Gum Bichromate Reality Check

[Jon Harwood]

Hi,
I am reposting this from the Alternative Process list at the request of Don Bryant.

I have been working with gum for several months and have gotten to the point that I can make a decent monochrome/duotone print but am not yet at the Tricolor level.

What I am interested is if I am more or less right in how I percieve the relative importance of the printing variables. Since gum is a tricky process to learn, having a decent vision of what is going on can help to make it more predictable.

I am using potassium dichromate and lana aquarelle cold press paper. I use compact fluorescent UV lamps to expose. I make digital negatives and apply Christina Z. Anderson's simple gum curve found in alternativephotography.com. I don't see the need to consider more sophisticated curves until I gain more skill with the process.

The most important variable appears to be time as exposure time determines the ability of the gum to stick to the paper and the extent to which the highlights are colored.

The second most important seems to be humidity. Here in North San Diego County, CA wild shifts in humidity can occur rapidly. I have to watch the National Weather Service and my hygrometer pretty carefully since humidity seems to dramatically affect printing times, and low humidity can make coating an awful experience. Below 40% I humidify my paper in a water based humidification chamber before coating and before exposure and I run a humidifier. This seems to bring a modicum of predictibilty to the exposure process (low humidity=long exposure-high humidity=short exposure.

Pigment and pigment concentrations seem to be the third most important factor and since each pigment varies in how it bleeds out, tends to affect exposure and God(dess) knows what else, I ran some simple exposure time test strips and use them as a rough guide as I slowly get to know each pigment.


There is also the issue of pigment concentration/exposure as high concentration, short exposure prints toward shadows while low concentration, long exposure prints toward highlights. I'm not sure this even fits into a hierarchy since it is a way of working with time and with pigment to affect the layers that make up a print. Perhaps the complexity of this and the difficulty of reducing the process to simple concepts points to one of the great beauties of the process, that is, how one must work intuitively yet methodically to handle it at all. Well, each successful print is a "small miracle" at least to me at this stage in my relationship to Gum.

Temperature of wash water and the ambient temperature seem to be the least important variables. There are variations with temperature and as summer approaches I expect to notice changes to the whole process. However if I were to compare temperature and humidity I would guess that humidity has a strong linear effect on the process while temperature has a weak linear effect.

There is also the issue of development as a variable It occupies its own special place in the list of variables as it can correct and compensate for errors in the others.

Anyhow those are the large factors I try to conceptualize and my way of thinking about them. I would be interested in any comments or suggestions for a better approach

Thanks!

 

[donbga]

Hi,
I am reposting this from the Alternative Process list at the request of Don Bryant.

I have been working with gum for several months and have gotten to the point that I can make a decent monochrome/duotone print but am not yet at the Tricolor level.

What I am interested is if I am more or less right in how I percieve the relative importance of the printing variables. Since gum is a tricky process to learn, having a decent vision of what is going on can help to make it more predictable.

I am using potassium dichromate and lana aquarelle cold press paper. I use compact fluorescent UV lamps to expose. I make digital negatives and apply Christina Z. Anderson's simple gum curve found in alternativephotography.com. I don't see the need to consider more sophisticated curves until I gain more skill with the process.

The most important variable appears to be time as exposure time determines the ability of the gum to stick to the paper and the extent to which the highlights are colored.

The second most important seems to be humidity. Here in North San Diego County, CA wild shifts in humidity can occur rapidly. I have to watch the National Weather Service and my hygrometer pretty carefully since humidity seems to dramatically affect printing times, and low humidity can make coating an awful experience. Below 40% I humidify my paper in a water based humidification chamber before coating and before exposure and I run a humidifier. This seems to bring a modicum of predictibilty to the exposure process (low humidity=long exposure-high humidity=short exposure.

Pigment and pigment concentrations seem to be the third most important factor and since each pigment varies in how it bleeds out, tends to affect exposure and God(dess) knows what else, I ran some simple exposure time test strips and use them as a rough guide as I slowly get to know each pigment.


There is also the issue of pigment concentration/exposure as high concentration, short exposure prints toward shadows while low concentration, long exposure prints toward highlights. I'm not sure this even fits into a hierarchy since it is a way of working with time and with pigment to affect the layers that make up a print. Perhaps the complexity of this and the difficulty of reducing the process to simple concepts points to one of the great beauties of the process, that is, how one must work intuitively yet methodically to handle it at all. Well, each successful print is a "small miracle" at least to me at this stage in my relationship to Gum.

Temperature of wash water and the ambient temperature seem to be the least important variables. There are variations with temperature and as summer approaches I expect to notice changes to the whole process. However if I were to compare temperature and humidity I would guess that humidity has a strong linear effect on the process while temperature has a weak linear effect.

There is also the issue of development as a variable It occupies its own special place in the list of variables as it can correct and compensate for errors in the others.

Anyhow those are the large factors I try to conceptualize and my way of thinking about them. I would be interested in any comments or suggestions for a better approach

Thanks!

Hi Jon,

Thanks for posting here. Are you sizing your paper. It's been my experience that sizing paper helps remove one of the variables of gum printing which is staining.

Second, I would definitely recommend that you derive your own adjustment curve instead of using a generic one. I think you will be much happier with the results.

Third, pigment concentration can be a can of worms, it's something that you will want to methodically experiment with. Certain pigments (say like thalo blue) print very intensely and the pigment concentration needs to be reduced.

Fourth, humidity is important and maintaining a constant humidity in your coating room will insure better success with digital negs, in fact any variable that you can control repeatedly is import IMO, when using digital negatives.

Don

 

[Jon Harwood]

Hi Don,

Yes I am sizing with gelatin hardened with glyoxal and that has given good results and has blessed me with freedom from staining so far. I also pre-shrink. I am developing a modified registration based on pencil corner marks and push pins and getting decent results although I will have to refine this as I go along.

I am keeping very detailed notes of pigment behavior so I can eventually have a good understanding of their behavior.

I think you are right that I will have to work out an individualized curve at some point. I have Mark Nelson's PDN book and I have read it, but I have been postponing curve testing and development until I gain some skill and consistency in printing. I have finally gotten to the point that I can set an intention for a print and then (after a few tries) render something that is more or less in the ballpark of the intention, so I suppose the next step will be testing curves.

I was hesitant to begin working with gum because it seemed more difficult than most alternative processes, but gum is the most addicting and wonderfully expressive method i have yet discovered. So I am finding that I am practically compelled to explore the possibilities and that will be including curves and step wedges. Thanks for the comments.


jh

 

[donbga]

Jon,

Yes I am sizing with gelatin hardened with glyoxal and that has given good results and has blessed me with freedom from staining so far.

I harden with glutaraldehyde and I like it much better than glyoxal. My sized paper is smooth as a babies bottom, but if glyoxal is working for you stick with it. I've never had any yellowing issues with glyoxal as some have reported but the sized surface is quite stiff.

I also pre-shrink. I am developing a modified registration based on pencil corner marks and push pins and getting decent results although I will have to refine this as I go along.

I also pre-shrink parent size sheets and size afterward. Try doing that with 100 sheets of paper.

>
I am keeping very detailed notes of pigment behavior so I can eventually have a good understanding of their behavior.
>

Good move you won't be disappointed.

but I have been postponing curve testing and development until I gain some skill and consistency in printing.

I think this is really a smart idea. When you get ready to start building your own curves start out with cyanotypes. It's a really easy and cheap process to build a curve for. Just be sure to test on sized paper if you want to use cyanotype for tri-color. A nice cyanotype curve will make beautiful prints in and of themselves. And this will help you become savvy with curve building.

I have finally gotten to the point that I can set an intention for a print and then (after a few tries) render something that is more or less in the ballpark of the intention, so I suppose the next step will be testing curves.

Sounds like you are working with the true spirit of gum printing. Don't be afraid to do physical developement if you need to or as a creative option.

but gum is the most addicting and wonderfully expressive method i have yet discovered.

Once you get rolling with gum the process is very addicting.

So I am finding that I am practically compelled to explore the possibilities and that will be including curves and step wedges. Thanks for the comments.

Good luck.

Don