Carbon printing vs Gum dichromate

[louisnel]

Hello,

I'm just wondering is anyone can shed light on why gum bichromate printing does not require transferring to a final support like carbon printing does.
As I understand it, when exposed the cross linking of the gelatine in carbon printing occurs top-down and this is why the final support is necessary, however, I would expect the same thing happens in gum printing. Why then does the gum print not come off the support in development as it does in carbon printing?

My best guess is that the use of Gum arabic instead of gelatine is the reason for this difference, that the gum pigment can diffuse out whereas the unexposed gelatine layer cannot... Would I be correct in that assumption?

[Vaughn]

I believe it is due to the thin heavier pigmented layer of gum rather than the thicker, lighter pigmented layer of gelatin in carbon printing.

There was a carbon process similar to gum, where the exposed gelatin is sort of rubbed off with a water-sawdust mixture and is similar to gum printing in a way (The Fresson process, commonly referred to as direct carbon printing). If one exposes the carbon tissue all the way to the support, one will not get a transfer. Generally, I believe the transferring can give carbon prints a sharper image than gum printing.

[Drew Wiley]

Casein is sometimes used analogously to gum. There have been all kinds of tweaks, including a commercialized kit product called Kwikprint. Fresson is a secretive direct carbon screen process that uses a high-output carbon arc enlarger. The distinctly gritty results can be exquisite despite the idiosyncratic hues of their old process color set : alizaron crimson (more scarlet than magenta), chromium oxide (more "poison green" than clean green), and a yellow more gold than actual yellow. Maybe Bob Carnie will chime in, since he specializes in hybrid versions of gum printing.

[sanking]

My best guess is that the use of Gum arabic instead of gelatine is the reason for this difference, that the gum pigment can diffuse out whereas the unexposed gelatine layer cannot... Would I be correct in that assumption?

There are some differences in the way various colloids adhere to different types of surfaces, but the issue is more complicated than gum arabic versus gelatin.

Gum is a type of a direct carbon type of process, which can also be done with gelatin, albumen, casein, or even PVA. In the direct carbon method the image is exposed through the negative from the top, and developed in place on the paper support. Direct carbon prints do not have good half tones as the highlight values, near the bottom of the coating, do not get enough exposure and wash away, wholly or in part. in order to solve this problem multiple printing of several thin layers of pigmented colloid are required. So in order to obtain both good shadow density and a full range of tonal values, multiple printing is required if some type of transfer procedure is not used.

We differentiate in this way between direct carbon printing, where no transfer is required, and carbon transfer. But without multiple printing no form of direct carbon, unless exposed through the back of the substrate, can compete with carbon transfer in terms of maximum shadow density and good highlight detail. And of course, multiple printing without good registration can result in loss of sharpness.

Sandy

[bob carnie]

I can only speak to gum printing, I have found that the secret for me at least is the ability for the light to harden the gum - pigment- Ammonium Dichromate mixture. where there is more light more hardening which in development in water releases the gum layer where there is less exposure in increments. It has been my experience that with normal exposure , it is impossible to get rid of some of the image no matter how long it is processed or even with brush work.
When I pull my prints from the water I am looking at the relief that is on the print which reminds me a lot of what one would see with a Carbon Transfer print, but as the print drys this relief sinks into the paper.
This is key , the gum is now a permanent part of the print once dry it is not coming off and rewetting will not affect the print so you either accept what you have or do another layer.
I am aware of some gum printers who do up to 9 - 12 layers of gum ... in fact right now I have Diana Bloomfields gum over cyanotype prints in my gallery and I must admit they are some of the nicest prints I have ever seen .


As Sandy pointed out carbon transfer has very dense shadow detail right off the hop. I feel gum could never match the sharpness of a tri colour carbon print that my friend John Bentley makes and or the colour correctness that Calvin Grier is working for, or the thick relief that Sandy gets with his mono carbon prints.
But for me I use a palladium base to create the detail and sharpness I want and I use the gum pigments to create an ambience , whether its correct or not I work on the layers until if feel its what I want to see in the print. It kind of reminds me of lith printing and solarization which I am very fond of, going into a series of images I feel that I am not 100% sure of what I am going to get, but as well with effort and a bit of colour savy one can create lovely prints.
For Mono Gum prints with some colour one can use tonal negatives from regions of the original scene which then can be used to place colour where one wants, much like tri tone or quad tone silver gelatin prints . In gum you have
much more control over the look of the print than I ever had with silver...