clearing dichromate stain (again): less nasty?

timeUnit · Jul 6, 2007

I understand that potassium metabisulfite or sodium metabisulfite can be used to clear the dichromate stain on gum prints. After a little research, it seems that the pot. or sod. variety is equal in "clearing power". Anyone to disagree?

Also, I wonder, is either one less "nasty"?

Last time I used pot. metabisulfite the odours were quite nasty -- made me cough a bit and I had a annoying taste in my mouth for a while after. Is it the same with sodium?

Thanks!

Henning

Lukas Werth · Jul 6, 2007

Try sodium sulfite - this is what I use. Just mix a new batch every day, it goes off.

Loris Medici · Jul 6, 2007

Hi Lukas,

Doesn't sodium sulfite cause softening of paper fibers and hardened gum?

Lukas Werth said:
Try sodium sulfite - this is what I use. Just mix a new batch every day, it goes off.

timeUnit · Jul 6, 2007

How long does it take to clear a print with sodium sulfite?

Katharine Thayer · Jul 6, 2007

My informal tests persuaded me that they (potassium metabisulfite and sodium bisulfite) are equally effective and equally odorous. I rarely get dichromate stain so don't often need to clear gum prints, but when I do, I put the trays outside.

I also found that sodium sulfite works, but my observation was that it takes significantly longer. I don't remember the times but could look it up if you're interested. My vague recollection is that it took somewhere in the neighborhood of an hour to clear to the same extent that the bisulfites clear in 5 minutes.

Lukas Werth · Jul 6, 2007

Loris,

truth is, I use sodium sulfite routinely to clear chrysotypes and pt/pd prints, as well as for cyanotypes (changes colour! - may be wishful, and clears prior bichromate very effectively), my pigment prints need clearing also only very occasionally.
I don't recall having ever seen any softening; perhaps this might happen with a much stronger solution than I use.

And, Katharine, I also never used a claring bath for gum/casein anywhere near an hour, so your finding surprises me. Do you use a freshly made-up solution?

Katharine Thayer · Jul 6, 2007

Loris Medici said:
Hi Lukas,

Doesn't sodium sulfite cause softening of paper fibers and hardened gum?

I missed this post in my first quick reading of the thread. I suspect this may be one of those gum "myths" that gets passed from person to person, but it's usually sodium bisulfite (or sodium metabisulfite) that is said to soften the gum, not sodium sulfite. (See Steven Livick's manual, for example).

When I compared potassium metabisulfite to sodium bisulfite for clearing gum, I could see no difference in clearing effectiveness or in their tendency to soften the gum. Since I've been told by chemists that functionally the two are interchangeable for the purpose, I find it hard to credit statements that say that one is "better" for clearing than the other, especially since my own observations don't show any perceptible difference.
kt

Katharine Thayer · Jul 6, 2007

Lukas Werth said:
Loris,

truth is, I use sodium sulfite routinely to clear chrysotypes and pt/pd prints, as well as for cyanotypes (changes colour! - may be wishful, and clears prior bichromate very effectively), my pigment prints need clearing also only very occasionally.
I don't recall having ever seen any softening; perhaps this might happen with a much stronger solution than I use.

And, Katharine, I also never used a claring bath for gum/casein anywhere near an hour, so your finding surprises me. Do you use a freshly made-up solution?

Hi Lukas,
I only ran that one set of tests with sodium sulfite, and yes I mixed it up fresh. At the time, I was curious about a statement someone made that sodium sulfite would not (could not) act as a clearing agent, because the pH is on the wrong side of neutral, or something like that. I didn't find that assertion (the assertion that sodium sulfite would not clear dichromate stain) to be supported by my tests, but I did find that the sodium sulfite took longer than the bisulfites take. As I said, I don't remember the times exactly, and it could well be that my vague recall of an hour for the sulfite may be incorrect. If you've used it routinely for clearing gum with consistently shorter times, then I'd be more inclined to credit your practical experience than my one-time test.
Katharine

Katharine Thayer · Jul 6, 2007

Okay, out of curiosity I looked up that comparison of sulfite to metabisulfite, and it did indeed take an hour in my test for the sulfite to clear as well as the bisulfite cleared in 5 minutes. But that's really neither here nor there, since I still say that Lukas's practical experience using sodium sulfite to clear in less time than this counts for more than this one test.
kt

Lukas Werth · Jul 9, 2007

Katharine Thayer said:
Okay, out of curiosity I looked up that comparison of sulfite to metabisulfite, and it did indeed take an hour in my test for the sulfite to clear as well as the bisulfite cleared in 5 minutes. But that's really neither here nor there, since I still say that Lukas's practical experience using sodium sulfite to clear in less time than this counts for more than this one test.
kt

On the other hand, you may very well be right that metabisulfite is much better for dichromate, an observation quite valuable for me. Is the stench troublesome?

timeUnit · Jul 9, 2007

It's quite irritating for the throat. It's a sort of stinging feeling in the throat, and I cough a bit from it. Should be done outside I think. If one has a decent mask on, I guess the problem would be slight.

Lukas Werth · Jul 9, 2007

timeUnit said:
It's quite irritating for the throat. It's a sort of stinging feeling in the throat, and I cough a bit from it. Should be done outside I think. If one has a decent mask on, I guess the problem would be slight.

Thank you for this answer, but what about sodium bisulfite?
As a rule, however, and as Katharine also seems to imply, I don't think that regular clearing is necessary for gum or casein, provided the right papers are used correctly sized.
For which process do you need to clear?

timeUnit · Jul 9, 2007

I cleared my gum prints when I did that process. This was in march last year. My teacher, Chia N-Löfqvist, taught me that this was common practice in the gum bichromate process. I'm a beginner, and didn't argue...

I think it was Kerik Kouklis who stated that dichromate stain could have to do with water quality. Anyway, we needed to clear the prints, as there was a visible stain. It also had a good effect on the clarity of the pigments, ie better saturation.

If one does not clear the prints, is there not a risk of staining over time, as the print is exposed to room light?

I have no experience with the sodium metabisulfite, but I gather it's about as nasty as the pot. variety.

Katharine Thayer · Jul 9, 2007

Lukas Werth said:
Is the stench troublesome?

As timeunit said, the fumes are acrid and will make you cough. This is true equally for potassium metabisulfite and sodium bisulfite, in my experience they are interchangeable in all respects as clearing agents for gum. That's why, when I do clear, I put the tray outside.

Whether clearing is recommended even if one doesn't need to clear visible stain, is an open question. Gum printers have been passing along this axiom for decades, but as far as I can determine, the belief has no evidentiary basis. It hasn't actually been established what dichromate stain is, chemically, which makes it rather difficult to know whether it could be harmful to paper.

timeunit's question of whether an uncleared print contains hexavalent chromium that could become reduced over time with exposure to light, resulting in an eventual dichromate stain even if there wasn't any visible stain to start with, is a good question. Again, we don't really know the answer, but I'm inclined to think that there's very little unreduced dichromate left in a print after development. In my experience, the unreduced dichromate leaves the print very readily, and if the print is placed in clear water to after the unreduced dichromate is washed out, (in other words if the print isn't left to stew in dichromate-laced water) the finished print should be effectively CrVI -free. I've been printing gum for over 20 years and have only rarely cleared a print, but I've not had any uncleared prints develop dichromate stain later.

kt

Jim Noel · Jul 9, 2007

Thanks Katharine.

That is the most authoritative answer to this question I have read anywhere.

Jim

Lukas Werth · Jul 10, 2007

Jim Noel said:
Thanks Katharine.

That is the most authoritative answer to this question I have read anywhere.

Jim

Same here. This exchange is really useful. One question to timeUnit, however: what is the quality of your local water? Soft, hard? Which paper have you been using for gum?

timeUnit · Jul 10, 2007

Ooh, a question for me!

Well, we generally have very soft water here in Göteborg. I'd also say that the quality is excellent. Some cities I've visited have a chlorine (or is it chloride) taste to the water, and others have ferrous taste (a bit like blood.. yuck!). Our water is very much tasteless, sometimes I find it a bit sweet.

I've only printed on Rives BFK from Arches. The paper was sized in gelatin twice, and hardened in a formaldehyde bath. The gelatin was 150 g to 5 liters water (boiled, not distilled), and I think she used 125 ml 37% formaldehyde solution to 5 liters of water (again, boiled).

timeUnit · Jul 31, 2007

I just recently cleared a test print in sod. metabisulfite 25 g and 1 litre water. I used a face mask (the cheap kind) and stood by the open door in my darkroom. I didn't find the fumes to be that nasty, actually I hardly noticed them.

Katharine Thayer · Jul 31, 2007

timeUnit said:
I just recently cleared a test print in sod. metabisulfite 25 g and 1 litre water. I used a face mask (the cheap kind) and stood by the open door in my darkroom. I didn't find the fumes to be that nasty, actually I hardly noticed them.

Henning, you didn't mention the concentration of the potassium metabisulfite that you didn't like before, but clearing agents are most commonly (as far as I know) mixed at 5%. The solution you describe here is 2.5%, half that strong. So it's hard to say whether it's the difference in the cation, or a difference in concentration, or the fact that you were wearing a face mask, that accounts for the experience of less smell. But if it cleared well in a reasonably short time, why not use half as much chemical and save a little money. At any rate, I'm glad you found a solution to the problem.

Katharine

timeUnit · Jul 31, 2007

Oh, I'm sorry! I wrote 25 g. I meant 50 g!

Katharine Thayer · Jul 31, 2007

timeUnit said:
Oh, I'm sorry! I wrote 25 g. I meant 50 g!

Well, never mind half my remarks then. At any rate, again, I'm glad you found a solution to the problem.
kt

clearing dichromate stain (again): less nasty?

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