones, where a continuous-tone negative
> would leave most of the area with a thick layer of soluble gum between the
> support and the hardened gum, the halftone-exposed gum should produce an
> approximately correct (if grainy) tone. The gum you are washing away isn't
> underneath the gum you want to stay, it's beside it.
>
> If I wanted a gum image on glass, I think I'd coat the glass with something
> that hardened gum sticks to very well. Perhaps a thin layer of hardened,
> unpigmented gum, using an intermediary subbing material between the glass
> and the hardened gum layer if necessary. At this point, one would have the
> same two choices we have already seen: coat pigmented gum onto the hardened
> gum layer and expose through the glass, or make a pigmented gum "tissue,"
> expose it normally, then adhere it to the hardened layer on the glass with
> cool water, let it dry, and then develop it in warm water. The former
> would make it difficult to do multiple layers. The latter might give
> problems with the continuing post-exposure hardening effect of dichromates,
> although perhaps a good rinse could be incorporated into the cool water
> adhering process to stop this.
>
> On the topic of chemical tooth: chemical tooth is a molecular effect.
> Very simply, good subs are molecules which have one end with an affinity
> for the support and the other with an affinity for whatever you want to
> adhere to the support. This can work excellently, and solves some thorny
> industrial problems. However, because the chemical tooth is only a
> molecule deep, I do not believe it could "reach through" the thickness of a
> gum layer to adhere the hardened gum away from the support if there were
> still soluble gum in between. So it might be a boon in conjunction with
> either of my suggestions above, but probably won't be a viable solution for
> any "support one surface and expose the other one" system.
>
> Best regards,
>
> etienne
Received on Mon Feb 21 12:07:34 2005