Started with Gumprinting but things don't work out.

[Uncle Goose]

I need some advice here because I fail to make a decent gumprint here.

The results are in no way consistent, even when I use the same amounts of sensitizer, gum, paint and exposure. I use 1 part gum to 1 part sensitizer (ammonium dichromate in a 10% solution) and a little lamp black paint. I tried to vary the exposure a few time to see what would happen but at 2.5min under Phillips UV lamp the image washed off for the most part, 6min was better but far from good. I don't know where the problem is. Anybody can help me??

 

[Chazzy]

Sounds as if you need longer exposures?

 

[smieglitz]

You don't mention the pigment content in the gum. Try reducing the pigment amount. Too much pigment can block the UV and cause underexposure. When the gum print is underexposed, the image washes away during processing.

 

[dmax]

You need a saturated dichromate solution for the sensitizer to work correctly. 10% is too dilute and won't harden the pigment/gum mixture sufficiently (assuming adequate exposure.) Insufficient hardening will then cause most of the emulsion to wash away.

 

[Katharine Thayer]

dmax, that's not quite correct, in fact it's been demonstrated in lab studies that a rather vanishingly small amount of dichromate will harden a given amount of colloid if the exposure is long enough. Not that any of us would want to bother with such a long exposure, but just to make the point that there's nothing wrong with 10% dichromate. Many people use such a dilution all the time and it hardens the gum just fine; the trick is to lengthen the exposure to match the more dilute dichromate.

I prefer the longer smoother tonal scale and shorter exposures of saturated ammonium dichromate, but that's a personal preference; others prefer the higher contrast (fewer steps but more separation between steps) of a more dilute dichromate.

For the present difficulty, I'd agree with others that the answer is most likely more exposure. Overpigmentation..... it's possible but lamp black is such an intense pigment that I think it would be difficult to overpigment it to the extent that it would block UV, without getting staining. Lamp black tends to stain if overpigmented, and if it's all washing off without leaving a pigment stain, then I'd guess overpigmentation isn't the problem.
Katharine

 

[epatsellis]

Katherine,
adding a comma, all the ruckus over Judy's grammatical corrections must be getting to you...


erie

 

[Katharine Thayer]

Funny but I think you've got the wrong forum. No, it wasn't that I was afraid of being criticized for a missing comma, but that I'm a writer, as well as a gum printer, and if I see where editing might improve the clarity or accuracy of my prose, I'm glad to have the ability to go back and make the correction. I really like the edit feature here.
kt

 

[epatsellis]

Sorry, should have added the alt-process list reference...


erie

 

[Uncle Goose]

OK, maybe a little follow-up here.

I got the bearings right now, I began to make teststrips because otherwise it was just like shooting in the direction of an enemy without knowing how far he sits, so you occasionally have a hit but to much got lost due failing accuracy. So finally I got the image staying on the paper, I switched to pigment instead of watercolor because it gives me the feeling I have more control on the dosage. I also used the recommended saturated Ammonium Dichromate solution and that worked good, the only thing now it a way to boost the contrast because it lacks quite some contrast. I still have that bottle of 10% solution so I could add more Ammonium Dichromate to see where it gets to a good contrast. I understand that having a less saturated solution will increase contrast so I might try it from there.

Thanx everybody for the info and tips are still welcomed.

 

[Katharine Thayer]

Hi, I'm glad you're getting better results, but it seems to me you're working against yourself here, perhaps by varying too many variables at once. If you like the stronger contrast of diluted dichromate, then you should stick with the diluted dichromate and keep increasing the exposure til you get the proper amount of hardening. If the soft contrast of the saturated ammonium dichromate doesn't please you, there's no reason to use saturated dichromate just to speed up the hardening, when you can get the harder contrast you want by using a more dilute dichromate and lengthening the exposure to match. It's just a matter of adjusting your practice to the requirements of the materials that suit you.
Katharine

 

[Uncle Goose]

Well, the 10% dilution works but the contrasts are just too harsh, it almost feels like a monographic image rather then a photograhic image. To work things out I made 2 additional dilutions, a 15% and a 20% and looking at the first results it seems that the 15% will do the trick, is has nice contrast that isn't too harsh. I might do the 20% too just to see what it gives but I will first practice a little bit more with the 15% till I got everything well under control.


BTW, does somebody ever tried to apply the coating with an Airbrush? That would be quite smooth but maybe the coating would be too thin to work with. It's not something I would try but it was just a thought.

 

[Katharine Thayer]

Well, that's always been my feeling about dichromate below a certain level, that I just don't like the harder contrast. But it's hard to be able to tell what another person will prefer; glad you're getting a dichromate dilution dialed in that's giving you a contrast you like.

As to applying the dichromated gum with an airbrush, a caution: dichromate is most dangerous when airborne and breathed into the bronchia and lungs, so I personally wouldn't apply it with an airbrush without a hood and mask.