Cyanotype color change w/ sodium bisulfite: what's going on?
Hi all, I'm looking for somebody who can explain to me the chemistry behind this reaction, and importantly, if the result be deemed "archival."
I put acrylic gloss medium over a VDB print a few months ago and got a better dmax and an interesting brush texture. Great. Last night I tried it on a cyanotype ("new" photo formulary kit) and had some really unusual results: the acrylic medium not only enhanced d-max, but it also boosted contrast significantly and shifted the tone toward deep purple!
From the acrylic medium MSDS ("Preservative - Non Hazardous At Level Used") I presumed that the active chemical was sodium bisulfite, and a different cyanotype treated in a 10% bath of sodium bisulfite exhibited a similar purple shift.
I'm about to start printing a series of large cyanotypes that I intend to show and possibly sell, and I find this effect desirable, but I'm wondering what's going on chemically here, where the purple cast represents a chemical shift in the pigment, or if it's the result of chemical damage. Or more directly, whether this process is archival.
FWIW: the attached image was 9 drops of sensitizer + 2 drops citric acid 40% on Arches Hot Press 140lb, bathed post-exposure in a bath of 1% citric acid before final wash.
I can't help you but it sure looks good.
Ammonia can boost cyanotype density and change color. Acrylics contain amines, close relatives to ammonia.
Just a wild guess.
I just googled acrylic gloss medium and found the same in regard to ammonia content. I also downloaded a MSDS for a popular acrylic gloss medium and varnish, and the pH was listed at between 8-9 making it an alkaline material.
The lavender color is probably from the reaction of the cyanotype with the high pH solution and unfortunately, it is also probably fugitive. In my experience, alkaline solutions also bleach cyanotypes so I would expect to see not only the color fade, but also the image density reduce.
You can observe the color change in cyanotypes and the bleaching effect by immersing a freshly printed cyanotype into a tray of water to which a few drops of ammonia (or film developer, borax or any other common household alkali, etc.,) has been added. It is a neat trick for saving overprinted cyanotypes and reducing their density. Washing the print in acidified water (add a few drops of stop bath to the tray) will restore the blue color but the print will have bleached somewhat.
I wish there was some way to fix the lavender color, but there is none that I know. I think the only toner that is marginally permanent with cyanotype is tannic acid (found in tea) which produces more neutral colors.
Forgot to mention that the boost in density and contrast that you are seeing is probably a physical change in reflection rather than chemical change. I think you would see the same thing density-wise by employing any sort of varnishing agent, gelatin, gum arabic solution, albumen, starch sizing, PVA, etc.
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Just an idea, but wouldn't sealing the print in the acrylic at least offset the fugitive nature of the change in ph? Anyone have any ideas on that?
Also, what would be the long term chemical and fading effects of the change in ph if it wasn't sealed? Would it revert back to a cyan tone?
Originally Posted by smieglitz
What an excellent reply, thank you! I suspected that the purple was fugitive. Having already been disappointed over the failure of the excellent warm tones I was seeing on my partially ferri-bleached silver-gel prints to survive a second fixing, I knew not to get too excited over this unexpected color shift.
I didn't want to complicate my original post, but to rule out the sulfur in sodium bisulfite, I tried another base, sodium hydroxide. I was expecting -- and observed -- a pretty strong reaction! A drop or two of NaOH at a medium strength took the cyanotype from dark blue to lavender to light purple to near-white within seconds.
I was using Art Advantage acrylic gloss varnish (http://www.art-advantage.com/msds/3240.pdf) which I doubt is archival in the first place. From what I understand, cyanotypes need paper with a neutral pH to be properly archived, basic or buffered environments are said to be hostile. Well, this acrylic medium fails on that point, so already the idea of sealing in the purple color with the coating is out.
At any rate, I plan to explore this further, so I'll keep APUG updated in this thread if anything emerges.
Further advice still welcomed, of course.
You can use Hydrocote Polyshield Clear Gloss polyurethane wood finish for the same purpose.
This material also is slightly alkaline and can give your cyanotypes a slight lavender cast *when used undiluted* but the cast reverse in a couple of days without any perceptible density loss and/or bleaching of highlights.
(Speculating ON) To be on the safe side, you can clear (not develop!) your prints in a mild (2%) citric acid bath. By doing so, citric acid molecules will be trapped inside the prussian blue lattice (this is highly probable -> see Mike Ware's book on Cyanotypes) and presumably protect the image from bleaching. (Speculating OFF)
I prefer to use it 1:1 diluted. That way, it will still improve dmax but without adding any (highly un-natural) gloss to the image. Of course the improvement won't be as much as when using it undiluted but it's sufficient to have convincing blacks...
BTW, Hydrocote can be considered as being archival since it doesn't yellow with time + it's original purpose is to protect wood in outdoor conditions. (Garden furniture, boats ect...)
Please note I'm not in the alternative processes yet, so beware of my possible ignorance, but in terms of archivability, isn't prussian blue supposed to be a highly stable pigment???
Originally Posted by Loris Medici
Prussian blue was one of the first non-natural pigments used in oil painting, and is still available to date as a major blue pigment for artists (I've used it several times myself). The artist quality oil paints are supposed to be able to last for centuries... so why is prussian blue in a cyanotype not archival?
Or is the archivability of oil paint "prussian blue" based on it's enclosure in the linseed oil?
It's only nonarchival in a basic/alkaline enviroment. I've had cyanotypes bleach out and look like acidwashed jeans if they've been left in hard tap water for an hour or two.
Makes matting an interesting challenge considering all mat board made these days is "acid free archival" and usually buffered with chalk too making it completely NOT archival for cyanotypes