New Cyanotype printed-out image is green

I have been printing with New Cyanotype for about five years. During that time I have made up at least twelve batches of 100-200 ml each. NC has a long shelf life when refrigerated, and I have never had my sensitizer go "bad." My current batch is 2 months old. I follow the Cyanomicon instructions in preparing the sensitizer, coating paper, exposing, and wet processing. I print exclusively on Arches Platine 145 gsm. I do not humidify or pre-treat the paper in any way.

Paper is always bright yellow after coating and remains bright yellow after it has dried for an hour at a constant humidity of 30%, a good sign that the paper does not contain any alkaline additives. When a print comes out of the exposure unit, the extreme highlights remain bright yellow. But yesterday the extreme highlights (and the overall printed-out image) had a distinctly greenish cast when the print came out of the exposure unit. I coated a second sheet of paper and allowed it to dry for 1.5 hours, but got the same result. The paper is from a recently-purchased package of 22x30 sheets, but I have made at least a dozen prints from the same stock before this happened.

Any idea as to what's happening?

I use the traditional cyanotype recipe. Never used the New Cyanotype. I made a print last week with solutions I mixed up several months ago. Got a lovely print on paper designed for Alt. What paper are you using? Have you tried mixing up a new batch of sensitiser?

New Cyanotype ≠ Classic Cyanotype so your experience using one doesn't translate to the other. My paper is Arches Platine 145 gsm. I mixed the sensitizer from raw chemicals 2 months ago, so it's relatively fresh (it keeps 18 months when refrigerated). I'm doing some testing so I make a lot of smallish prints. There were no problems with any of these prints until boom!, this happened. I suspect I some paper from my stock may be the problem. I'll treat some paper with sulfamic acid to see if that helps.

I didn't mention it, but the print itself came out just fine.

Yes, I know all about the New Cyanotype, and it's formula. I went with the traditional cyanotype, because it less toxic... I teach my photography students cyanotype, and my school's administrators like the sound of that You are using Arches Platine, so it shouldn't require any acidification. I dumped that paper after I got a package from the notorious bad batch from a few years back... Bob Carnie also fell prey to it as well. I think he switched over to Hahnemuhle Platinum Rag, as did I. The bad batch that I have works really well though, for my carbon transfer printing Well, after all that, if your print came out fine then all is well!

I have heard about the notorious bad batch. What are the symptoms when you print with paper from the bad batch?

jisner said:

I have heard about the notorious bad batch. What are the symptoms when you print with paper from the bad batch?

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If you hold the developed print up to the light, you can see many black spots. Some of the are visible on the surface of the image. That was from making Kallitypes, and image it would be the same for the New Cyanotype formula, as it contains iron salts, too.